Energy Systems and Fatigue
Muscular Strength
& Endurance
(Testing)
1
Outline
• Muscle strength versus endurance
• Energy systems and fatigue
– what limits muscle strength and endurance
• Reasons for strength and endurance
evaluation
• Types of muscle contraction and testing
equipment
• Problems with muscular fitness testing
2
Muscle Activity Definitions
• Muscular Strength
 the amount of force that a muscle, or muscle
group, can produce in a single maximal effort.
 One repetition max, dynamometer
• Muscular Endurance
 the ability of a muscle group to perform repeated
contractions against a light load for an extended
period of time.
 Partial curl ups, push ups, repetitions of a % of body
weight
• Muscular Power
 the ability to produce force quickly.
 Work (force * distance) over time, or force *
velocity
 Vertical jump
3
Power = Force x Velocity
Maximum power
occurs at about 1/3
maximal velocity
of shortening and
about 1/3
maximum
concentric force?
Force
Power
Force
Velocity
4
Fatigue
• Fatigue- inability to maintain a given exercise intensity
– rarely completely fatigued - can maintain lower intensity
output
– Observe reduction in force and velocity and a prolonged
relaxation time
• Causes of muscle fatigue have been classified into
central and peripheral
• Central - includes CNS, motivation and psychological
factors
– restoration of force with external stimulation of muscle indicates central fatigue
– NH3, hypoglycemia, reticular formation
• Peripheral - PNS to muscle - EC coupling, energy
supply and force generation
– CP depletion, Glycogen depletion, proton buildup
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Energy Systems
• Maintenance of ATP levels is required for muscle
contraction to continue
• ATP can be maintained through a variety of pathways
determined by factors such as hormone levels,
internal cellular environment (O2 levels, fuel, acidity,
calcium) and demand
• All pathways are operational, there is a shift in
dominance and ATP output as a result of the
influence of these factors
• If ATP production or muscle contraction can not be
maintained, the muscle is said to be fatigued
6
NCSA Essential of Strength Training and Conditioning, 2008
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Immediate Energy System
• intracellular ATP and Creatine Phosphate
(CP) stores
• CP + ADP --> C +ATP
– enzyme for reaction is Creatine Kinase
• Very high power output but low capacity
• Exhausted in ~10 seconds of maximal effort
– Weight lifting, shot put, vertical jump
• Fatigue due to CP and ATP depletion
– Eg. End of a weight training exercise set
8
Anaerobic Glycolysis
• Breakdown of glucose (2 ATP) or glycogen (3
ATP) to pyruvate
– Pyruvate then converted to lactate to allow glycolysis
to continue anaerobically
• Fairly fast, moderate yield
– Predominates for all out activity that can last between
30 seconds and 2-3 minutes
– Ideal for test of anaerobic power to be maximal and
end within this time frame
• Fatigue due to H+ (acidity) from glycolysis
–
–
–
–
–
Inhibits PFK (enzyme in glycolysis)
Inhibits ATPase and ATP hydrolysis
Inhibits Calcium binding to troponin C
Inhibits SR ATPase - reducing calcium reuptake and release
Increased threshold of free Calcium required for contraction
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Aerobic systems
• Fats, carbohydrates and some protein
oxidized to CO2 and H2O in mitochondria
– Slow production - fats theoretically unlimited ATP
production - glycogen has limited stores
• Predominant for activity lasting > 2-3 min
• Power (VO2 max)limited by oxygen delivery to
working muscle
– Cardiac output - Hb content
• Performance limited by respiratory capacity of
muscle - mitochondrial content
• Fatigue can result from
– glycogen depletion (1.5 to 2 hours high intensity)
– Uncoupling of mitochondria (calcium buildup)
– CNS involvement (central fatigue)
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General Characteristics of the
Energy Systems
ATP/PC System
Lactic Acid System
Aerobic System
Anaerobic
Anaerobic
Aerobic (oxygen)
Very rapid
Rapid
slow
PC and ATP utilized
Glycogen/glucose used
Glycogen, fats, and
protein used
Very limited production
Limited production of ATP Unlimited production of
ATP
Limited stores in muscle fatigue with depletion
By product - lactate and
acidity leads to fatigue
No fatiguing by products
Used in sprint and highpower short duration
activity
Activities that are
maximal for 1-3 minutes
Long duration activities, 5
minutes plus
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Capacity and Power of the Three
Energy Systems
For ATP Production
(Untrained Male Subjects)
Energy System
Capacity (total Moles)
Power (moles / min)
Phosphagens (ATP /PC)
0.6
3.6
Anaerobic glycolysis
1.2
1.6
Aerobic
Unlimited (if including
fats)
1.0
Endurance time decreases with increasing force
14
Fig. 1. Estimation of critical power (CP) in a representative subject
Critical Power highest constant
work rate
that can be
maintained
without fatigue
Jones, A. M. et al. Am J Physiol Regul Integr Comp Physiol 294: R585-R593 2008;
doi:10.1152/ajpregu.00731.2007
15
Copyright ©2008 American Physiological Society
Skeletal muscle Fiber types
• Fiber types have different capacities for each
energy system - specialized for different types
of activity
• Variability in size of fibers and motor units,
speed and strength of contraction,
capillarization, enzyme and fuel content
• Fibers recruited based on demand
– Type I - light activity
– Type I and II a - moderate activity
– All fibers types - high intensity demand - high
power output - contribution of II b will be high
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Fiber Recruitment
• CNS driven stimulation of number of motor
units required for contraction
• Motor unit - motor neuron and all muscle
fibers that it stimulates - all fibers within a
motor unit are of the same type
– Variable threshold for stimulus of different motor
units - type I, IIa, IIb
• Asynchronous recruitment - endurance
activity, alternate recruitment of motor units to
allow recovery and sustained low %
contractions
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Muscular Strength Testing
• Purpose
–
–
–
–
Assess muscular fitness
Identify specific areas of weakness
Monitor progress in a rehabilitation program
Measure effectiveness of a resistance training
program
• Correlation between Endurance and Strength
is greater than .90
– These measures are clearly related
– We know, however,that training programs can
result in specific improvements in strength or
endurance depending on design
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Strength Training Benefits
•
•
•
•
Increased muscle fiber size (hypertrophy).
