Sports Med
DOI 10.1007/s40279-015-0305-z
REVIEW ARTICLE
Muscle Quality in Aging: a Multi-Dimensional Approach
to Muscle Functioning with Applications for Treatment
Maren S. Fragala • Anne M. Kenny
George A. Kuchel
•
Ó Springer International Publishing Switzerland 2015
Abstract Aging is often accompanied by declines in
physical functioning which impedes older adults’ quality of
life, sense of independence, and ability to perform daily
tasks. Age-related decreases in skeletal muscle quantity,
termed sarcopenia, have traditionally been blamed for
these physical decrements. However, recent evidence
suggests that the quality of muscle tissue may be more
functionally relevant than its quantity. ‘Muscle quality’ has
been emerging as a means to elucidate and describe the
intricate intramuscular changes associated with muscle
performance in the context of aging and sarcopenia. While
muscle quality has most commonly been defined in terms
of muscle composition or relative strength, at the core,
muscle quality really describes muscle’s ability to function.
Skeletal muscle displays a strong structure–function relationship by which several architectural characteristics
factor into its functional capacity. This review describes
the structural, physiological, and functional determinants
of muscle quality at the tissue and cellular level, while also
introducing other novel parameters such as sarcomere
spacing and integrity, circulating biomarkers, and the
muscle quality index. Muscle qualitative features are described from the perspective of how physical exercise may
improve muscle quality in older adults. This broad, multidimensional perspective of muscle quality in the context
of aging and sarcopenia offers comprehensive insights for
M. S. Fragala (&)
Institute of Exercise Physiology and Wellness,
University of Central Florida, 4000 Central Florida Blvd,
Orlando, FL 32816, USA
e-mail: maren.fragala@ucf.edu
M. S. Fragala A. M. Kenny G. A. Kuchel
University of Connecticut Center on Aging, University
of Connecticut Health Center, Farmington, CT, USA
consideration and integration in developing improved
prognostic tools for research and clinical care, while also
promoting translational approaches to the design of novel
targeted intervention strategies designed to maintain
function and mobility into late life.
Key Points
Despite the detrimental contributions of aging on
skeletal muscle, no consensus definition of muscle
quality presently exists.
A multi-dimensional understanding of muscle
qualitative factors contributing to functional
impairments may lead to improved efforts in
identifying risk factors and developing effective
treatments.
Physically exercising under-utilized muscles appears
to be the leading strategy for improving muscle
quality and function.
1 Introduction to Skeletal Muscle Quality
As many as half of the older adult population in the USA are
affected by sarcopenia, a progressive, debilitating musculoskeletal condition [1]. Initially, the term ‘sarcopenia’ was
introduced to describe the age-related reduction in skeletal
muscle mass—somewhat similar to how osteoporosis
affects bone. However, unlike osteoporosis, which remains
asymptomatic until an unfortunate catastrophic event, sarcopenia manifests itself in regressive mobility and
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functional impairments [2]. While declines in physical
function and loss of physiological resilience have traditionally been attributed to the age-related decreases in
muscle mass, recent evidence indicates that the quality of
the muscle tissue may have relatively greater functional
relevance. Muscle quality refers to the tissue’s capacity to
perform its various functions, including contraction, metabolism, and electrical conduction. Many dimensions of
muscle quality have relevance to muscle’s capacity to
perform its various functions, spanning from the broad
aspect of whole muscle force production to muscle composition and morphology to the level of the sarcomere (the
basic contractile element). In clinical research, muscle
quality has often been measured in terms of strength normalized to muscle mass as determined by dual-energy x-ray
absorptiometry (DXA) or computed tomography (CT).
Recently, there has also been a growing interest in defining
the structural, physiological, and biological determinants of
muscle quality at the level of muscle tissues, individual
muscle cells, cellular components, or metabolic pathways
(Fig. 1). While several prior reviews have eloquently and
thoroughly described aspects of muscle quality assessment
[3, 4], functional deficit [5], endocrinology [6], pharmacology [7], treatment [8], or exercise training [9], none have
yet integrated the clinical, mechanistic, and treatment approaches into a multi-dimensional approach as intended in
this review. This article reviews the complex indices and
dimensions of skeletal muscle and highlights the concept
that its function stems from the interaction of several
qualitative characteristics. We also briefly highlight the
available data on functional and clinical implications to
support targeted interventions for improving muscle quality. Recent studies by our laboratory and others may have
important clinical relevance as they demonstrate that even
without increases in muscle mass, exercise interventions
may be beneficial to improving other features of muscle
quality. Thus, multidimensional perspectives of muscle
quality may offer insights towards improved prognostic
tools for research and clinical care, while also promoting
translational approaches to the design of novel targeted
intervention strategies.
1.1 Skeletal Muscle in Aging and Physiological
Resilience
Advancing biological age is frequently accompanied by
diminished physiological reserve and decreased physical
functioning which adversely impact perceived quality of
life and well-being with aging. Amongst the most detrimental consequences associated with such deterioration is
the presentation of the frailty phenotype. An overarching
common dimension to several conceptualizations of frailty
involves decreased physiological resilience and increased
vulnerability to adverse events leading to an enhanced risk
of future disability and mortality [10, 11]. The Fried frailty
phenotype is characterized by weight loss, fatigue, weakness, and vulnerability to other adverse events associated
with morbidity and mortality [12, 13]. While the physiological basis for frailty is generally viewed to be multifactorial, sarcopenia characterized by musculoskeletal deterioration appears to represent a central element [14]. Not
only is skeletal muscle primarily important for physical
functioning, but homeostatic imbalances resulting in altered energy metabolism and protein degradation may be a
critical mediating factor for the onset of frailty. While the
impact of physiological and functional decrements are
progressively burdensome in hindering older adults’ ability
to perform activities of daily living required for independence [15], both disability and frailty can increase susceptibility to severe catastrophic events [16–18]. Thus,
understanding features of skeletal muscle quality changes
with aging, sarcopenia, and frailty may be key to preserving independence and reducing catastrophic events.
