Muscle Structure and Contraction
What happens when you contract your muscle? First of, let’s outline the structure of skeletal muscle, which is surprisingly orderly and analogous. A muscle group (e.g. the biceps, the pecs, the glutes) is surrounded by epimysium, deep fascia that serves as the outermost layer of the entire muscle. Underneath the muscle group is the perimysium, connective tissue that surrounds individual bundles of muscle fibers. These bundles that make up the muscle group are called fasciculus or fascicles. These fascicles are, in turn, made up of individual muscle fibers, which are surrounded by yet another set of connective tissue, the endomysium. Muscle fibers are thin, elongated cylinders, sometimes running the entire length of the muscle. They have a membrane called the sarcolemma, many nuclei situated on the periphery of the cell, and a cytoplasm called the sarcoplasm. The sarcoplasm contains contractile components, dominated by myofibrils that run the entire length of the fiber. Myofibrils are further divided into sarcomeres, the smallest functional unit of skeletal muscle. Sarcomeres contain the contractile proteins myosin and actin, which are the proteins that bind during muscle contraction. Six actin filaments surround each myosin filament, and each actin filament is surrounded by three myosin filaments—an arrangement that gives muscle its striated appearance under magnification.
Muscle contraction begins with the alpha motor neuron producing an action potential that propagates down its axon to the neuromuscular junction (there is only one neuro-muscular junction per muscle fiber), causing a change in the electrical potential of the membrane of the cell. An electrical nerve impulse releases the neurotransmitter acetylcholine, which excites the sarcolemma. This activates the sarcoplasmic reticulum, parallel to and surrounding each myofibril, to release the calcium it stores into the myofibril, causing tension development in the muscle. Perpendicular to the sarcoplasmic reticulum, the T-tubules between the myofibril and the sarcolemma causes the action potential to arrive nearly simultaneously from the surface to the depths of the fiber to ensure coordinated contraction among the myofibrils.
Myofibrils contain the myofilaments mainly responsible for the contraction of the muscle fiber, myosin and actin. The calcium released by the sarcoplasmic reticulum binds with the protein molecule Troponin along the actin filaments and shifts Tropomyosin, another protein molecule, away from myosin binding sites, thereby uncovering a way for actin and myosin to bind. According to the sliding filament theory, myosin heads then grab the actin filaments, creating cross-bridges, allowing actin to slide over myosin. This causes the sarcomere (H-Zone, I-Band) to shorten. The force capability of a muscle, the amount of force produced by a muscle, is directly related to the number of cross-sectional cross-bridges it has at the level of the myofibril. It takes time for all the potential myosin heads to make contact with actin filaments; hence pre-loading is needed, which must involve load since the inertia of the weight will arouse sufficient force in the muscle, activating the myosin and actin filaments.
Once myosin is attached to actin, mitochondria produce ATP to provide energy for cross-bridge flexion. Only a very small displacement of actin occurs with each flexion of the myosin cross-bridge. Hence rapid, repeated flexions must occur in many cross-bridges throughout the muscle fiber to produce measurable movement. Force develops if there is resistance to pulling of myosin and actin; hence the need for load. If force is developed to exceed the load, a concentric contraction initiates in which myosin and actin move toward the center of the sarcomere (M-Line) through a series of ATP-consuming exertions. Another molecule of ATP is needed to detach the myosin head from the actin filament so that it can do it all over again.
The force capability of a muscle is greatest at its resting (v. contracted or stretched) length, when the myosin and actin filaments are at their starting distance from each other. When they are stimulated by the motor neuron, all muscle fibers of a motor unit (the motor neuron and all the muscle fibers it innervates) contract and develop force at the same time (all-or-none principle). The extent of control of a muscle, a matter altogether different from a muscle’s force production, depends on the number of muscle fibers within each motor unit. Precision is greater the lower the proportion between the motor neuron and the muscle fibers, this proportion determining the number of individual fibers the neuron must direct.
An action potential results in a short period of activation of the muscle fibers within the motor unit which may lead to muscle contraction, also known as a twitch. Although calcium release during a twitch is sufficient to allow optimal activation of myosin and actin, and thereby maximal force of the fibers, calcium is removed by the sarcoplasmic reticulum so that it can be recycled before force reaches its maximum, leading the muscle to relax. Similarly, a stronger action potential cannot produce a stronger contraction. Nonetheless, there are ways to increase force production. The time interval between twitches can be decreased (a method important for smaller muscles) so that force from the two twitches summates, which is greater than the force produced by a single twitch. If the frequency of twitches is rapid enough, twitches merge and possibly even fuse, leading to tetanus in which the maximal amount of force of the motor unit is developed. Additionally, the fact that all muscle fibers of a motor unit contract at the same time does not mean that all motor units are immediately and simultaneously activated. Another way to increase force output, then, is to recruit more motor units through pre-loading (to activate fibers) and by performing activities with different physiological needs (to recruit and simultaneously employ different muscle fiber types).
Source: NSCA’s Essentials of Strength Training and Conditioning