The Scientific Principles of Strength Training
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Muscular Strength: The amount of
force a muscle can produce with a
single maximal effort
Mechanical Strength: the maximum
torque that can be generated about a
joint
Torque about the elbow joint
Strength determined by:
• Absolute force developed by muscle
• Distance from joint center to tendon insertion
• Angle of tendon insertion
Shoulder joint torque as a function of arm position
Structural organization of skeletal muscle
From Principles of
Human Anatomy (7th
edition), 1995 by
Gerard J. Tortora, Fig
9.5, p 213
From Basic
Biomechanics
by Susan Hall
(3rd edition),
Fig 6.6, page
153
6-6
From Skeletal Muscle:
Form and Function (2nd
ed) by MacIntosh,
Gardiner, and McComas.
Fig 1.4, p. 8.
From Basic Biomechanics by Susan
Hall (3rd edition), Fig 6.5, page 152
6-5
From Basic Biomechanics by
Susan Hall (3rd edition), Fig
6.3, page 150
6-3
From Exercise
Physiology: Theory and
Application to Fitness
and Performance (6th
Edition) by Scott K.
Powers and Edward T.
Howley. Fig 8.6 P. 147
A motor unit: single motor neuron and all
the muscle fibers it innervates
From Basic Biomechanics
Instructors manual by Susan
Hall (2nd edition, 1995), Fig TM
31
From Basic Biomechanics by Susan
Hall (3rd edition), Fig 6.7, page
154
6-7
From Basic Biomechanics by Susan Hall
(3rd edition), Fig 6.8, page 154
6-8
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Types of muscle fiber: Fast twitch vs Slow Twitch
Type I
ST Oxidative
(S0)
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Contraction speed
Time to peak force
Fatigue rate
Fiber diam.
Aerobic capacity
Mitochondrial conc.
Anaerobic capacity
slow
slow
slow
small
high
high
low
Type IIa
FT Oxidative -
Type IIb
FT Glycolytic
Glycolytic (FOG)
fast (2xI)
fast
inter.
inter.
inter.
inter.
inter.
(FG)
fast (4xI)
fast
fast
large
low
low
High
Sedentary people – 50% slow/50% fast, whereas elite
athletes may differ
e.g., cross country skiers – 75% slow 25% fast
sprinters
- 40% slow 60% fast
Factors affecting force Production
1. Cross-sectional
area
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Hypertrophy: increase
in the # of myofibrils
and myofilaments
Hyperplasia: increase
in the number of
fibers???
2. Rate Coding – frequency of stimulation
From Basic Biomechanics by Susan Hall
(3rd edition), Fig 6.9, page 155
3. Spatial recruitment
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Increase # of active motor units (MUs)
Order of recruitment
I ---> IIa -----> IIb
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Henneman's size principle: MUs are recruited in
order of their size, from small to large
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Relative contributions of rate coding and spatial
recruitment.
• Small muscles - all MUs recruited at approximately 50%
max. force; thereafter, rate coding is responsible for
force increase up to max
• Large muscles - all MUs recruited at approximately 80%
max. force.
The force-velocity
relationship for
muscle tissue:
When resistance
(force) is negligible,
muscle contracts
with maximal
velocity.
Force
4. Velocity of shortening: Force inversely
related to shortening velocity
(Low resistance,
high contraction
velocity)
Velocity
isometric
maximum
Force
The force-velocity
relationship for
muscle tissue: As
the load increases,
concentric
contraction velocity
slows to zero at
isometric
maximum.
Velocity
Force-Velocity Relationship in different muscle fiber types
Type II fiber
Type I fiber
Effect of Temperature on Force-Velocity relationship
(22oC, 25oC, 31Co, and 37oC)
Force -Velocity Relationship (Effect of strength-Training)
Force-velocity Relationship During Eccentric
Muscular Contractions
Force/Velocity/Power Relationship
Force/velocity curve
Power/velocity
curve
Force
Power
30%
From Basic Biomechanics
by Susan Hall (3rd
edition), Fig 6.25, page
175
30%
Velocity
Effect of Muscle Fiber Types on Power-Velocity Relationship
Consequences of the force-velocity relationship for
sports practice
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When training for sports that require power, train
with the appropriate % of 1 RM that will elicit the
most power.