However, in examining muscle qualitative changes that
frequently accompany aging and ultimately frailty, it is
important to consider the large physiological variation in
elders of a similar age [19]. Large variations imply that
many of the qualitative changes that we observe with aging
Fig. 1 Skeletal muscle
qualitative features can span
from the level of whole muscle
functioning to features of the
basic contractile elements
Muscle
fiber
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Relave
strength
Sarcomere
Z-line
Myofibril
Skeletal
muscle
Whole
muscle
funcon
Acn
Myosin
Muscle
quality
index
Muscle
composion
Muscle
architecture
Muscle
ultrastructure
Basic
contracle
elements
Muscle Quality in Aging
may actually be more of a product of disuse than aging per
se [20]. Thus, throughout this review, aging is considered
the process of cellular and systemic senescence in normal
conditions that ultimately lead to frailty. Although broad,
normal conditions are considered in the absence of disease
or high-level athletic training that can interfere with the
process of what is considered normal aging.
1.2 Skeletal Muscular Quantity Versus Quality
The most widely acknowledged changes in skeletal muscle
with aging entail reductions in muscle mass. Muscle mass
decreases approximately 3–8 % per decade after the age of
30 years, with rates accelerating after the age of 60 years in
an epidemiologic population [21]. This age-related loss in
skeletal muscle mass, termed sarcopenia, is often attributed
to the disability and morbidity observed in the older adult
population [22] and is a primary determinant of the agerelated declines in skeletal muscle strength [23, 24]. Another
term ‘dynapenia’ was more recently introduced to describe
and distinguish the age-associated loss of strength from loss
of mass (sarcopenia) [25]. While some discrepancy remains
as to whether or not to distinguish dynapenia from sarcopenia
or whether muscle weakness should be incorporated into
clinical definitions of sarcopenia, our recent work has revealed that muscle weakness is an important factor in distinguishing older adults with mobility impairment [26].
Regardless of the terminology, muscle weakness is collectively attributed to alterations in muscle quantity, muscle
contractile quality, and neural activation [27].
Although muscle mass is an important underlying factor
contributing to muscle strength [28], mounting evidence
has emerged showing that muscle strength and power are
strongly related to mobility, functional status, and mortality
in frail elderly [29–32] even when adjusting for muscle
mass [27]. Interestingly, several studies have demonstrated
a mismatch in the rates of change involving these parameters, where the rate of decline in muscle strength is
much more rapid than the concomitant loss of muscle mass
[23, 33–37]. In addition, studies have shown that leg
muscle power (a measure of the rate that force is developed) seems to be more important than muscle strength in
determining the ability to perform daily activities [29] or
the risk of hip fracture [38]. Such findings imply that agerelated skeletal muscle changes involve more than just
skeletal muscle mass loss and suggest that additional features of quality and neuromuscular innervation are factors
in muscle strength and functioning.
1.3 Skeletal Muscular Changes Associated with Aging
As muscle mass and functioning decline with age, several
changes occur locally within individual muscles, which
affect the quality of the muscle. With aging, muscle fibers
decrease in both size and number, particularly in type II
(high force) fibers [36, 39, 40]. In fact, autopsy studies
reveal 25 % fewer muscle fibers in the medial vastus lateralis of older (72 years) than in younger (30 years) individuals [41]. Moreover, biopsy studies also show a
changing fiber type distribution with age where the percentage and area of type II fibers in the vastus lateralis is
reduced [42]. As fiber type distribution changes, so does
the oxidative enzyme activity and muscle capillarization,
which decrease [9]. Interestingly, aging has differential
effects on various human skeletal muscle groups [43], depending on their function and fiber composition [43]. For
example, type II fibers of the vastus lateralis, an important
muscle in the leg for locomotion, become very small and
less circular with age, whereas in the masseter, an important muscle for chewing, fibers maintain circularity and
only decreases in size [43]. Additionally, age-related
muscular changes at the cellular level appear to be impacted by sex, where declines in the type II fiber area
appear restricted to men [44].
Several studies have demonstrated the presence of impaired protein synthesis and decreased muscle anabolism
with aging [45–48]. Declines in protein synthesis impair
muscle contractile function, strength, and protein quality
[25, 46, 49]. Several factors may contribute to the disproportional protein turnover observed in aged muscle. The
age-associated increased levels of pro-inflammatory cytokines [50–52] may be part of the mechanisms that cause
the interference with protein synthetic pathways [53] and
contribute to the declines in muscle strength [50, 54], mass
[55], and disability [51, 52]. Furthermore, more recent
evidence also points to an age-related increase in myofibrillar protein glycation that may interfere with contractile
protein structure and function [56]. In addition, muscle
fibers of older adults contain more lysosomes [57], indicating greater potential for protein degradation than
younger muscle. Regardless of the mechanism, it remains
unclear as to whether age-related changes in muscle protein
synthetic rates and contractile characteristics are preventable and whether protein synthesis measurements actually translate into contractile protein hypertrophy in
muscle. (For further detail regarding protein metabolism
with aging, please refer to prior reviews by Burd et al. [58]
and Yarasheski [59]).
With age, the contractile characteristics of muscle fibers
change. Muscle fibers transition to take on slow, type I,
characteristics (fatigue resistant, slow contraction velocity)
[60]. The selective loss of type II fibers, which possess
greater contraction velocity [61], driving this observed
transition impacts the muscle’s ability to generate maximal
force [62]. The mechanisms underlying the slowed contractile speed in the muscle fibers appear to differ between
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slow and fast fibers. In fast-twitch muscle fibers, the decreased speed of contraction is likely due to an age-related
impairment of intrinsic sarcoplasmic reticulum function
and alteration in sarcoplasmic reticulum volume [57, 63].
However, in slow fibers, the age-related decrease in contractile speed appears to be due to a change in the properties of the myosin protein [64]. Additionally, since it is
known that muscle fibers take on the characteristics of the
nerves innervating them [65], it is likely that fiber type
changes associated with aging are attributable to changes in
neural input to the muscle fibers contained within each
motor unit. With aging, motor units are lost [66–68]. As a
result, the size of each motor unit increases with age [68].
