Neuromuscular changes of the aged human hamstrings

Eric A. Kirk, Kevin J. Gilmore, Charles L. Rice

Despite the life-long importance for posture and locomotion, neuromuscular properties of the hamstrings muscle have not been explored with adult ageing. The purpose was to assess and compare age-related effects on contractile function, spinal motor neuron output expressed as motor unit (MU) discharge rates in the hamstrings of 11 young (26 {plus minus} 4 y) and 10 old (80 {plus minus} 5 y) men. Maximal voluntary isometric contractions (MVC), stimulated contractile properties, surface and intramuscular electromyography (EMG) from sub-maximal to MVC were recorded in the biceps femoris (BF) and semimembranosus-semitendinosus (SS) muscles. MVC torque was ~50% less in the old with both age groups attaining {greater than or equal to}93% mean voluntary activation. Evoked twitches in the old were ~50% lower in amplitude and >150% longer in duration as compared to young. At successive voluntary contractions of 25, 50 and 100% MVC, MU discharge rates were up to 45% lower in old, with no differences in relative submaximal surface EMG between age groups. Furthermore, the old had significantly lower MU discharge rates in the SS at all contraction intensities compared to the BF muscle. Men in their 8th to 10th decades of life demonstrate substantially lower strength and MU discharge rates in this functionally important large lower limb muscle group, with greater age-related effect on discharge rates in the medial hamstrings. These findings in comparison to other muscles studied highlight that the neuromuscular properties of limb muscles, and indeed within functionally similar portions of a muscle group, are not all affected equally by the ageing process.