Final answer:
Parallel muscles have fascicles aligned with the muscle's long axis, commonly found in skeletal muscles. They can be flat or spindle-shaped (fusiform) with a noticeable middle section called the belly that bulges upon contraction.
Step-by-step explanation:
Fundamental Characteristics of Parallel Muscles
When discussing parallel muscles, we refer to those with fascicles aligned with the long axis of the muscle. This configuration is common among the skeletal muscles in the body. These muscles can be flat bands or have a round profile with tendons at the ends. Notably, a parallel muscle with a large midsection is commonly called the belly, and it might present a spindle-shaped form, also known as fusiform.
The action of these muscles is illustrated by the biceps brachii, where during contraction, the muscle's belly enlarges as the contractile fibers shorten. Muscles' function also depends on their arrangement with their synergists and antagonists, effectively dictating the force, speed, and range of movement. Fascicle arrangement contributes significantly to a muscle's ability to generate force, shape the muscle's overall action, and is a factor in naming muscles, such as the biceps brachii or the triceps brachii.