As neural innervation changes with aging, muscle fiber
types that are generally dispersed in the muscle in younger
adults tend to group together in older inactive adults [57,
69], reflecting reduced reinnervation. Also, older muscle
fibers tend to be angulated in shape, reflecting denervation
of muscle fibers [69]. Motor nerves also demonstrate agerelated changes where nerve fibers decrease in diameter
[70] and nerve conduction slows [71]. In addition, the
neuromuscular junction (NMJ) where the motor neuron and
the muscle fibers meet changes with aging where presynaptic nerve terminal branching and post-synaptic distribution of neurotransmitter receptor sites are increased
and the NMJ has limited capacity to adapt [72]. This may
affect the ability of aged muscle cells to modulate calcium
release with muscle contraction. Thus, age-related changes
in neuromuscular activation appear to largely contribute to
declines in strength and power [73, 74].
2 Describing Muscle Quality
Muscle quality describes the physiological functional
capacity of muscle tissue. In quantifying muscle quality,
muscle’s contractile function is often assessed as the muscle’s ability to generate force measured as strength, power, or
function. Measuring the whole muscle’s ability to function or
generate force represents an index of muscle quality. Such
indices of muscle quality include measures of relative
strength and muscle quality index (MQI), as described in
Sect. 2.1. However, muscle quality indices are ultimately
dependent on the qualitative features of the muscle tissue,
including the composition, architecture/morphology, and
ultrastructure of the contractile apparatus, each of which are
described in Sect. 2.2. Additionally, the assessment of
muscle qualitative features is dependent on instruments and
technologies. There are several emerging technologies
including applications of ultrasound to measure muscle
architecture, second harmonic generation (SHG) to qualify
ultrastructure, and circulating biomarkers to reflect muscle
status, which are also described.
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2.1 Indices of Muscle Quality
2.1.1 Relative Strength
Typically, muscle quality is defined indirectly as muscle
strength relative to the muscle quantity or the amount of
muscle mass generating the force [75, 76]. When defined in
this way, like muscle strength and mass, muscle quality
declines with aging [77, 78]. In addition, muscle quality
(relative strength) has been shown to be a stronger predictor of performance than strength, mass, or body composition alone in older adults [79–81]. Interestingly,
muscle quality (as relative strength) has been shown to be
inversely related to skeletal muscle mass [79, 82]. However, several complexities exist in interpreting and comparing muscle quality when defined as relative strength
such as movement type (isotonic vs. isometric vs. isokinetic) [35, 83] and anatomical location (upper vs. lower
body) [75].
2.1.2 Muscle Quality Index
Recently, Barbat-Artigas et al. [4] suggested the MQI as an
assessment of muscle quality based on a functional test for
older adults. The MQI estimates muscle power from body
anthropometrics and timed chair rises [84]. It elaborates on
the typical chair rise test by accounting for the anthropometric measures of body mass and leg length that have
previously been shown to alter the relationship between
chair rise performance and leg strength [84]. As a clinically
feasible potential assessment of muscle quality, we compared the sensitivity to change of the MQI to other functional tests, and found that that the MQI increases with
resistance exercise training in older adults to a greater
magnitude and presents higher reliability than other functional measures (gait speed, grip strength, get up and go)
[85]. Thus, the MQI should be considered as a potentially
informative clinical outcome in interventional studies in
older adults. While indices of relative strength are more
informative than measures of muscle mass alone, such
definitions of muscle quality do not account for other
muscular qualitative features such as architecture or composition or functions such as mobility.
2.2 Dimensions of Muscle Quality
2.2.1 Muscle Composition
Age-related changes in body composition [28, 86] can
affect muscle performance both functionally and physiologically. Functionally, the low lean mass to total body
mass ratio can impede functional performance [87] and
muscle strength [88], and lead to frailty [89] and nursing
Muscle Quality in Aging
home admission [90]. Physiologically, excessive body fat
is related to lipid infiltration [91, 92] in the muscle which
can impede muscle functioning (voluntary force production). Measures of total and regional muscle mass used to
assess body composition and quantify muscle quality are
often accurately measured with DXA. Although informative for identifying muscle mass thresholds associated with
muscle weakness, such technology lacks the sensitivity to
distinguish muscle composition. DXA measurements of
skeletal muscle mass assume that all non-fat and non-bone
fat-free mass is skeletal muscle mass [93], and cannot
detect fat that infiltrates the muscle. Thus, DXA fails to
identify intramuscular adipose deposits, the visible fat beneath the muscle fascia and between the muscle groups
[94], as well as intramyocellular lipid droplets located
between muscle fibers. The measurement of muscle composition in vivo requires more sophisticated and less
available technologies [95] such as magnetic resonance
imaging (MRI) or CT [96]. Most studies assessing muscle
composition have used CT, which quantifies muscle density based on attenuation characteristics attained with
compiled x-rays to discern fat and muscle tissue within the
muscle [97]. Such measures indicate a positive association
between muscle density and strength, regardless of size
[92], indicating that lipid accumulation in the muscle
(lower muscle density) hinders muscle quality.
With aging the density of skeletal muscle decreases
[92, 98, 99], indicating lipid accumulation in the muscle
[100–102]. Older men have 59–127 % more fat in the
muscle compartments of the thigh than younger men [98],
with an annual increase of 18 % shown in longitudinal
measures [103]. Excessive lipid infiltration in skeletal
muscles is associated with low muscle strength and poor
physical performance [28, 55, 104], independent of crosssectional area (CSA) of the muscle. Age-related fat infiltration in the muscle hinders the contractile ability of
the muscle [105] and functionality in older adults [106,
107].
Intramuscular lipid stores include both intramyocellular
lipids, spherical droplets located between the muscle cells,
and extramyocellular lipids, strands of adipocytes surrounding muscle fibers [108, 109]. Both stores of lipid
have different metabolic roles, where intramyocellular
lipid droplets are more readily available in the muscle to
provide an energy substrate during exercise while extramyocellular lipids have a slower turnover and serve as
a longer-term storage site [110]. Older adults have increased intramyocellular lipids [111], characterized by
larger droplet size [112] and a reduced oxidative capacity
in comparison to younger adults [113]. This may result
from the continuous supply of fat to the muscle without
concomitant oxidation, resulting in the accumulation of
intramuscular lipids [111].
