Isotonic contraction, Isometric contraction, Motor unit

1. Isometric, Isotonic
contractions and motor unit
Dr. Sai Sailesh Kumar G
Associate Professor
Department of Physiology
RDGMC

2. Introduction
 Muscle contraction involves the shortening of
contractile elements.
 Tension is the force of contraction of muscle acting
on the objects.
 Load is the force exerted on the muscle by the
object.

3. Types of contraction
 Depending on changes in the length of the muscle,
there are three types of muscle contraction.
 Isometric contraction ( same length).
 Isotonic contraction (tone constant)
 Isokinetic contraction.

4. Isometric contraction
 When both the ends of muscles are fixed and stimulated.
 The muscle goes into isometric contraction ( same length).
 The length of muscle remains unchanged.
 The tension of muscle increases but tension is less than
load.
 Hence, load can not be lifted.
 Muscle is not shortening so work done = Zero

5. Isometric contraction
 Muscle is not shortening so work done = Zero
 Oxygen consumption increases proportionally to muscle tension.
 During this contraction, the maximum load will be on cardiovascular
function.
 Increase in blood pressure.
 Hence, isometric exercises are not recommended for heart patients.
 It is a type of strength training in which muscle length and joint angle do
not change.
 Example: Pushing the wall, several yoga poses

6. Isometric contraction

7. Isotonic contraction
 Muscle shortens without any change in tension.
 Tension that develops is more than the load.
 Hence, load is lifted.
 Work is done.
 Load imposed on CVS is less.
 This type of exercise is recommended for heart patients.
 Example: walking, running, lifting a bucket

8. Isotonic contraction
 Two types.
 Concentric contraction- muscle tension rises to meet the
resistance then remains same as the muscle shortens.
 Example: Lifting a bucket
 Eccentric contraction- muscle lengthens due to the
resistance being greater than the force the muscle is
producing.
 Example: walking downstairs, unloading the load.

9. Isotonic contraction

10. Isokinetic contraction
 Muscle shortens during a contraction but at a
constant speed of contraction.
 This is a strength training exercise useful for toning
weak musculature and building strength in all the
muscles.

11. Work done
 Workdone = force x distance
 Isometric contraction do not produce work.
 No change in length
 Work done = force x 0 = 0

12. Similarities in isometric and isotonic contractions
 Muscle excitation
 Cross bridge formation
 Filaments sliding

13. Isometric contraction
1. Muscle does not shorten
2. Contract against force with
out shortening ( fixed length)
3. Does not involve joint
movement
4. Tension increases
5. No work done
6. Less release of heat
Isotonic contraction
1. Muscle shorten
2. Shorten against fix load
3. Involve joint movement
4. Tension constant
5. Work done
6. More release of heat

14. Benefits of isometric exercise
 Main benefit is increased muscle strength
 Can easily target a specific muscle or group of
muscles
 If you start exercise with holding a weight of 5kg for
20 seconds, as you progress you can increase
time and also weight

15. Motor unit
 Motor neurons are present at anterior gray horn of spinal
cord.
 Each motor neuron can activate a group of the muscle
fibers through its axon terminals.
 A single motor neuron and the muscle fibers it innervates,
together is called motor unit.
 It is referred as physiological or functional unit of the
muscle.

18. Types of motor units
 There are three types of motor units.
 Fast motor unit (IIB).
 Slow motor unit (I).
 Intermediate motor unit (IIA).
 This classification is based on type of muscle fibers.
 In a motor unit, only one type of muscle fibers are
present.

19. Size of the motor unit
 The size of motor unit depends on the number of muscle fibers
that a motor neuron innervates.
1. Small motor unit - muscles concerned with fine skilled
movements, number of muscle fibers in a motor unit varies from
2-6. example- middle ear muscles, ocular muscles.
2. Large motor unit – In muscles with gross and crude
movements, the number of muscle fibers in a motor unit varies
from 100-600. example- muscles of back and leg

20. Number of motor units in a muscle
 The number of motor units in a muscle fiber vary
from few to few hundreds depending on the size of
the muscle.

21. Recruitment of motor units
 This depends on force of contraction required.
 When there is a need for less force of contraction, fewer and smaller
motor units are activated.
 When there is a need for more force of contraction, large and several
motor units are activated.
 During prolonged low intensity activity, slow motor units are recruited
 During short, high intensity activity, fast motor units are activated.
 Motor units are activated asynchronously and this will help in smooth
contractions and delays early onset of fatigue.

23. Recruitment of motor units
 It minimizes fatigue by allowing most fatigue-resistant
muscle fibers to be in use.
 More fatigable fibers are kept in reserve until needed
to achieve higher forces
 Cause smooth contraction and relaxation of muscle
 The recruitment of motor units can be studied using
electromyography.

24. Force summation
 Adding together individual twitch contractions to
increase the intensity of overall muscle contraction.
 It can occur by
 Increasing number of motor units contracting
simultaneously- multiple fiber summation
 Increasing the frequency of contraction- frequency
summation- leads to tetanization.

25. Force summation
 When weak signals were sent to the muscles to contract
from CNS, only small motor units are activated.
 As there is an increase in the strength of the stimulus,
larger and larger motor units are recruited.
 The phenomenon of activation of large motor units as the
strength of stimulus increase is size principle.

26. Frequency summation
 As the frequency increases, there comes a point where each
new contraction occurs before the preceding one is over.
 As a result, second contraction is added to first
 Total strength of contraction raises progressively, with
increase in frequency.
 Tetanization – at higher frequency muscle is only remains in
contraction and no relaxation.

27. Muscle unit
 Group of muscle fibers, innervated by a single
motor neuron is called a muscle unit.

28. Macromotor unit
 Seen in poliomyelitis.
 Motor neurons are destroyed and leads to
paralysis of corresponding muscle fibers
 The neighboring nerve fibers give collateral
branches that supply paralyzed muscle fibers
 Results in large motor unit-macromotor unit

29. Motor point
 Point on the muscle at which the nerve enters the
muscle to establish contact with the individual
muscle fiber.
 A good response of the muscle is obtained when
the stimulus is applied at this point.

30. THANK YOU