The force a muscle generates is dependent on the length of the muscle and the shortening velocity of the muscle. These two fundamental properties of muscles affect biomechanical properties including limiting running speed, strength, and jumping distance and height.
The force-length relationship Figure 2 in muscle indicates that muscles generate the greatest force when at their resting, or ideal length, and the least amount of force when shortened or stretched relative to the resting length. When a skeletal muscle fiber contracts, myosin heads attach to actin to form cross-bridges followed by the thin filaments sliding over the thick filaments as the heads pull the actin, and this results in sarcomere shortening, creating the tension of the muscle contraction. The cross-bridges can only form where thin and thick filaments already overlap, so that the length of the sarcomere has a direct influence on the force generated when the sarcomere shortens. This is also called the length-tension relationship.
Resting length is often the ideal length of a muscle and the length at which it can create the greatest active force. This active force decreases with deviations from such ideal length, either by shortening or stretching. The ideal length of a sarcomere to produce maximal tension occurs at 80 percent to 120 percent of its resting length, with 100 percent being the state where the medial edges of the thin filaments are just at the most-medial myosin heads of the thick filaments (Figure 3). This length maximizes the overlap of actin-binding sites and myosin heads. If a sarcomere is stretched past this ideal length (beyond 120 percent), thick and thin filaments do not overlap sufficiently, which results in less tension produced. If a sarcomere is shortened beyond 80 percent, the zone of overlap is reduced with the thin filaments jutting beyond the last of the myosin heads and shrinks the H zone, which is normally composed of myosin tails. Eventually, there is nowhere else for the thin filaments to go and the amount of tension is diminished.
If the muscle is stretched to the point where thick and thin filaments do not overlap at all, no cross-bridges can be formed, and no tension is produced in that sarcomere. This amount of stretching does not usually occur, as accessory proteins (such as titin) and connective tissue oppose extreme stretching. Such muscles work to maintain the ideal length and thus peak active force for muscles.
The force-velocity relationship Figure 1 in muscle relates the speed at which a muscle changes length (usually regulated by external forces, such as load or other muscles) to the force it can generate. Force declines in a hyperbolic fashion relative to the isometric force as the shortening velocity increases, eventually reaching zero at some maximum velocity. The reverse holds true for when the muscle is stretched-force increases above isometric maximum, until finally reaching an absolute maximum. This has strong implications for the rate at which muscles can perform mechanical work (power). Since power is equal to force times velocity, the muscle generates no power at either isometric force (due to zero velocity) or maximal velocity (due to zero force). Instead, the optimal shortening velocity for power generation is approximately one-third of maximum shortening velocity. These two fundamental properties of muscle have numerous biomechanical consequences including limiting running speed, strength, and jumping distance and height.