Introduction to Sports Biomechanics: Analysing Human Movement ...

Figure 6.14 Tension–time relationship.
Muscle stiffness
THE ANATOMY OF HUMAN MOVEMENT
The mechanical stiffness of a muscle is the instantaneous rate of change of force with
length – it is the slope of the muscle tension–length curve. Unstimulated muscles
possess low stiffness (or high compliance). This rises with time during tension and is
directly related to the degree of filament overlap and cross-bridge attachment. At high
rates of change of force, such as occur in many sports, muscle is stiff, particularly in
eccentric contractions for which stiffnesses over 200 times those for concentric contractions
have been reported. Stiffness is often considered to be under reflex control with
regulation through both the length component of the muscle spindle receptors and the
force–feedback component of the Golgi tendon organs. The exact role of the various
reflex components in stiffness regulation in fast human movements in sport remains to
be fully established as do their effects in the stretch–shortening cycle (see below). It is
clear, however, that the reflexes can almost double the stiffness of the muscles alone at
some joints. Furthermore, muscle and reflex properties and the central nervous system
interact in determining how stiffness affects the control of movement.
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