it is very useful content for the basic knowledge of motor units . Dedicated to my father shri satyaveer singh , my mother bateri devi and the entire lnipe family .

1. CONCEPT OF
MOTOR UNIT
By : sandeep choudhary , Email
kumarsandeep4028@gmail.com
Sandeep choudhary (Dept of sports biomechanics lnipe gwalior ) 1

2. Sandeep choudhary (Dept of sports biomechanics lnipe gwalior )
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 A motor unit is made up of a motor neuron and
the skeletal muscle fibre innervated by that
motor neurons axonal terminal.
 All the muscle fibres in a motor unit are of
same fibre type .

3. Sandeep choudhary (Dept of sports biomechanics lnipe gwalior )
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 Classification of motor units on the basis of
number of cells.
Large motor units small motor units
(no of cells >100) (about 10 cells)

4. Sandeep choudhary (Dept of sports biomechanics lnipe gwalior )
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5. Sandeep choudhary (Dept of sports biomechanics lnipe gwalior )
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 The number of muscle fibres within each motor
unit can vary in particular muscle .
 (a) muscle that acts on large body masses has
more number of muscle fibres .
 (b) muscle that acts on small body masses
have less number of muscle fibres

6. Sandeep choudhary (Dept of sports biomechanics lnipe gwalior )
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 Alpha motor neuron innervating head and
neck are found in brain stem .
 Remaining alpha motor innervating rest of
body are found in spinal cord

7. Sandeep choudhary (Dept of sports biomechanics lnipe gwalior )
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 Alpha motor neuron receives signals (input)
from a number of sources which includes.
1. Sensory neuron
2. Upper motor neuron
3. Inner motor neuron

8. Sandeep choudhary (Dept of sports biomechanics lnipe gwalior )
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 Motor unit contraction is maintained for the
duration of 10-15 seconds .
 Whereas for muscle hypotrophy the duration is
40-60 seconds.
 The synapse b/w efferent neuron and a
muscle fibre always release enough
acetylcholine to depolarise the muscle above
threshold .

9. Sandeep choudhary (Dept of sports biomechanics lnipe gwalior )
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 Calcium ions are responsible for formation of
cross bridges b/w actin and myosin filament.

10. Types of motor units
Sandeep choudhary (Dept of sports biomechanics lnipe gwalior )
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 Motor units are classified into following types.
1. Slow motor units
2. Fast fatigable motor units
3. Fast fatigue resistant motor units

11. Slow motor units
Sandeep choudhary (Dept of sports biomechanics lnipe gwalior )
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 It innervates small red muscle fibres.
 Small red muscle fibres contracts slowly and
generates relatively small amount of force .
 It has plentiful mitochondria.
 Reach myoglobin content .
 Reach capillary beds.
 Slow motor units are required for sustained
muscular contraction .
 Eg. Maintenance of upright posture .

12. Fast fatigable motor units
Sandeep choudhary (Dept of sports biomechanics lnipe gwalior )
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 Large alpha motor neuron innervates large
number of muscle fibres .
 It generates more amount of force .
 It has less number of mitochondria .
 Easily fatigable in nature.
 Especially important for greater exertion that
require large forces such as running or
jumping etc.

13. Fast fatigue resistant motor
units
Sandeep choudhary (Dept of sports biomechanics lnipe gwalior )
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 Innervates intermediate size of muscle fibres .
 Not as fast as fast fatigable motor units .
 More resistant to fatigue as in comparison to
fast fatigable motor units .
 Generates twice amount of force than slow
motor units.

14. Properties of motor units
Sandeep choudhary (Dept of sports biomechanics lnipe gwalior )
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Characteristics Slow motor units Fast fatigue
resistant motor
units
Fast fatigable
motor units
function
function slow Intermediate Intermittent
Fatigue resistance High Medium low
Force gradation Fine Intermediate Coarse
Force Lower peak Medium peak High peak
Mitochondria (in
numbers )
High low low
Metabolic optimum
activity
Isometric Shortening Shortening
Recruitment
threshold
60-120 m/s 10-50 m/s 10-50 m/s

15. Sandeep choudhary (Dept of sports biomechanics lnipe gwalior )
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Characteristics Slow motor units Fast fatigue
resistant motor
units
Fast fatigable
motor units
function
Phase Tonic phasic Phasic
Duty cycle Long and
continuous
Intermediate Intermittent

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