Ageing of human muscles and tendons.

At whole muscle level, the reduction in intrinsic force observed with ageing is probably the result of the combined effect of changes in: (i) muscle architecture, (ii) tendon mechanical properties, (iii) neural drive (reduced agonist and increased antagonist muscles' activity), and (iv) single fibre specific tension. Only recently have alterations in muscle architecture and in tendon mechanical properties been shown to contribute to the reduction in intrinsic muscle force, and tendon stiffness changes play an important role. Of note is the fact that most of these changes may be reversed by 14 weeks of resistive training, for both fibre fascicle length and tendon stiffness were found to be increased by 10% and 64%, respectively. Surprisingly, however, training had no effect on the estimated relative length-tension properties of the muscle, indicating that the effects of increased tendon stiffness and increased fascicle length cancelled out each other. It seems that natural strategies may be in place to ensure that the relative operating range of muscle remains unaltered by changes in physical activity, and perhaps age.