Whole muscles are composed of motor units containing muscle fibers of the same type. The proportions of fiber types determine a muscle's contraction speed, strength, and fatigability. The total tension a muscle produces depends on the tension of individual fibers and the number contracting. The nervous system controls tension and shortening velocity by regulating the number of active motor units through recruitment. Recruitment increases the excitatory input to motor neurons, activating more motor units and fibers. A muscle's shortening velocity is influenced by its load and the fiber types of its motor units.

1. Whole-Muscle Contraction whole muscles are made up of many muscle fibers organized into motor units. All the muscle fibers in a single motor unit are of the same fiber type.

2. fiber type designation to the motor unit and refer to slow-oxidative motor units fast-oxidative motor unit fast glycolytic motor units.

3. Depending on the proportions of the fiber types present, muscles can differ considerably in their maximal contraction speed, strength, and fatigability Eg. the muscles of the back and legs, muscles in the arms

4. Control of Muscle Tension The total tension a muscle can develop depends upon two factors: the amount of tension developed by each fiber. the number of fibers contracting at any time.

6. By controlling these two factors, the nervous system controls whole-muscle tension, as well as the shortening velocity.

7. The number of fibers contracting at any time depends the number of fibers in each motor unit (motor unit size), the number of active motor units.

8. Motor unit size varies considerably from one muscle to another. The force produced by a single fiber, as we have seen earlier, depends in part on the fiber diameter— the greater the diameter, the greater the force.

9. The process of increasing the number of motor units that are active in a muscle at any given time is called recruitment. achieved by increasing the excitatory synaptic input to the motor neurons

10. The greater the number of active motor neurons, the more motor units recruited, and the greater the muscle tension. Motor neuron size plays an important role in the recruitment of motor units.

11. Control of Shortening Velocity the velocity at which a single muscle fiber shortens is determined by: the load on the fiber whether the fiber is a fast fiber or a slow fiber.

12. Translated to a whole muscle, these characteristics become: the load on the whole muscle the types of motor units in the muscle