Measurements of muscle stiffness, the electromyogram and activity in single muscle spindles of human flexor muscles following conditioning by passive stretch or contraction.

In experiments on adult human subjects we examined the effect on passive mechanical properties of a muscle by conditioning it with either an isometric contraction or passive muscle extension. The test measurement was the amount of muscle displacement (stiffness) and the accompanying EMG in response to a brief torque pulse. Two muscles were tested, flexor digitorum profundus (FDP) and brachialis. In FDP the discharge of single muscle spindles was recorded as well. After muscle extension and return to the initial length, passive stiffness was less than after an isometric contraction. The changes in stiffness were accompanied by changes in pattern of EMG and in the responses of muscle spindles. It is suggested that in resting muscle there are stable cross bridges between actin and myosin filaments of muscle fibres which largely determine the passive stiffness. Muscle extension leads to detachment of these cross bridges which then re-form at the longer length. Return of the muscle to its starting length leads to development of slack in muscle fibres because, stiffened by the presence of the stable cross bridges, they are unable to shorten. Slack in muscle fibres lowers their measured stiffness. Muscle contraction, on the other hand, will result in any preexisting slack being taken up by the actively shortening muscle fibres, thereby raising muscle stiffness. Stiffness in intrafusal fibres is likely to follow a similar pattern to that in extrafusal fibres, leading to changes in stretch responsiveness of muscle spindles and consequently in the reflex EMG. It is concluded that the changes in stiffness and accompanying reflexes observed in this study are likely to be seen, at least under some conditions, in normal movements.