The Scientific Principles of Strength TrainingThe Scientific Principles of Strength Training

Muscular StrengthMuscular Strength: The amount of : The amount of force a muscle can produce with a force a muscle can produce with a single maximal effortsingle maximal effort

Mechanical Strength: Mechanical Strength: the maximum the maximum torque that can be generated about torque that can be generated about a jointa joint

Torque about the elbow jointTorque 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 positionShoulder joint torque as a function of arm position

Structural organization of skeletal muscleStructural organization of skeletal muscle

From Principles of Human Anatomy (7th edition), 1995 by Gerard J. Tortora, Fig 9.5, p 213

6-6

From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.6, page 153

From Skeletal Muscle: Form and Function (2nd ed) by MacIntosh, Gardiner, and McComas. Fig 1.4, p. 8.

6-5From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.5, page 152

6-3

From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.3, page 150

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

6-7

From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.7, page 154

6-8

From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.8, page 154

Types of muscle fiber: Fast twitch vs Slow TwitchTypes of muscle fiber: Fast twitch vs Slow Twitch

Type I Type IIa Type IIbType I Type IIa Type IIb ST OxidativeST Oxidative FT Oxidative -FT Oxidative - FT FT

GlycolyticGlycolytic

(S0)(S0) Glycolytic (FOG) (FG)Glycolytic (FOG) (FG)

Contraction speed slow fast (2xI) Contraction speed slow fast (2xI) fast (4xI) fast (4xI) Time to peak force slow fast fastTime to peak force slow fast fast Fatigue rate slow inter. Fatigue rate slow inter. fast fast Fiber diam. small inter. largeFiber diam. small inter. large Aerobic capacity high inter. lowAerobic capacity high inter. low Mitochondrial conc. high inter. lowMitochondrial conc. high inter. low Anaerobic capacity low inter. HighAnaerobic capacity low inter. High

Sedentary people – 50% slow/50% fast, whereas elite Sedentary people – 50% slow/50% fast, whereas elite athletes may differ athletes may differ

e.g., cross country skiers – 75% slow 25% faste.g., cross country skiers – 75% slow 25% fast sprinters - 40% slow 60% fastsprinters - 40% slow 60% fast

Factors affecting force ProductionFactors affecting force Production

1. Cross-sectional 1. Cross-sectional area area Hypertrophy: increase Hypertrophy: increase

in the # of myofibrils in the # of myofibrils and myofilamentsand myofilaments

Hyperplasia: increase Hyperplasia: increase in the number of in the number of fibers??? fibers???

2. 2. Rate Coding – frequency of stimulationRate Coding – frequency of stimulation

From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.9, page 155

3. Spatial recruitment3. Spatial recruitment

Increase # of active motor units (MUs) Increase # of active motor units (MUs)

Order of recruitmentOrder of recruitment

I ---> IIa -----> IIbI ---> IIa -----> IIb

Henneman's size principle: MUs are recruited in Henneman's size principle: MUs are recruited in order of their size, from small to largeorder of their size, from small to large

Relative contributions of rate coding and spatial Relative contributions of rate coding and spatial recruitment.recruitment.• Small muscles - all MUs recruited at approximately 50% Small muscles - all MUs recruited at approximately 50%

max. force; thereafter, rate coding is responsible for max. force; thereafter, rate coding is responsible for force increase up to maxforce increase up to max

• Large muscles - all MUs recruited at approximately 80% Large muscles - all MUs recruited at approximately 80%

max. force. max. force.

4. Velocity of shortening:4. Velocity of shortening: Force inversely Force inversely related to shortening velocityrelated to shortening velocity

The force-velocity The force-velocity relationship for relationship for muscle tissue: muscle tissue: When resistance When resistance (force) is negligible, (force) is negligible, muscle contracts muscle contracts with maximal with maximal velocity.velocity.

Velocity

For

ce

(Low resistance, high contraction velocity)

The force-velocity relationship for muscle tissue: As the load increases, concentric contraction velocity slows to zero at isometric maximum.

