Figure 5.3 Factors that affect the worker’s ability to maintain a position while doing a task. The limb rotates around the joint, under control of at least one agonist and one antagonist muscle. Each muscle contributes passive elasticity (pe), crossbridge force (cb), elasticity from structures in series with the crossbridges (se), and viscosity (v). Its activity is controlled by reflexes and volition. The limb has inertia. The task itself may exert forces on the limb, or load the limb with extra mass or elasticity (modified from Winters et al ., 1988).

Muscle stiffness and viscosity arise mostly from its crossbridges (Ford et al., 1977; Rack and Westbury, 1974) (Figure 5.3). These crossbridges are elastic, and any movement that lengthens the muscle must stretch them; this gives an apparent stiffness. Also, the crossbridges detach and reattach in a way that depends on the speed of stretch; this gives an apparent viscosity. Some of the stiffness arises from the presence of the stretch reflex, which activates a muscle when it is lengthened, increasing its resistance to stretch (Nichols and Houk, 1973). The stretch reflex is most important at low to intermediate values of force. Some also arises from muscle’s passive elasticity. When a muscle is inactive and unloaded, it assumes a certain length (its ‘rest length’). If you stretch the muscle beyond its rest length, for example by hanging a weight on it, the muscle exerts a force opposing the stretch. If the muscle crosses a joint, this force causes a torque at that joint. This torque is familiar to you if you have ever tried to increase your ‘flexibility’ – you will have felt a strong resistance as you tried to rotate the joint beyond its normal range. The passive muscle torque around human joints is different for different joints, but always tends to restore the muscle towards its rest length. It increases sharply at the ends of the range of motion (Mansour and Audu, 1986; Yoon and Mansour, 1982). Passive forces are not constant, and the joint tends to become less stiff if it is stretched repeatedly (Lakie et al., 1984); this effect, called ‘thixotropy’, arises from the properties of both muscle and joint (Wiegner, 1987). Passive elasticity does not affect stiffness much, except at the extremes of the range of motion.