Age-related muscle activation profiles and joint stiffness regulation in repetitive hopping

It is well documented that increasing effort during exercise is characterized by an increase in electromyographic activity of the relevant muscles. How aging influences this relationship is a matter of great interest. In the present study, nine young and 24 elderly subjects did repetitive hopping with maximal effort as well as with 50%, 65%, 75% and 90% intensities. During hopping joint kinematics were measured together with electromyographic activity (EMG) from the soleus, gastrocnemius medialis, gastrocnemius lateralis and tibialis anterior muscles. The results showed that agonist activation increased in both age groups with increasing intensity. The highest jumping efficiency (EMG ratio of the braking phase to the push off-phase activation) was achieved with moderate hopping intensities (65-75%) in both the young and in the elderly. Age-comparison showed that elderly subjects had high agonist preactivation but thereafter lower activation during the braking phase. Antagonist coactivation was minimal and did not show age- or intensity-specificity. The elderly had more flexed knees at the instant of ground contact. When intensity increased, the elderly also plantarflexed their ankles more before ground contact. Ankle joint stiffness was lower in elderly subjects only in high hopping intensities (90% and Max). These results confirm that age-specific agonist muscle activation profiles exist during hopping even when exercise intensities are matched on the relative scale. The results suggest further that the elderly can adjust their reduced neuromuscular capacity to match the demands set by different exercise intensities.