The difference in stiffness of the active plantarflexors between young and elderly human females.

The purpose of this investigation was to determine the difference in the intrinsic stiffness of the active plantarflexor muscles between young and elderly human females, and to determine changes in this stiffness associated with a 6-week strengthening exercise program. Intrinsic muscle stiffness was determined by measuring the resistance in the first 62 milliseconds offered by the muscle to multiple single stretch trials at 20% to 60% of maximal isometric contraction. Stiffnesses were related to pre-stretch torque (K/PST) by a linear fit. The K/PST relationship of the elderly subjects exhibited a steeper slope, indicating a greater increase in stiffness with an increase in pre-stretch torque. The effect of testing subjects at identical relative, but different absolute pre-stretch torque levels precluded definitive conclusions on possible stiffness differences between the age groups. After the strengthening exercise program, the plantarflexors showed an increase in maximum produced torque, and a decrease in stiffness as well as a decrease in the slope of the K/PST relationship.