The force-velocity relationship

The force capability of a muscle is related to the cross-sectional area of the muscle. Muscles that have larger cross-sectional areas have larger numbers of sarcomeres in parallel, more potential cross-bridge heads in contact with actin molecules, and greater potential for applying force.

On the other hand, more sarcomeres in series (i.e. one after another – a longer muscle) increases the potential velocity of shortening. This may possibly occur because all sarcomeres shorten at about the same time (for example, two sarcomeres in series could shorten the ends of the muscle two times as far in the same amount of time as only one sarcomere). Longer muscles with more sarcomeres in series have greater potential for fast contractions.

In addition, force production is inversely related to velocity of shortening during concentric actions – during faster movements, less force production is possible; and when lifting heavier loads, slower movements occur. This is probably due to a smaller number of cross-bridge contacts on actin filaments at any instant as the velocity of shortening increases.

The relationship is different for eccentric actions. As the velocity of eccentric action increases, maximal force production also increases. The force capabilities are typically 120% to 160% greater in eccentric actions than in concentric actions. This means that when overloading eccentrically, very heavy resistances may be needed, but when training for explosive concentric movements, relatively light resistances may be more suitable.

Not all muscles have sarcomeres in series, aligned along the long axis of the muscle. Pennate muscle has muscle fibres that align obliquely with the tendon, and the angle of pennation with the tendon can affect the number of sarcomeres per cross-sectional area and thus the maximal force capabilities. Any factor that affects angle of pennation would therefore affect strength and velocity of shortening, as long as the cross-sectional area remains the same. Muscles with greater pennation (muscle fibres at a greater angle from parallel to the tendon) have more sarcomeres in parallel and fewer sarcomeres in series. They are therefore better able to generate force, but have a lower maximal shortening velocity than nonpennate muscle. Angle of pennation can vary, depending on hereditary factors and even training, which could help account for some of the differences in strength and speed seen in individuals who seem to have muscle of the same size.

N.B. Longer muscles with more sarcomeres in series (e.g. fusiform) have greater potential for fast contractions, but produce less force; whereas muscles with more sarcomeres in parallel (e.g. pennate muscles) have the potential to produce larger force, but at slower velocities.