Mechanical properties of human ankle extensors after muscle potentiation

The contractile properties of a muscle depend on the activation history of its motor units. At the same time as fatigue seems to impair muscle excitation-contraction coupling, post-tetanic potentiation can augment force production. The effects of post-tetanic potentiation on the mechanical muscle properties of the intact human ankle extensor muscles were investigated by a 4 degree dorsiflexion of the ankle joint during a sustained contraction. The contraction was elicited by 10 Hz electrical stimulation of the tibial nerve. The changes in the contraction torque and in the intrinsic muscle stiffness of the ankle extensors before and after prolonged electrically elicited muscle activation were measured. From the onset of continuous synchronized 10 Hz stimulation to the attainment of maximal torque, the ankle joint torque increased by 47%. At matched background contraction, the prolonged electrically elicited contraction increased the intrinsic muscle stiffness by 49%. The first stretch after prolonged stimulation gave rise to a 17% yield in the background contraction and a 73% yield in the torque increment. The findings imply that with fatigue an increase in the intrinsic stiffness of the pre-stretched muscle might operate as a "safety factor" to compensate for a reduced reflex-induced stiffness, keeping the total muscle resistance at a high level in the active muscle.