Dr. Aivaras Ratkevicius

Human strength

SR1502

Human strengthPlan

• Assessment of muscle strength• Cross sectional area of muscles• Muscle fibre types• Motor units• MU recruitment during exercise

• Reading list:• 1. McArdle W.D. et al. Exercise Physiology: energy,

nutrition, human performance. Lippincott Williams & Wilkins, 2001. p. 362-382, 501-510

• 2. Wilmore J.H. & Costill D.L. Physiology of sport and exercise. Human Kinetics, 2004, p. 33-57.

Muscle strengthDynamometry

• Muscle strength is measured as maximal force generated by a particular muscle group

• Popular methods for assessment of muscle strength:1) Dynamometry (example in the figure)2) 1 repetition maximum (1 RM) method(1 RM is the maximal weight that one can lift)

Muscle strength Assessment

• Force increases in repeated maximal voluntary contractions (MVCs)

• Warm up and familiarization are important !!!

Force time curve

Fo

rce

(N)

Action potentials of muscle fibres

0.25 0.75 1.25 1.75 2.25 2.75Time from movement initiation in the Brain (s)

0

Force curve

Electromyogram(mV)

• Maximal force can not be exerted instantaneously !!!• Time is needed to:• 1) design the motor program (Brain: CNS time)• 2) conduct APs to the muscle fibres (AP conduction via axons in

CNS and periphery)• 3) generate force in muscle fibres (Ca2+ release and cross

bridge formation)

Muscle strength Assessment instructions

• 1. Clear instructions (motor programme)• 2. Adequate practise should be allowed

(warm up, familiarization)• 3. One contraction is performed every 2

min (fatigue prevention)• 4. Time (~3-5 s) should be allowed to

reach maximal force levels (time for muscle force generation)

• 5. Strong verbal encouragements are needed (motivation)

Muscle strengthCross sectional area

• Muscle strength is proportional to cross sectional area of skeletal muscles

Muscle strengthCross sectional area

• Cross sectional area of vastus lateralis decreases with age

• Muscle mass decreases with age

• This is referred to as sarcopenia (loss of flesh)

Lexell et al. 1983

Magnetic Resonance Imaging (MRI)Cross sectional areas (CSA) of thigh muscles

• Older people often have thinner muscles and more subcutaneous fat in their limbs

• This is partly due to decreased levels of physical activity

Muscle strength

Fm = specific tension x C.S.A.

Specific tension = 30 N/cm2

Muscle CSA (cm2)

Biceps Brachii 5.8

Brachialis 7.4

Brachioradialis 2.0

Triceps Brachii 23.8

From Edgerton, Apor & Roy (1990)

Predicted Force

174

222

60

536

• Pennate muscles (fanshaped):• 1) Large physiological cross

section, optimal for force generation

• 2) Small movement amplitude and low velocity of shortening

• Fusiform muscles• 1) Small physiological cross

section and low force generation• 2) Large movement amplitude

optimal for high velocity of shortening

• Note: Hamstrings (fusiform muscles) are prone to injury

PennateFusiform

Muscle shapePhysiological cross sectional area

• Muscles samples (biopsies) can be obtained applying needle biopsy technique (Bergstrom 1961)

Muscle fibre histochemistry

• Muscle fibre cross sections were stained at different pH• Muscle fibres are divided into TWO major types:

Type I (oxidative, slow) & Type II (glycolytic, fast)IIA, IIB, IIC are subtypes of type II

• Contraction speed: Slowest I => IIC => IIA => IIB Fastest

Muscle fibre histochemistry

• Motor unit is the functional unit of the neuromuscular system

• Motor unit is composed of motor neuron ( MN), axon and muscle fibres

• -MN innervates <3000 muscle fibres

• -MNs are in the spinal cord

• Motor neuron pool is a group of MNs that innervates a muscle

Motor unit

McArdle et al. 2001

• Small a MNs innervate small, slow, “aerobic” muscle fibres (type I)• Big a MNs innervate big, fast “anaerobic” fibres (type II)

SlowFatigue Resistant

FastFatigue Resistant

FastFast Fatigable

Types of MUs

McArdle et al. 2001

Type IType IIAType IIB

<= Increase in neural activation

S FRFFSlow

(S type) FastFatigueResistant (FR type)

FastFatigable (FF type)

1st recruited 2nd recruited 3rd recruited

MUs receive common neural input and are recruited according to their sizes !!!

THREE major types of motor neurons:

S type are small “high” excitability FR type are big “average” excitabilityFF type are very big “low” excitability

Motorneuron

Muscle fibres

MU recruitment• Recruitment of muscle

fibres during exercise:• Light intensity exercise:

Type I (slow)• Medium intensity exercise:

Type I + type IIA (FR)• High intensity exercise:

Type I + Type IIA + Type IIB (FF)

• Important observation: Type I fibres are always recruited during exercise

• Muscle fibre type composition:• Power athletes have high content of type II (fast

twitch, FT) fibres and average maximal oxygen consumption

• Endurance athletes have high content of type I (slow twitch, ST ) fibres and high maximal oxygen consumption

Human strengthSummary

• Clear instructions, adequate practise, sufficient time and motivation are all essential for assessment of muscle strength in humans

• Muscle strength is directly proportional to physiological CSA of skeletal muscles

• Type I (slow) and type II (fast) are two main fibre types in human muscles

• Motor unit (MU) is composed of motor neuron ( MN), axon and muscle fibres

• Slow muscle fibres (MUs) are recruited at low forces and fast fibres (MUs) at high forces