Final answer:
Muscles fire asynchronously to enable smoother movements through a process called recruitment, where smaller motor units fire first for precision and larger units are recruited for more strength as needed. This is managed by the nervous system and facilitated by the lever systems of muscle and bone interactions.
Step-by-step explanation:
The majority of muscles fire asynchronously because it's a smoother movement. This coordination in muscle movement is due to the nervous system, which controls different motor units within the muscle. Muscles are made up of different motor units which vary in size and strength. Small motor units, with fewer muscle fibers and lower-threshold motor neurons, are more excitable and fire first, resulting in finer, more precise movements. As more force is required, larger motor units with bigger, higher-threshold motor neurons are recruited to generate more contractile strength through a process called recruitment. In the case of multipennate muscle like the deltoid, stimulation of individual fascicles can change the direction of the muscle's pull, allowing for complex movements like arm abduction combined with flexion.
Moreover, muscles operate in pairs, with one acting as a flexor and the other as an extensor, to facilitate movement through contraction. Skeletal muscles are attached to bones via tendons close to joints, which function like simple machines with a mechanical advantage, where the input force (muscle contraction) is much greater than the output force (limb movement). This system allows for efficient and smooth movements within various ranges of force, enabling both delicate and powerful actions by the same muscle groups.