The force an activated skeletal muscle can sustain depends on how fast it is shortening or lengthening. This is the force–velocity relationship.

Concentric shortening: heavier loads shorten more slowly

During a concentric contraction, the muscle shortens against an external load.

At a light load, cross-bridges can cycle while the filaments slide relatively rapidly. As the opposing load increases, shortening becomes slower.

At the largest load that the activated muscle can hold without changing length, shortening velocity reaches zero and the contraction is isometric.

Qualitatively,

concentric shortening speed
        high |\
             | \
             |  \
             |   \
           0 +----\------> opposing force
                  isometric limit

This inverse relation does not mean each myosin head literally moves more slowly in direct proportion to load. Macroscopic velocity emerges from the kinetics and mechanics of many cross-bridges interacting with a moving filament system.

Eccentric lengthening: active force can exceed isometric force

If the external load exceeds the force that the activated muscle can hold isometrically, the muscle can be forced to lengthen while remaining active.

Over part of this eccentric regime, muscle can resist a force larger than its maximum concentric or isometric force. Cross-bridges and elastic components of muscle can bear force while being stretched.

Thus the complete relationship is asymmetric:

• faster concentric shortening is associated with lower sustainable force;
• zero velocity corresponds to an isometric condition;
• active lengthening can support comparatively high resisting force.

Other variables must be controlled when comparing velocities

Force–velocity is not the only determinant of muscle force. Starting length and the degree to which the contractile apparatus is activated can also change measured force, and prolonged activity can change the muscle's force capacity over time.

A meaningful force–velocity comparison therefore holds those other conditions as constant as practical.

The central mechanical result is that skeletal muscle is neither an ideal force source nor an ideal constant-speed actuator. Its force capacity and movement velocity constrain one another because both arise from the dynamics of actin–myosin interactions.