24 weeks of:
a). heavy weight-training b. Explosive strength
training
From Science
and Practice
of Strength
Training (2nd
edition) V.M.
Zatsiorsky
and W.J.
Kraemer
(2006) Fig
2.19 P. 39)
Why do elite weight lifters start a barbell lift
from the floor slowly?
They try to accelerate maximally when the bar
is at knee height. Two reasons:
1. At this position, the highest forces can be
generated as a result of body posture
2. Because force
decreases when
velocity increases,
barbell must
approach the most
favored position at
a relatively low
velocity to impart
maximal force to
the bar.
From Science and Practice of Strength
Training (2nd edition) V.M. Zatsiorsky and
W.J. Kraemer (2006) Fig 2.20 P. 40)
Adaptations associated with strength training
1. Activates protein
catabolism. This
creates conditions for
enhanced synthesis of
contractile proteins
during the rest period
(break down, build up
theory)
From R.L. Leiber (1992). Skeletal
Muscle Structure and Function. Fig
6.1, p. 262.
2. Neural adaptations occur to improve
intra-muscular and inter-muscular
coordination.
• Intra-muscular coordination – affects the
ability to voluntarily activate individual fibers in
a specific muscle
• Inter-muscular coordination – affects the
ability to activate many different muscles at
the appropriate time
Intra-muscular coordination changes with
training
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Untrained individuals find it difficult to
recruit all their fast-twitch MUs. With
training, an increase in MU activation
occurs
Strength training also trains the MUs to
fire at the optimal firing rate to achieve
tetany
MUs might also become activated more
synchronously during all out maximum
effort
Consequently, maximal muscular
force is achieved when:
1. A maximal # of both FT and ST motor
units are recruited
2. Rate coding is optimal to produce a
fused state of tetany
3. The MUs work synchronously over the
short period of maximal effort.
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Psychological factors are also of importance
CNS either increases the flow of excitatory stimuli, decreases
inhibitory stimuli, or both
Consequently, an expansion of the recruitable motor neuron
pool occurs and an increase in strength results
Hidden strength potential of human muscle can also be
demonstrated by electrostimulation
Muscle strength deficit (MSD) =
(Force during electrostimulation-Maximal voluntary force) x 100
Maximal voluntary force
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Typically falls between 5-35%
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Electrostimulation
• Possibility exists to induce hypertrophy through
electrostimulation
• However, does not train the nervous system to
recruit motor units
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Bilateral Deficit
• During maximal contractions, the sum of forces
exerted by homonymous muscles unilaterally is
typically larger than the sum of forces exerted by the
same muscles bilaterally
• Bilateral training can eliminate this deficit, or even
allow bilateral facilitation
Other benefits of strength training
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Increase in resting metabolic rate
• Each additional pound of muscle tissue increases
resting metabolism by 30 to 50 calories per day = 10,950 to
18,250 calories a year = 3-5 lb of fat
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Increase in bone mineral content and, therefore, bone density
Increases the thickness and strength of the connective tissue
structures crossing joints such as tendons and ligaments –
helps prevent injury
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Increased stores of ATP, Creatine Phosphate (CP), and glycogen
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Aids rehabilitation from injury
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Aging gracefully! Less falls in latter years
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Looking better, feeling better. Greater self-esteem
Metabolic stress of resistance
training
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Classed as only light to moderate in terms
of energy expenditure per workout
Standard weight-training does not
improve endurance or produce significant
cardiovascular benefits like aerobic type
activity does
Circuit-training increases metabolic stress
Delayed onset of muscle soreness
(DOMS)
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The intensity and the novelty of a workout influence how sore you
become
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Lactate does not cause muscle soreness due to:
• 1. Lactate returns to baseline within an hour of exercise
• 2. After exercise, lactate is in equal amounts within the muscle
and the blood
• 3. DOMS is specific, not generalized
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Muscle soreness is due to the physiological response to muscle
fiber and connective tissue damage (microtears)
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White blood cells enter the muscle tissue, clean up the debris of
broken proteins, and then initiate the regeneration phase
Muscle Soreness (continued)
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Edema (increase in fluid) to the area accompanies
the above response
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The pressure from edema is thought to produce
the sensation of soreness
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Also, metabolic by-products released from the
macrophages may sensitize pain receptors
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Next stage is the proliferation of satellite cells help form new myofibrils
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Eccentric contractions cause the greatest amount
of soreness