Movement of your muscles requires contraction of hundreds of skeletal muscle fibers in unison. These contractions involve a complex coordination of signals from the brain and spinal cord to the nerve endings on your muscle fibers. When the muscle fibers receive this signal, a series of molecular and conformational changes to the muscle proteins trigger contraction. Since every movement you make is different, the brain must calculate how much muscle force is needed to execute the particular movement. For example, your muscles require more force to pick up a brick than a pencil. The strength of a muscle contraction depends on the number of motor units activated, stimulation frequency of the motor units, muscle fiber length and the speed of contraction.
Motor Unit Activation
A motor unit consists of an alpha-motor neuron and the muscle fibers it innervates. Depending on the size of the motor unit, the alpha-motor neuron connects to between 10 and 1,000 muscle fibers and sends a signal to trigger simultaneous contraction of all the fibers in that motor unit. This synchronized contraction allows the muscle to make coordinated movements. For movements requiring little force, such as picking up a pencil, you recruit small motor units in your arm muscles. For high-force movements, such as picking up a brick, you will recruit large motor units in addition to the small ones to execute the movement.
Stimulation Frequency
A single stimulus of the muscle fiber from the nervous system will produce a small amount of muscle force, followed by a muscle relaxation as the fiber returns to baseline. However, if the nervous system delivers several stimuli before the fibers can fully relax, the muscle fibers produce more force than they would in response to a single stimulus. Continual nervous system stimulation of the motor unit and muscle fibers yields the maximal force possible of the given muscle fibers, increasing the strength of the muscle contraction.
Muscle Fiber Length
Your muscles have thin and thick filaments, which are organized into contractile units called sarcomeres. Within each sarcomere, thick filament proteins slide and bind to proteins in the thin filament during muscle contraction. The sarcomeres have an optimal length at which the number of possible binding sites between the filaments is maximized. If your muscle fibers are shorter or longer than this optimal length, they do not have as much force-producing potential because there are fewer available binding sites between the filaments. For example, when your elbow is fully bent, the length of your biceps muscle fibers is shorter and less capable of producing force than when your elbow is extended.
Contraction Speed
Muscle contraction speed determines the force-producing capacity of your muscle. For a concentric muscle contraction in which the muscle fibers shorten, your muscles' force-producing capacity decreases at faster contraction speeds. Conversely, an eccentric contraction in which the muscle lengthens produces greater force at faster contraction speeds, and force-production capacity is always greater in an eccentric contraction compared to a concentric contraction. For example, you may be capable of lowering a heavy barbell to your chest during a chest press, but cannot lift it off your chest. This is because the chest and shoulder muscles eccentrically contract to lower the weight and contract concentrically to lift the weight.
References
- "Physiology of Sport and Exercise, 4th Edition"; Jack H. Wilmore, David L. Costill and W. Larry Kenney; 2008
- "Skeletal Muscle From Molecules to Movement"; David Jones, Joan Round and Arnold de Haan; 2004
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