2.2.2 Muscle Architecture
Muscle contractile function is related to its architectural
characteristics [114–116]. Amongst the architectural characteristics relevant to the force-producing capacity of
muscles are both the muscle fiber length and arrangement
in relation to the direction of force produced by the whole
muscle [117]. A muscle is advantaged in force production
when fibers are oriented at a large pennation angle from the
direction of force production where more fibers can be
packed into a greater volume [115, 118]. Accordingly,
amongst the age-associated architectural changes in
skeletal muscle, muscle in older adults has reduced muscle
fiber length [119, 120] and altered pennation angles [121],
likely due to fiber atrophy. Such architectural changes
contribute to reduced force production capacity. We recently reported on muscle architectural changes to resistance exercise in a cohort of healthy adults using
physiological CSA (PCSA) of the thigh [122]. In contrast
to traditional cross-sectional area measures, muscle PCSA
measures of the vastus lateralis included a composite of
architectural measures including CSA, echo intensity,
pennation angle, and fascicle length. Muscle architecture
measured as PCSA was significantly correlated with leg
extension strength and change in leg extension strength
[122].
2.2.3 Muscle ‘Ultrastructure’
Despite general consensus regarding age-related decreases
in whole muscle strength and function with age, some
discrepancy exists in the literature when examining single
human skeletal muscle fibers. Some findings indicate ageassociated increases in muscle fiber stiffness [123], reduced
contractility [123], and reduced specific tension [44].
Others have reported preserved single muscle fiber contractile function despite significant decreases in whole
muscle level strength [124]. Such discrepancies have been
attributed to physical activity, where active older adults
have preserved single fiber contractile properties [125], or
compensation by surviving fibers to partially correct muscle size deficits in an attempt to maintain optimal forcegenerating capacity [124]. Regardless, more in-depth
evaluation of muscle contractile apparatus at the ultrastructural level may yield greater insights into qualitative
changes in skeletal muscle with age.
2.2.4 Sarcomere: The Basic Contractile Element
While most focus on aging muscle has been at the macrostructural level, it is important to consider that age-related
muscle quality changes involve qualitative changes in
muscle ultrastructure [40, 126]. Muscle cells are comprised
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of myofibrils or cylindrical structures, containing repeated
complexes of organized specialized proteins (sarcomeres)
[127] (Fig. 1). Sarcomeres contain overlapping thin (actin)
and thick (myosin) filaments [128], which represent the
basic contractile elements for muscle contraction [129].
The actin filaments are anchored to multi-protein complexes known as the Z-disks [130], which border each
sarcomere, maintain structure, and transmit tension during
contraction [131]. Thus, the length of the sarcomere is most
prominently defined by the Z-disk boundaries [132]. The
length of the sarcomere quantifies the overlap between thin
and thick filaments, and hence the potential for force
generated from the actin and myosin interactions [133].
Muscle contraction ultimately occurs when the Z-disks are
pulled together as myosin cross-bridges link to actin filaments, shortening the sarcomeres in series [128]. The force
generated by the contracting sarcomeres within the muscle
is transmitted throughout the muscle by the extracellular
matrix.
Some evidence exists to indicate that sarcomeric changes
may occur with aging in a manner that may inhibit muscle
functioning. Animal comparison models show evidence that
differences in myofilament lengths are related to functional
demands of muscle [134]. Electron microscopic analysis of
human skeletal muscle biopsies reveal myofibrillar disorder,
Z-line streaming, and dilatation in aged skeletal muscle [57].
In addition, aged skeletal muscle shows a variety of changes
within the extracellular matrix, including collagen accumulation and altered elasticity [135]. Since the order and arrangement of the myofilaments play a crucial role in force
generation [136], age-related alterations in sarcomere spacing and integrity and extracellular matrix functioning may
contribute muscle force-generating capacity. The reduced
muscle fiber length in aged skeletal muscle may be due to a
loss of sarcomeres in series since sarcomeres are lost as a
result of decreased muscle tension from disuse [137]. Some
data from human biopsy samples indicate that sarcomere
length is reduced in older compared with younger adults
[138]. From a functional point of view, a loss of sarcomeres
in parallel and in series alters both the length–tension as well
as the force–velocity relationships [116]. Although research
has implicated optimal sarcomere lengths in muscular force
generation [133, 139], how such characteristics relate to
muscle quality, physical performance, and aging is emerging
as a research priority as technologies improve to allow such
evaluations.
3 Future Directions and Technologies in Muscle
Qualitative Assessments
While existing assessments and approaches to muscle
quality assessment have been broad and informative, many
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have been limited by costs, accessibility, feasibility, and
the information that they can provide. New directions,
technologies, and applications of existing technologies may
enable the evaluation of hypotheses so far unanswered and
increase the clinical applications and accessibility to
muscle qualitative information. Among such future directions are the applications of safe and simple ultrasound
imaging to assess muscle architecture, use of imaging tools
to enable the in vivo assessment of sarcomere structure and
alignment, and evaluation of novel biomarkers of muscle
status that may provide new information that can be
assessed through the blood.
3.1 Skeletal Muscle Ultrasound
The ability to quantify muscle size and composition requires elaborate medical imaging equipment that is not
readily available for routine analysis. However, recent
advances have supported the use of ultrasound technologies to non-invasively measure and quantify skeletal
muscle. Ultrasound provides a safe and more widely
available technique to accurately quantify and qualify
regional muscle mass. Data from cadaver [140] and MRI
studies [141] reveal that ultrasound is a valid and reliable
tool to assess muscle CSA, thickness, and volume. Interestingly, we recently found that local muscle hypertrophy may be detected on ultrasound before changes in
DXA are apparent [122]. In addition to measuring local
muscle size, muscle quality may be assessed on ultrasound using post hoc analysis of density by quantifying
echo intensity. Echo intensity is a measure of the reflectivity of the sound waves emitted to the tissue. Connective tissue and lipid are more reflective than more dense
muscle tissue and are depicted in the image as lighter
gray [142].
