Influence of age on motor control accuracy during static ramp contractions.

We investigated the influence of the ageing process on the performance of the motor control system accuracy during a challenging motor task throughout the analysis of force output oscillations. The force signal of the first dorsal interosseous during linearly varying static contraction, 0-100-0% of the maximal volitional abduction in 15 s, was studied in 11 young and older adults. The relative error between the target and the actual force as well as several parameters of the force oscillations (corrections) were estimated. To understand the experimental results, we analyzed the force output generated by a set of computational simulations of a pool of motor units controlled by a proportional-integral-derivative system. Compared to young adults the older subjects presented larger errors and a lower number of corrections with longer duration and larger relative amplitude. The motor control system modelling varied the error update frequency (UF) of the controller (from 1 to 2.5 Hz) as well as the range of contraction time (CT) of the recruited motor unit (30-90 ms and 60-120 ms reflecting young and old ranges, respectively). The simulation generated force profiles with parameters similar to experimental recordings in young (UF = 1.5; CT 30-90 ms) and older (UF = 1; CT 60-120 ms) adults. Interestingly, the results of the simulations suggested that the improvement in the error update frequency of the controller was not able to compensate for the contractile changes in the motor unit twitches. In conclusion, the peripheral contractile changes with age can influence motor unit control strategies and represent a crucial phenomenon in the generation of larger force oscillations in older adults.