FACTORS INFLUENCING FORCE OF CONTRACTION

MOTOR UNIT RECRUITMENT

FATIGUE

RK Goit, Lecturer

Department of Physiology

The Muscle Twitch

the mechanical response of a muscle fiber to a

single action potential

starts about 2 ms after the start of depolarization

of the membrane

Peak tension

an ocular muscle- 7.5 msec (fast twitch fibers)

gastrocnemius muscle- 40 msec (mixture of fast

& slow twitch fibers)

soleus muscle- about 90msec (slow twitch fibers)

ocular movements must be extremely rapid to

maintain fixation of the eyes on specific objects

gastrocnemius muscle must contract moderately

rapidly to provide sufficient velocity of limb

movement for running & jumping

soleus muscle is concerned with slow contraction

for continual, long-term support of the body

Classification of skeletal muscle fiber types

Type I Type II

Other names Slow; Oxidative; Red Fast; Glycolytic; White

Myosin ATPase activity Low High

Ca2+ Pumping capacity of

sarcoplasmic reticulum

Moderate High

Diameter Moderate Large

Glycolytic capacity Moderate High

Oxidative capacity High Low

Twitch duration 100 ms 7.5 ms

Number of fibers Few (100-500) Many (1000-2000)

Contraction velocity Moderate Fast

Fatigability Low high

Motor unit size Small Large

Size of motor neuron Small Large

Function Concerned with strong,

gross, sustained

movements

Concerned with fine,

rapid, precise movement

Frequency-tension relation

increase in muscle tension from successive action

potentials -summation

a maintained contraction in response to repetitive

stimulation -tetanus

at low stimulation frequencies, the tension may

oscillate as the muscle fiber partially relaxes

between the stimuli-unfused tetanus

a fused tetanus, with no oscillation, is produced at

higher stimulation frequencies

Fatigue

decrease in performance due to continuous & prolonged activity

can occur in the whole organism or in isolated preparations

is a reversible phenomenon & there is no permanent functional or structural damage to the tissue

if a muscle is allowed to rest after the onset of fatigue, it can recover its ability to contract upon restimulation

Major sites of fatigue

Central nervous system

encouragement & motivation increases the

performance of the subject

encouragement stimulates the frontal lobe of the

cortex that increases the activity in the motor

cortex

Neuromuscular junction

exhaustion of ACh

fast twitch muscle fibers seem to be more prone

to fatigue while slow fibers get fatigued later on

Muscle

Depletion of energy reserves

Depletion of creatine phosphate & glycogen

Accumulation of metabolites

↑ lactic acid concentration

↓ pH

↓ affinity of Ca++ for troponin

Inhibits some key glycolytic enzymes (glycogen phosphorylase & phosphofructokinase)

Accumulation of Ca++ in T-tubules

Motor nerve

nerve is theoretically unfatiguable

Recruitment

the process of increasing the number of motor units that are active in a muscle at any given time

slow twitch motor units tend to be small (100 to 500 muscle fibers) & is easily excited

fast twitch motor units tend to be large (1000 to 2000 muscle fibers) & are more difficult to excite

slow twitch motor units tend to be recruited first

fast twitch motor units are recruited when more & more force is needed

process of increasing the force of contraction by

recruiting additional motor units is called

summation

Factors determining force of contraction

1. Action potential frequencies (frequency-

tension relation)

2. Fiber length (length-tension relation)

3. Fiber diameter

4. Fatigue

5. Number of active fibers

References

Berne & Levy Physiology, 6/E

Ganong Review of Medical Physiology, 23/E

Textbook of Medical Physiology, 12/E Guyton &

Hall

Understanding Medical Physiology, 4/E Bijlani &

Manjunatha

Vander’s Human Physiology: The Mechanisms of

Body Function, 11/E

Thank You