Motor output relative error during static linear maximal torque ramp: influence of the age

Aim To evaluate if the torque output relative error during linearly varying static contractions can be influenced by possible changes in motor unit properties and activation/deactivation strategies (MUAS and MUDS) due to the ageing process. Method Eleven young (Y, 23.90 ± 3.72 years) and eleven old (O, 69.63 ± 4.34 years) subjects were recruited for the study. After the first dorsal interosseous (FDI) maximal voluntary contraction (MVC) during static abduction, the subject performed one triangular isometric contraction (0-100-0 %). The rate of the up-going (UGR) and down-going ramp (DGR) was 13.3% MVC/s. The requested output tension (% MVC target) was provided on a PC screen together with the force from the subject for the necessary visual feedback. The global, positive and negative relative errors (E%) were calculated for the whole triangle and for the up going and down-gong phases separately according to the formula E%= absolute value of (exerted - requested target force)/requested force*100. The filtered force signal (bandwidth: 0-20 Hz) was analyzed using the Spike Shape Analysis (SSA) technique. Statistical significance of the calculated parameters between Y and O was set at p<0.05. Results As reported in the table the calculated E% were always greater in old subjects. The SSA reported lower number of longer spikes, with similar absolute amplitude, in O with respect to Y subjects. When the spike amplitude is scaled to the individual MVC the spikes of O subjects are greater than in Y. Conclusion MUAS and MUDA adopted by Y and O subjects determine a different a more precise motor output in Y than in O. The key factors may be: I. the larger number of error correction (reflected in the spike number) and II. the larger motor unit size due to the muscle fibers reinnervation process with age (reflecting the larger relative spike amplitude in O compared to Y).