Ultrasound has been shown to be a reliable tool for
assessing muscle quality as echo intensity [143, 144] (a
proxy of muscle composition), which may be considered an
index of muscle quality. Analyzing the reflectivity via
gray-scale analysis provides a useful low-cost, easily accessible, and safe method to evaluate the muscle quality
[145]. Although some evidence suggests a relationship
between muscle echo intensity and strength [146], our
preliminary study did not find significant changes in muscle echo intensity with acute resistance exercise when
significant strength changes occurred [122]. As skeletal
muscle ultrasound applications to assessing muscle
qualitative features is a newer approach, further studies are
warranted to determine whether it may be a useful application for clinical incorporation. In addition to providing
qualitative and quantitative information on skeletal muscle
integrity, size, and density, ultrasound technology also
enables the assessment of muscle architecture.
Muscle Quality in Aging
3.2 Second Harmonic Generation Technology
Insights contributing to mechanical concepts underlying
physical performance can be attained at the sarcomeric
level [128] and some evidence reveals disordered sarcomeres in aged muscle [57]. Currently, studying the
contractile structures of human skeletal muscle is difficult
and invasive, but can be done through muscle biopsy
procedures, where a grain size sample of muscle is removed from a superficial muscle by a hollow needle. This
muscle sample can be processed and analyzed for muscle
cell histology, including fiber type, arrangement, and organization using microscopy techniques. In spite of the
ability of electron microscopy (EM) to provide important
qualitative information regarding muscle ultrastructure,
this classical approach can be confounded by significant
contraction artifacts associated with tissue fixation [147].
Moreover, the very nature of EM analysis, whereby individual ultrathin 2-dimensional sections are studied, precludes any meaningful quantitative analysis addressing
changes taking place within a complex 3-dimensional tissue [148].
Novel imaging technology involving SHG imaging is
emerging as a means of visualizing muscle ultrastructure in
3-dimensions, followed by objective quantification of sarcomere numbers, dimensions, integrity, and spacing within
intact muscle [149–153]. SHG is a non-linear signal generated from the recombination of two photons on noncentro-symmetric molecular arrays into one photon of half
the energy [149]. The signal is determined by the orientation, polarization, and local symmetry of chiral molecules. Both the collagen in the basal lamina surrounding
muscle fibers and the coiled rod region of myosin thick
filaments in the myofibrils are strong emitters of the SHG
signal [150–153]. These characteristics allow the integrity
of the sarcomeric structure to be evaluated with such
techniques. Further description of the physical basis of
SHG as tool for in vivo imaging of thick structural proteins
can be found in the work of Mohler et al. [154].
The applications of SHG techniques for qualifying the
skeletal characteristics of muscle have been shown by the
work by Plotnikov et al. [138] who describe sarcomeric
disruptions with second harmonic imaging techniques in
dystrophic and aged muscle. Such work uses Helmholtz
equations to determine sarcomere pattern quantification,
indicating preliminary sarcomere pattern differences between older and young adults [138]. More recently, Liu
et al. [155] developed a tool to quantify skeletal muscle
irregularities based on SHG-generated images and
Buttgereit et al. [156] used SHG to demonstrate fiber regeneration. Unlike other means of imaging, SHG allows
high-resolution quantitative 3-dimensional imaging of unstained bundles of muscle fibers at the molecular level with
minimal tissue sampling processing [157]. Such features
provide potential for the adaptation of SHG optics for
in vivo imaging during medical examination [158–160].
Llewellyn et al. [161] and Cromie et al. [162] have successfully used needle-sized microendoscopes to visualize
sarcomeres in human skeletal muscle in vivo. Hence, this
quantitative microscopic assessment of muscle sarcomere
characteristics may become useful as a complementary
diagnostic tool to identify a more precise and quantifiable
subcellular target for sarcopenia, frailty, and disability interventions. However, before such advancements can be
made, this technology should be used to quantify such
measures of muscle quality in frail and healthy human
muscle and relate such findings to physical performance. In
addition, beyond the costs and sophistication of SHG apparatus, some technical challenges remain for in vivo use
such as live tissue motion, low signal, and coupling of the
objective to the tissue for in vivo imaging [160, 162].
3.3 Circulating Biomarkers
Circulating biomarkers may provide an alternative measure
of skeletal muscle quality that is both informative and
clinically feasible and overcomes some of the barriers to
muscle quality assessment. The International Working
Group on Sarcopenia [163] previously has reported on
biomarkers reflective of muscle status. Such markers have
included anabolic hormones [e.g., testosterone, growth
hormone (GH), insulin-like growth factor-1 (IGF-1)], inflammatory biomarkers (e.g., C-reactive protein, interleukin-6, tumor necrosis factor-a), and products of
oxidative damage (e.g., advanced glycation end-products,
protein carbonyls, oxidized low-density lipoproteins)
[163]. In addition, several excellent reviews (hormones
[164], sex steroid hormones [165], IGF-1 [166], GH [167],
and cytokines [168]) have covered the circulating
biomarkers of sarcopenia more in depth, focusing mostly
on the hormonal milieu to support anabolic processes.
Here we focus selectively on the emerging biomarkers
that we have recently studied. As we [122] and other researchers [169] have shown, measures of muscle mass may
give an incomplete picture of the functional scope of sarcopenia and may not be sensitive to changes. Besides the
anabolic hormones and inflammatory markers, which
nicely reflect the hormonal milieu to support anabolic
processes, but may or may not translate to actual protein
synthesis, other biomarkers that can be measured in the
blood have emerged that can be reflective of actual tissue
remodeling. These candidates include N-terminal peptide
of procollagen type III (P3NP) and C-terminal agrin fragment (CAF). P3NP is a peptide fragment cleaved from the
procollagen III molecule during type III collagen synthesis
[170]. Type III collagen is a subtype of collagen located in
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Several treatment strategies have been evaluated for attenuating the age-related declines in skeletal muscle.
