In order for the human body to move, skeletal muscles must contract. Each muscle in the body is composed of muscle fibers, organized into groups called motor units. These motor units receive signals from the brain when movement is desired. The ability to vary the amount of force produced by skeletal muscles is essential for smooth and coordinated performance of patterned movement. In the human body, various mechanisms are in place in order to efficiently graduate the amount of strength needed to perform these tasks efficiently.
Motor Unit Recruitment and Firing Frequency
A motor unit is the functional unit of the neuromuscular system. The amount of muscle fibers innervated by a motor unit varies from fewer than 10 fibers for small muscles to more than 100 for larger muscles. When stimulated, all the fibers innervated by the stimulated motor unit contract. To increase strength, more motor units are recruited to contract or the frequency of the signal to contract is increased.
The ability of muscle to generate force is determined in part by the speed at which a muscle is contracting. For concentric muscle contractions, or when a muscle shortens, as the speed of a contraction increases the force-producing capability of the muscle decreases in a hyperbolic fashion. During eccentric, or muscle lengthening, contractions the force-producing capability of the muscle is increased with greater speed.
Muscle fibers are composed of thick and thin filaments organized into sarcomeres, or the smallest contractile unit of muscle. The length of a muscle impacts its force-producing capability because of the corresponding overlap of thick and thin filaments at varying lengths. When a muscle fiber is stimulated to contract and that muscle is at an optimal length, indicated by the greatest possible overlap of thick and thin filaments, maximal strength is produced. If a muscle is too short or too long, less than optimal force is produced due to the lack of filament overlap and binding site availability.
Muscle Fiber Type
Not all muscle fibers are equal. Skeletal muscle fibers fall into one of two categories based on the alpha-motor neuron that innervates them: type I muscle fibers, or slow-twitch fibers, and type II fibers, or fast-twitch fibers. Motor units containing type II fibers are typically larger than motor units containing type I fibers. This difference in motor unit size means that when a single type II motor unit is stimulated, more muscle fibers contract than when a type I motor unit is stimulated. Since more fibers are stimulated to contract in type II motor units, more force is produced by type II fibers.