Reduced percent body fat
Increased muscle contractile strength.
Increased bone mineral density and tensile
strength.
• Increased ligament tensile strength.
• Improved physical functioning with old age
• These changes improve our physical
capacity, physical appearance, metabolic
function and injury risk.
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Types of Muscular Contraction
•
•
•
•
Isometric Contraction (static)
Concentric isotonic Contraction (shortening)
Eccentric isotonic Contraction (lengthening)
Isokinetic Contraction (joint angular velocity
remains constant)
Muscle Action
 Prime movers
 Antagonist
 Stabilizers
21
Static Strength Tests
• Hand grip dynamometers
• Leg and back strength dynamometers
• Cable tensiometers
Advantages -- good reliability, good
objectivity, norms available.
Disadvantages - angle specific, boring,
increase SBP, validity ?
22
Force-Length Curve of Muscle
Physiological Range
Tension
Lo
Length of Contractile element
Torque (Nm)
Torque vs. Angle
Forearm Flexion
80
60
40
20
0
0
20
40
60
Angle (degrees)
80
100
24
NSCA fig 4.7 - change in moment arm and mechanical advantage
during a bicep curl
25
Rotational velocity advantage
Torque advantage
Fig 4.9 NSCA - impact of insertion on moment arm and torque26
Dynamic Strength Testing
•
•
•
One-repetition maximum tests.
Batteries of tests have been developed. bench
press, squat, overhead press and others.
Variable-rep maximum tests.
 1 RM = (weight lifted) / [1.0278 - (n X 0.0278)]
 n = repetitions to failure (not to exceed 10)
•
•
•
 Safer than using 1-RM, especially with beginners.
Fixed weight tests: YMCA bench press test
(Females 35-lbs, males 80-lbs)
Elderly tests of ADL - activities of Daily Living
Alternate RM estimations using 45% of body mass
 Women 60-70 yrs
 1RM = (.92 * wt(kg)) + (.79 * reps) - 3.73
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Isokinetic Strength Tests
• Expensive and therefore less practical.
 Strength -- 30o- 60o per second
 Endurance -- 120o - 180o per second
 Power -- 120o - 300o per second
 Eg: Canadian Ski Team cybex knee extension
test is performed at 100o/s.
• Many isokinetic systems (e.g. Omnitron)
have their own protocols.
• Disadvantages
• humans do not move at constant velocity
• isokinetic tests use isolated joint movements
• may not be reflective of performance ability
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Problems Associated With
Muscular Fitness Testing
• Strength & endurance are specific to
the muscle group, the type and speed
of contraction, and the angle being
tested (how applicable are isometric
tests?).
• Caution should be used in selecting
test items to measure muscular
strength (often really measuring
endurance!).
• Most tests of strength & endurance
require a maximal effort (motivation?).
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Problems Associated With
Muscular Fitness Testing (cont.)
• Because strength is related to lean body
weight of the individual, the tests should be
expressed in relative terms.
• Performance on some endurance tests (e.g.
chin-ups and push-ups) is highly dependent
on the strength and strength to weight ratio
of the individual.
• There is a lack of up-to-date endurance
norms for men and women (especially >25).
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Strength vs Age & Gender
Grip Strength ((kg)
120
100
80
60
40
20
0
0
10
20
30
40
50
60
70
80
Age (yrs)
31
Musculoskeletal Health in
Older Adults
•
•
•
•
•
•
Muscle function is reduced in older adults as a
result of aging and inactivity
Largest impact on lower limb and trunk extensors
lower limb flexors and upper limb muscles are also
affected
There is a selective loss of Type II muscle fibers
and a proportionately larger decrease in power
and strength
Strength to mass ratio also declines
Ability to adapt to a resistance training program is
not affected by aging
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Muscular Strength and Endurance
Lab
•
Preparation
 Determine % of body weight requirements for weight
room portion of lab prior to start of lab session
 see lab manual 2
 Bring proper clothing to perform strength and endurance
assessments in both Pipers gym and lab room.
 Be prepared to travel between these facilities quickly during
lab session.
•
•
•
Instructions for weight room in lab manual
Instructions for lab room in CPAFLA manual
Enter all results in log book
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