Treatment strategies have mainly focused on hormone replacement therapies, dietary interventions, pharmaceuticals, and various types of physical exercise. Despite such
research efforts, interventions using pharmaceutical or
nutritional supplements to treat sarcopenia have lacked
efficacy in improving muscle function and size due to inconclusive findings or limited evidence [178]. In addition,
thus far, relatively few interventional strategies have focused on muscle quality-related outcomes. On the other
hand, exercise training (especially resistance-type exercise)
has consistently proven to be safe and highly effective
intervention for increasing muscle mass, strength, and
quality in older adults [178].
hormonal interventions have been implemented, they seldom demonstrate efficacy for improving muscle functioning. Moreover, studies administering hormonal treatments
for treatment of sarcopenia in older adults have seldom
reported muscle qualitative outcomes. Thus, although
serum levels of the adrenal steroid hormone dehydroepiandrosterone (DHEA) [181] are associated with
frailty characteristics in older adults [179], administration
of exogenous DHEA supplementation does not appear to
benefit measures of physical function or performance
[182]. Similarly, while circulating estradiol is associated
with skeletal muscle mass in older women [180], estrogen
replacement therapy does not appear to protect against the
muscle loss of aging [183]. In addition, long-term estrogen
therapy does not appear to affect appendicular muscle
mass, body composition, or physical performance in older
women [184, 185]. On the other hand, androgens have
shown some anabolic effects in older adults. Bioavailable
testosterone is significantly associated with both higher
skeletal muscle mass [180] and less skeletal muscle fat
infiltration [186] in older men. When administered to older
adults, testosterone has been shown to be able to effectively increase circulating testosterone levels and favorably
affect body composition [187, 188]. However, despite the
anabolic effects of testosterone administration, its effects
on muscle quality and function have yet to be confirmed.
Studies that have evaluated the effects of androgen therapy
on muscle quality [188] and physical function in older men
[187] did not show promising effects. However, androgen
replacement therapy may be most effective in older men
who are hypogonadal [189]. Nevertheless, the marginal
benefits must be weighed in terms of the risks as higher
doses are associated with a fear of accelerating prostate
cancer [189]. Similarly, GH replacement therapy in older
adults is associated with a high incidence of adverse effects, [189] and it does not appear to be any more beneficial
to muscle strength than resistance training [189]. Finally, in
one hormonal study that did measure muscle quality as
both relative strength and CT attenuation, circulating
concentrations of estrogen, DHEAS, and IGF-1 were correlated with muscle quality while testosterone and DHT
(dihydrotestosterone) were not [190]. Regardless of the
relationships, the efficacy of these hormones as exogenous
treatment to improve muscle quality remains to be
examined.
4.1 Hormonal Interventions
4.2 Future Directions in Pharmaceuticals
Hormonal interventions have typically been explored as a
treatment for muscle wasting based on the cross-sectional
associations between circulating hormone concentrations
and skeletal muscle mass and characteristics of frailty in
older men and women [179, 180]. However, when
To date, few drugs have been developed for sarcopenia
specifically [191], due likely to the complexity of the
muscle qualitative features associated with the condition
along with the lack of a uniform set of clinical criteria to
diagnose the condition [192]. In light of this need, an
skeletal muscles that provides a structural framework of the
muscle for the alignment and growth of the myoblasts in
muscle repair [171]. Previous research has indicated that
older adults have reduced levels of circulating P3NP [172]
and that changes in serum P3NP levels are predictive of
changes in lean body mass and strength [172]. CAF is also
a peptide fragment cleaved from the nerve-derived protein
agrin in neuromuscular remodeling. Normally, the agrin
protein is important to maintaining the NMJ [173]. However, excessive cleavage of agrin by the enzyme neurotrypsin leads to functional disintegration at the NMJ
[173, 174]. Circulating CAF concentrations are elevated in
older adults with sarcopenia [175] and frailty [176], potentially indicating breakdown of the NMJ.
We evaluated changes in circulating concentration of
P3NP and CAF in response to a preliminary intervention
designed to increase muscle quality but not mass [177]. We
found that circulating CAF increased to a clinically
meaningful magnitude while changes in circulating P3NP
were less clear, but appeared to reflect muscle hypertrophy
[177]. While assessment of P3NP and CAF from blood
samples may provide minimally invasive and clinically
informative measures of skeletal muscle status in older
adults, further research is needed to elucidate whether
P3NP, CAF, or other biomarkers can reflect muscle
qualitative adaptations with larger and longer studies.
4 Treatment Strategies
123
Muscle Quality in Aging
international task force recently met to address the need for
pharmaceutical trials for sarcopenia [193]. The task force
reported that issues of safety and research developments
have restricted the clinical applications of pharmaceutical
interventions [193]. Nevertheless, with the increased understanding of the pathways contributing to sarcopenia and
muscle quality, several potential targets for drug development have been identified [194]. Of the potential targets for
drug development, circulatory, metabolic, neuromuscular,
anabolic, catabolic, force generation, and signaling pathways have all been targeted, which are described in brief
here. For further information on the pharmacological
aspects of sarcopenia, please refer to a prior review by
Brotto and Abreu [7].
Several physiological targets for drug development to
combat sarcopenia and its qualitative consequences are
being explored. Circulatory targets of drugs to treat sarcopenia have focused on ACE inhibitors that have shown
some efficacy in improving physical performance [195,
196]. Mitochondrial targets have focused on promoting
mitochondrial biogenesis and calcium metabolism with
peroxisome proliferator-activated receptor-c coactivator 1a
[197] and cyclophilin D inhibitors [198]. Anabolic targets
have focused on selective androgen receptor modulators
(SARMs), which are able to selectively target androgen
receptors in the target tissue, thereby sparing androgen
receptors in the prostate and myocardial tissue [199, 200].
