1. FACTORS INFLUENCING FORCE OF CONTRACTION
MOTOR UNIT RECRUITMENT
FATIGUE
RK Goit, Lecturer
Department of Physiology

2. 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

4.  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

5. 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

6. Frequency-tension relation

8.  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

9. 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

10. 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

11.  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

12. 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

13.  process of increasing the force of contraction by
recruiting additional motor units is called
summation

14. 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

15. 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

16. Thank You