Velocity

For

ce

isometric maximum

Force-Velocity Relationship in different muscle fiber typesForce-Velocity Relationship in different muscle fiber types

Type II fiber

Type I fiber

Effect of Temperature on Force-Velocity relationship Effect of Temperature on Force-Velocity relationship (22(22ooC, 25C, 25ooC, 31CC, 31Coo, and 37, and 37ooC)C)

Force -Velocity Relationship (Effect of strength-Training)Force -Velocity Relationship (Effect of strength-Training)

Force-velocity Relationship During Eccentric Force-velocity Relationship During Eccentric Muscular ContractionsMuscular Contractions

Force/Velocity/Power RelationshipForce/Velocity/Power RelationshipForc

e

Velocity

Pow

er

30%

30%

Force/velocity curve

Power/velocity curve

From Basic Biomechanics by Susan Hall (3rd edition), Fig 6.25, page 175

Effect of Muscle Fiber Types on Power-Velocity RelationshipEffect of Muscle Fiber Types on Power-Velocity Relationship

Consequences of the force-velocity relationship for Consequences of the force-velocity relationship for sports practicesports practice When training for sports that require power, train When training for sports that require power, train

with the appropriate % of 1 RM that will elicit the with the appropriate % of 1 RM that will elicit the most power. most power.

24 weeks of:24 weeks of:

a). heavy weight-training b. Explosive strength a). heavy weight-training b. Explosive strength training 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 Why do elite weight lifters start a barbell lift from the floor slowly? from the floor slowly?

They try to accelerate maximally when the bar They try to accelerate maximally when the bar is at knee height. Two reasons: is at knee height. Two reasons:

1. At this position, the highest forces can be 1. At this position, the highest forces can be generated as a result of body posturegenerated as a result of body posture

2. Because force 2. Because force decreases when decreases when velocity increases, velocity increases, barbell must barbell must approach the most approach the most favored position at favored position at a relatively low a relatively low velocity to impart velocity to impart maximal force to maximal force to the bar.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 trainingAdaptations associated with strength training

1. 1. Activates protein Activates protein catabolism. This catabolism. This creates conditions for creates conditions for enhanced synthesis of enhanced synthesis of contractile proteins contractile proteins during the rest period during the rest period (break down, build up (break down, build up theory)theory)

From R.L. Leiber (1992). Skeletal Muscle Structure and Function. Fig 6.1, p. 262.

2. Neural adaptations occur to improve 2. Neural adaptations occur to improve intra-muscular and inter-muscular intra-muscular and inter-muscular coordination.coordination.

• Intra-muscular coordination – affects the ability Intra-muscular coordination – affects the ability to voluntarily activate individual fibers in a to voluntarily activate individual fibers in a specific muscle specific muscle

• Inter-muscular coordination – affects the ability Inter-muscular coordination – affects the ability to activate many different muscles at the to activate many different muscles at the appropriate time appropriate time

Intra-muscular coordination changes withIntra-muscular coordination changes withtrainingtraining

Untrained individuals find it difficult to Untrained individuals find it difficult to recruit all their fast-twitch MUs. With recruit all their fast-twitch MUs. With training, an increase in MU activation training, an increase in MU activation occurs occurs

Strength training also trains the MUs to Strength training also trains the MUs to fire at the optimal firing rate to achieve fire at the optimal firing rate to achieve tetany tetany

MUs might also become activated more MUs might also become activated more

synchronously during all out maximum synchronously during all out maximum effort effort

Consequently, maximal muscular Consequently, maximal muscular force is achieved when:force is achieved when:

1. 1. A maximal # of both FT and ST motor A maximal # of both FT and ST motor units are recruitedunits are recruited

2. Rate coding is optimal to produce a 2. Rate coding is optimal to produce a fused state of tetanyfused state of tetany

3. The MUs work synchronously over the 3. The MUs work synchronously over the short period of maximal effort. short period of maximal effort.