Neuromuscular targets have targeted the agrin protein
within the NMJ, which is involved in maintaining neuromuscular integrity [174]. Additional drug targets have focused on attenuating myonuclear apoptosis through
myostatin inhibition [201] and attenuating myofibrillar
degradation via proteasome inhibitors [202]. In addition,
new drugs targeted at troponin activation may provide a
potential therapeutic mechanism for increasing muscle
force and strength [203]. Furthermore, more recent targets
have focused on micro-RNAs that are able to that bind to
complementary regions on numerous messenger RNAs
(mRNAs) to modulate signaling cascades [204]. Several
micro-RNAs have been associated with aging [205].
Treatment targets have focused on the role micro-RNAs in
the control of myogenic commitment [206] and maintenance of the NMJ [207]. While research on these and other
neuromuscular pathways are underway, these potential
drugs are some way away from being available to the
clinician and the patient and potential adverse effects are
yet to be determined.
4.3 Nutritional Interventions
Nutritional interventions provide a more accessible means
to intervene in the age-related decrements in skeletal
muscle as prescriptions are not necessary and adverse
effects are fewer and less severe. Nutritional interventions
for sarcopenia have mostly focused on protein supplementation due to the protein synthetic pathways stimulated
by the essential amino acids contained within them. In
addition, some, but far less, research has investigated the
effects of other nutritional supplements such as omega-3
fatty acids (O3FAs), creatine, b-hydroxy-b-methylbutyrate
(HMB), and b-alanine on muscle factors in older adults.
Interestingly, however, some research has shown that older
adults may be less susceptible to the anabolic effects of
protein than younger adults in what has been considered
‘anabolic resistance’. Although protein supplementation is
a potent stimulator of protein synthesis in the skeletal
muscle of younger adults, older adults’ muscle appears
resistant to the anabolic stimulus [208, 209], possibly due
to reduced expression and activation of anabolic signaling
pathways [209]. Nevertheless, although dietary protein can
improve protein synthesis [210], increasing dietary protein
does not appear to improve muscle composition responses
to resistance training in older adults any more than the
benefits seen from resistance training alone [211].
Some studies by our laboratory and others have shown
some efficacy in improving muscle strength, size, fatigue
threshold, and function with supplementation with creatine
[212, 213], b-alanine [214], and HMB [215]. Creatine is a
common supplement used by athletes to increase stores of
phosphocreatine for short-term energy production [216].
We have found that short-term supplementation with creatine in older women was able to increase muscle strength
and performance without any adverse effects [213].
Similarly, Stout et al. [212] reported significant increases in
grip strength and physical working capacity with creatine
supplementation in older men and women. b-Alanine is an
amino acid important for increasing muscle carnosine
synthesis and improving muscle metabolic buffering capacity [217]. We have shown that a protein drink fortified
with b-alanine may improve physical working capacity,
muscle quality, and function in older men and women
[214]. HMB is a metabolite of the essential amino acid
leucine that induces acute muscle protein anabolism
through increasing synthesis and attenuating degradation
[218]. Stout et al. [215] reported that long-term supplementation with HMB in older adults can improve muscle
strength and muscle quality. Despite some encouraging
findings, other nutrients, such as self-reported O3FA intake
[219], have shown no associations with lower extremity
function. Similarly, Fiatarone et al. [30] showed that a
multi-nutrient supplementation without concomitant exercise does not reduce muscle weakness or physical frailty.
While some nutritional strategies look encouraging for
targeting various muscle quantitative and qualitative features, further research is needed to identify the optimal
nutritional approach for preventing and treating sarcopenia
123
M. S. Fragala et al.
and investigating muscle qualitative features more
thoroughly.
4.4 Exercise and Physical Activity
Physical activity, especially in the form of resistance exercise training, has been consistently shown to be a feasible
and effective means of counteracting muscle weakness and
physical frailty and improving muscle strength, size, and
function [30, 220, 221], even at a very old age ([80 years)
[222]. Although varying exercise protocols have shown
effectiveness for increasing muscle quantity, quality, and
strength, the dosage in terms of intensity, frequency, and
volume ultimately dictates the expected outcomes [9].
Sufficient resistance training intensity (70–90 % of
1 repetition maximum) [223, 224], frequency, and duration
(three times a week for 8–12 weeks) [224] is well-tolerated
in older adults [225] and required to elicit the desired
muscular responses. The American College of Sports
Medicine (ACSM) [226] recommends that older adults
participate in a variety of exercises including endurance,
resistance, flexibility, and balance exercises to increase
active life expectancy and to limit the development and
progression of disabling conditions. Each mode of exercise
stimulates specific muscular adaptations. While physical
activity in general is important for maintaining physical
functioning, resistance (strength) exercise training remains
the most effective intervention for increasing muscle mass
and strength in older people [189, 227]. A 2009 Cochrane
review of 121 randomized controlled trials involving 6,700
participants showed that progressive resistance exercise
training is an effective and safe intervention for improving
physical functioning in older people with few adverse
events [228].
Similar to younger adults, the plasticity of skeletal
muscle in response to resistance exercise training is
somewhat maintained with aging [229]. Resistance training
leads to increases in muscle strength and power of both the
upper and lower body in older adults [230]. Even frail older
adults aged [90 years have shown profound strength improvements of 174 % in response to resistance exercise
training [231]. While the gains appear continual with
training, the greatest gains are often seen within in the first
3 months of training [232]. However, in contrast to
younger adults who experience both marked increases in
neural function and hypertrophy with training, gains in
muscle strength and power in older adults are predominantly determined by neural adaptations [233]. Neural
adaptations resulting in strength and power improvements
involve increased voluntary activation of the agonist
muscles and reduced coactivation of the antagonist muscles
[233]. In addition, long-term exercise promotes the
123
reinnervation of muscle fibers and may spare the loss of
fibers due to denervation by selective recruitment to slow
fibers [69]. Some newer evidence also suggests that high
threshold motor units may be preserved with oscillatory
contractions [234]. Older adults also demonstrate a
preservation of thigh girth [235], evidence of muscle hypertrophy at the fiber level [236], and an increase in myofibrillar protein turnover [237] that accompany strength
gains in response to resistance training. In addition, resistance training is able to stimulate a similar shift in the
expression of myosin heavy chain (MHC) isoforms (from
MHC IIb to MHC IIa) in older adults to that in younger
adults [229].
