Reduced corticospinal responses in older compared to younger adults during submaximal isometric, shortening and lengthening contractions.

The aim of this study was to assess differences in motor performance, as well as corticospinal and spinal responses to transcranial magnetic and percutaneous nerve stimulation, respectively, during submaximal isometric, shortening and lengthening contractions between younger and older adults. Fifteen younger (26 ± 4 yrs, 7 females) and 14 older (64 ± 3 yrs, 5 females) adults performed isometric, and shortening and lengthening dorsiflexion on an isokinetic dynamometer (5°·s-1 ) at 25 and 50% of contraction type specific maximums. Motor evoked potentials (MEPs) and H-reflexes were recorded at anatomical zero. Maximal dorsiflexor torque was greater during lengthening compared to shortening and isometric contractions (p<0.001), but were not age-dependent (p=0.158). However, torque variability was greater in older compared to young (p<0.001). Background electromyographic (EMG) activity was greater in older compared to younger individuals (p<0.005) and was contraction type dependent (p<0.001). As evoked responses are influenced both by the maximal level of excitation and background EMG activity, the responses were additionally normalised ([MEP/Mmax ]/RMS and [H/Mmax ]/RMS). The (MEP/Mmax )/RMS and (H/Mmax)/RMS were similar across contraction types, but were greater in young compared to older adults (p<0.001). Peripheral motor conduction times were prolonged in older adults (p=0.003), whilst peripheral sensory conduction times and central motor conduction times were not age-dependent (p≥0.356). These data suggest that age-related changes throughout the central nervous system serve to accommodate contraction type specific motor control. Moreover, a reduction in corticospinal responses and increased torque variability seem to occur without a significant reduction in maximal torque producing capacity during older age.