The number of motor units stimulated, as well as the rate at which the motor units are stimulated, will cause the force developed by the muscle to vary. In this activity you will compare motor unit stimulation in resting muscle (muscle tone) and in the same muscle contracting under different conditions.
Play the animation to review details of muscle motor units recruitment.
The amount of force developed by a muscle is dependent on the number of motor units activated (recruitment) and the frequency of motor unit stimulation (wave summation).
Normal resting muscles exhibit muscle tone due to the stimulation of a small number of motor units. Motor unit activation is rotated among all the different motor units within a muscle. This asynchronous activation keeps all the motor units within a muscle in shape.
A muscle that is responsible for an action is called an agonist or prime mover. The muscle that has an action opposite the agonist is called the antagonist, and is usually on the opposite surface from the agonist. The major muscles located on the arm, the biceps brachii and triceps brachii, are good examples. On the anterior surface of the arm is the biceps brachii, the agonist for forearm flexion, and on the posterior surface of the arm is the triceps brachii, the antagonist for this action. The triceps brachii is the agonist for arm extension and the biceps brachii is the antagonist for this action.
During isotonic contraction, muscle length changes while the tension remains constant. There are two types of isotonic contractions, concentric and eccentric.
Concentric isotonic contraction involves muscle shortening. An example of this is when the biceps brachii muscle contracts and shortens during arm flexion.
Eccentric isotonic contraction involves muscle lengthening when muscle tension resists an action. An example of this is when the biceps brachii muscle resists arm extension.
In isometric contraction, the length of the muscle remains fixed while the muscle is developing tension. An example of this is when the biceps brachii muscle is maintaining the arm in a flexed position.
Recruitment is the process of increasing the number of active motor units to increase force developed by a muscle. Usually, recruitment involves activation of different motor units to help delay fatigue. The weaker, more fatigue resistant motor units (slow oxidative motor units) are recruited first, with stronger motor units (less fatigue resistant) added if more force is needed in response to greater muscle load (weight moved).
When a motor unit is stimulated to contract, each muscle fiber within the motor unit produces an electrical signal (action potential) along the sarcolemma (muscle fiber plasma membrane). These electrical signals can be picked up by electromyography and recorded as an electromyogram (EMG). An EMG contains spikes, changes in measured voltage, that correspond to action potentials within motor units. Many factors including motor unit size, number of motor units activated, and frequency of motor unit firing affect the amplitude and frequency of EMG spikes. Therefore an increase in amplitude and frequency of EMG spikes is correlated with increased motor unit activation. A correlation can be drawn between increased amplitude and frequency of EMG spikes and greater muscle tension.
