Older Adults exhibit Greater Positional Variability during Ankle Dorsiflexion than Index Finger Abduction

Abstract

The impaired ability of older adults to control force and position during low intensity contractions has been extensively demonstrated. Nonetheless, it is not clear whether these age-associated differences get influenced by the limb and load lifted during positional tasks. PURPOSE: To determine the interaction of age and limb on motor output variability during positional tasks. METHODS: Eleven young adults (25.6 ± 4.9 yrs, 7 women and 4 men) and ten older adults (76.7 ± 5.6 yrs, 6 women and 4 men) were asked to accurately match and maintain a horizontal target line with 5° abduction of their index finger and 5° dorsiflexion of their ankle. The loads were 2% (5% for the leg), 15%, 30%, and 50% of the maximal load (1 RM) that could be lifted with each limb. Each trial lasted 20 s and was performed 3 times at each condition. The visual gain was kept constant at 1° (visual angle) for all trials. Positional variability was quantified as the standard deviation (SD) from the detrended position trace of each limb. The neural activation of the involved agonist muscles was quantified as the EMG amplitude and modulation of the first dorsal interosseus (index finger abduction) and tibialis anterior (dorsiflexion) muscles. RESULTS: The heaviest load (50% 1 RM) significantly increased (P = 0.019) positional variability for both limbs compared with the lighter loads (2-30% MVC). Although positional variability was greater in older adults for both limbs (P = 0.004), age differences were significantly greater for the dorsiflexion task (young: 0.064 ° ± 0.011 vs. older: 0.143 ° ± 0.013) compared with the abduction of the index finger (young: 0.048 ° ± 0.003 vs. older: 0.082 ° ± 0.009) task (age × limb interaction; P = 0.012). The amplified positional variability in older adults was related to differential agonist modulation. Furthermore, the exacerbated positional variability during dorsiflexion in older adults was related to their inability to modulate power from 30-60 Hz in the tibialis anterior muscle. CONCLUSIONS: These results provide novel evidence that older adults may exhibit greater impairments in motor control with the leg compared with the hand because of their inability to modulate agonist muscle activity from 30-60 Hz. Supported by NIH Grant R01 AG031769 to EA Christou.