Strength training alters the viscoelastic properties of tendons in elderly humans

The effect of strength training for 14 weeks on patella tendon viscoelastic properties was investigated in a group of elderly individuals. Participants were assigned to training (age [mean +/- SD] 73.6 +/- 3.4 years; n = 7) or control (age 66.4 +/- 1.7 years; n = 7) groups. Training was performed three times per week and consisted of two series of 10 repetitions of leg-extension and leg-press exercises at 80% of the 5-repetition maximum. Tendon elongation during an isometric knee-extension contraction-relaxation was measured using ultrasonography. Tendon stiffness was calculated from the gradient of the estimated force-elongation relationship and mechanical hysteresis was calculated as the area between loading-unloading curves. Knee-flexor coactivation, estimated from biceps femoris muscle electromyographic activity, was unaltered (P > 0.05) after the training and control periods. No changes (P > 0.05) were observed in stiffness or hysteresis after the control period. In contrast, tendon stiffness increased from 1376 +/- 811 to 2256 +/- 1476 N x mm(-1) (P < 0.01) and hysteresis decreased from 33 +/- 5 to 24 +/- 4% (P < 0.05), after training. These training-induced adaptations have implications for maximal muscle force, rate of force development, and metabolic cost of locomotion.