(1) Different muscles can differ in origin and insertion, and therefore in action, such as the biceps brachii and brachialis muscles. (2) Synergistic muscles acting together on a joint can produce stronger or more coordinated movements than a single larger muscle working alone. (3) Even if similar in origin and insertion, two muscles may differ in physiological type, such as the slow oxidative soleus muscle and fast glycolytic gastrocnemius muscle on the dorsal side of the leg. One muscle can be better adapted for quick responses and the other for fatigue resistance in prolonged exercise.
Once stretched, inelastic muscles would not recoil to their resting length. A biceps brachii muscle stretched by elbow extension, for example, would become flaccid when the elbow was flexed.
For each of the following muscle pairs, state which muscle you think would have the higher percentage of fast glycolytic fibers: (a) Muscles that move the eyes or muscles of the upper throat than initiate swallowing? (b) The abdominal muscles employed in doing sit-ups or the muscles employed in handwriting? (c) Muscles of the tongue or the skeletal sphincter muscle of the anus? Explain each answer.
(a) The eye muscles; (b) the muscles used in handwriting; (c) muscles of the tongue. The reason in all cases is that these muscles must act more quickly than the others named in the question, so they require a fast glycolytic (fast-twitch) metabolism.
Botulism results in depressed somatic reflexes and flaccid muscle paralysis, because motor neurons cannot release ACh and stimulate the muscles to contract. Suffocation is a common cause of death in botulism, resulting from inability of the intercostal and phrenic nerves to stimulate the muscles of respiration.