Psychological factors are also of importancePsychological factors are also of importance

CNS either increases the flow of excitatory stimuli, decreases CNS either increases the flow of excitatory stimuli, decreases

inhibitory stimuli, or both inhibitory stimuli, or both

Consequently, an expansion of the recruitable motor neuron Consequently, an expansion of the recruitable motor neuron pool occurs and an increase in strength resultspool occurs and an increase in strength results

Hidden strength potential of human muscle can also be Hidden strength potential of human muscle can also be demonstrated by electrostimulationdemonstrated by electrostimulation

Muscle strength deficit (MSD) = Muscle strength deficit (MSD) =

((Force during electrostimulation-Maximal voluntary forceForce during electrostimulation-Maximal voluntary force) x 100) x 100

Maximal voluntary forceMaximal voluntary force

Typically falls between 5-35%Typically falls between 5-35%

ElectrostimulationElectrostimulation

• Possibility exists to induce hypertrophy through Possibility exists to induce hypertrophy through electrostimulationelectrostimulation

• However, does not train the nervous system to However, does not train the nervous system to recruit motor unitsrecruit motor units

Bilateral DeficitBilateral Deficit

• During maximal contractions, the sum of forces During maximal contractions, the sum of forces exerted by homonymous muscles unilaterally is exerted by homonymous muscles unilaterally is typically larger than the sum of forces exerted by the typically larger than the sum of forces exerted by the same muscles bilaterallysame muscles bilaterally

• Bilateral training can eliminate this deficit, or even Bilateral training can eliminate this deficit, or even allow bilateral facilitationallow bilateral facilitation

Other benefits of strength trainingOther benefits of strength training Increase in resting metabolic rateIncrease in resting metabolic rate

• Each additional pound of muscle tissue increases Each additional pound of muscle tissue increases resting metabolism by 30 to 50 calories per day = 10,950 to resting metabolism by 30 to 50 calories per day = 10,950 to

18,250 calories a year = 3-5 lb of fat18,250 calories a year = 3-5 lb of fat

Increase in bone mineral content and, therefore, bone density Increase in bone mineral content and, therefore, bone density

Increases the thickness and strength of the connective tissue Increases the thickness and strength of the connective tissue structures crossing joints such as tendons and ligaments – helps structures crossing joints such as tendons and ligaments – helps prevent injury prevent injury

Increased stores of ATP, Creatine Phosphate (CP), and glycogenIncreased stores of ATP, Creatine Phosphate (CP), and glycogen

Aids rehabilitation from injuryAids rehabilitation from injury

Aging gracefully! Less falls in latter yearsAging gracefully! Less falls in latter years

Looking better, feeling better. Greater self-esteemLooking better, feeling better. Greater self-esteem

Metabolic stress of resistance Metabolic stress of resistance trainingtraining

Classed as only light to moderate in terms Classed as only light to moderate in terms of energy expenditure per workoutof energy expenditure per workout

Standard weight-training does not improve Standard weight-training does not improve

endurance or produce significant endurance or produce significant cardiovascular benefits like aerobic type cardiovascular benefits like aerobic type activity doesactivity does

Circuit-training increases metabolic stressCircuit-training increases metabolic stress

Delayed onset of muscle soreness Delayed onset of muscle soreness (DOMS)(DOMS)

The intensity and the novelty of a workout influence how sore you The intensity and the novelty of a workout influence how sore you becomebecome

Lactate does not cause muscle soreness due to:Lactate does not cause muscle soreness due to:

• 1. Lactate returns to baseline within an hour of exercise1. Lactate returns to baseline within an hour of exercise• 2. After exercise, lactate is in equal amounts within the muscle 2. After exercise, lactate is in equal amounts within the muscle

and the bloodand the blood• 3. DOMS is specific, not generalized3. DOMS is specific, not generalized

Muscle soreness is due to the physiological response to muscle Muscle soreness is due to the physiological response to muscle fiber and connective tissue damage (microtears)fiber and connective tissue damage (microtears)

White blood cells enter the muscle tissue, clean up the debris of White blood cells enter the muscle tissue, clean up the debris of broken proteins, and then initiate the regeneration phasebroken proteins, and then initiate the regeneration phase

Muscle Soreness (continued)Muscle Soreness (continued) Edema (increase in fluid) to the area accompanies Edema (increase in fluid) to the area accompanies

the above responsethe above response

The pressure from edema is thought to produce The pressure from edema is thought to produce the sensation of sorenessthe sensation of soreness

Also, metabolic by-products released from the Also, metabolic by-products released from the

macrophages may sensitize pain receptorsmacrophages may sensitize pain receptors

Next stage is the proliferation of satellite cells - Next stage is the proliferation of satellite cells - help form new myofibrilshelp form new myofibrils

Eccentric contractions cause the greatest amount Eccentric contractions cause the greatest amount of soreness of soreness