Despite the somewhat maintained neuromuscular plasticity with training in aged muscle, the muscular anabolic
response is somewhat attenuated with age [230, 238]. The
reason for the dampened response is likely attributable to
endocrine or neuromuscular impairments [230]. In comparison to younger adults, untrained older adults show an
attenuated hormonal response to exercise [239] and downregulation of the skeletal muscle IGF-1 system [240],
which may impair hypertrophic capacity. However, resistance training has shown to counteract these changes by
enhancing the hormonal response to resistance exercise
(i.e., decreased resting cortisol and increase circulating
testosterone) [239] and reversing the age-related downregulation of the skeletal muscle IGF-1 system [240],
which may promote anabolic potential. Also, within the
muscle, age-related alterations in myonuclei may impair
hypertrophic capacity [241, 242]. Changes in myonuclear
content represent a mechanism for modulating protein
content and hypertrophy [243–245]. Since myonuclei regulate gene expression and protein synthesis of the surrounding muscle fiber [246], the age-related reduced
responsiveness of skeletal muscle may relate to alterations
in myonuclear number, distribution, and function [241,
242]. Animal models indicate that the myonuclei of aged
type II muscle fibers demonstrate a decrease in pre-mRNA
transcription, processing, and transport rate compared with
younger adult animals [242]. However, the myonuclear
domains of type I and IIa are smaller than in IIb fibers
[245], theoretically providing greater potential for proportional hypertrophy within them without exceeding the
myonuclear domain. Regardless, resistance training has
shown to reverse the age-related decline in myonuclei
precursor satellite cells [247], thereby enhancing the potential for enhanced myonuclear dictated hypertrophy.
In addition, the positioning of myonuclei along muscle
fibers is plastic and influenced by blood supply [248]. With
greater blood supply, the number of myonuclei is higher in
slow than in fast fibers, despite the greater absolute size of
fast fibers [249–251]. Interestingly, myonuclear content is
Muscle Quality in Aging
correlated with mitochondrial content [251], which is also
reduced in aged human skeletal muscle. The downregulation of mitochondrial biogenesis in older individuals
is believed to be due to reduced metabolism and chronic
inflammation [252]. Moreover, older adults have a lower
proportion of intramyocellular lipids in contact with mitochondria [112], perhaps as a result of reduced physical
activity since exercise causes a proximal subcellular shift
of lipid droplets to the mitochondria [253]. Endurance
exercise training in older adults elicits a proliferation of
muscle capillaries and an increase in oxidative enzyme
activity within the muscle [9]. In addition, resistance exercise can improve mitochondrial capacity [254] and reverse the age-related declines in mitochondrial function
[255]. Hence, although some anabolic mechanisms may be
dampened with aging, many of the age-associated reductions in muscle quality may be attenuated by physical
activity.
Mechanisms through which exercise may influence
muscle quality and functional status are complex. However, qualitative improvements in muscle appear to account
for the adaptations more so than muscle hypertrophy [23,
256] or other muscle parameters [257]. Evidence of muscle
qualitative improvements stem from interventional studies
that show disproportionate improvements in muscle function to muscle size [258]. Muscular qualitative improvements include attenuating age-related intramuscular
adipose infiltration [103], increasing muscle fiber area
[220], muscle power [30], relative strength [259], MQI
[85], PCSA [122], neuromuscular functioning [222], and
increasing contractile protein [260]. The stimuli for regeneration of myofibrils involve the mechanical disruption
during exercise [261]. Interestingly, some evidence suggests that older men experience greater muscle disruption
following eccentric exercise than young men, which may
be due in part to the smaller muscle mass [262]. Although
older individuals have reduced protein synthesis as compared with younger adults during the resting state, muscle
protein synthesis capacity stimulated by exercise is preserved in old age [46, 48, 263]. More specifically, resistance exercise increases the synthesis rate of major
contractile proteins, MHC, actin, and mixed muscle proteins [46] and shifts the expression of MHC isoforms from
MHC IIb to MHC IIa [229]. Resistance exercise is associated with increased presence of developmental isoforms
[261], implicating muscle regeneration through activation
of new myogenic precursors in frail elders. In addition,
exercise may reduce intramuscular lipid accumulation via
enhanced lipolysis stimulated by muscle contraction and
epinephrine release [264, 265]. Enhanced hormonal profiles [238], including increased IGF-1 [261], likely mediate
protein synthesis for new or hypertrophied myofibril formation in the muscle of older adults in response to
resistance exercise. Moreover, exercise training increases
resting fascicle length due to the addition of sarcomeres in
series [120, 266], potentially reversing aging-induced reductions in fascicle length.
5 Conclusions
Despite the detrimental contributions of aging on skeletal
muscle, no consensus definition, diagnostic tests, or medical treatments currently exist for sarcopenia [267], dynapenia, or muscle quality. Understanding muscle
qualitative factors contributing to functional impairments
at the most basic physiological level will ultimately help
both clinicians and patients by identifying risk factors and
developing interventions and treatments. Given the role
that muscle quality plays in function, more direct measures
at an ultrastructural level could potentially provide prognostic information. Emerging technology holds promise to
provide a means of assessing muscle quality changes at an
ultrastructural level. By measuring muscle quality, strategies to identify individuals at ‘high risk’ of functional decline and frailty can be developed and more aggressive care
and interventions can be implemented. Muscle disuse is a
feasible target for intervention for the onset of muscle
weakness and frailty in older adults. Putting under-utilized
muscles to work in a resistance exercise training program is
repeatedly shown to be an effective and feasible means to
maintain and improve function and improve muscle quality. Although some mechanisms are attenuated with aging,
benefits can still be achieved in function and performance
from exercise training through alternative adaptations in
fiber characteristics and muscle quality.
Acknowledgments The authors have no potential conflicts of
interest that are directly relevant to the content of this review. No
sources of funding were used to assist in the preparation of this
review.
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