Force depression following muscle shortening in sub-maximal voluntary contractions of human adductor pollicis

Example of average (full-wave rectified and smoothed) EMG (AEMG (A), raw EMG (B), thumb adduction force (C), and metacarpophalangeal joint angle (D) as a function of time during 30% of maximal voluntary contractions (MVC) of human adductor pollicis for an isometric reference and an isometric-30° shortening-isometric test contraction. Subjects were asked to control activation (activation control) at 30% MVC (see Methods). Isometric reference force (F Iso ref) was measured for a 2-s period in the final, steady-state part (labelled by the horizontal dotted arrow in C) for comparison with the corresponding values obtained from the isometric contractions following active shortening (F Iso short). Contractions were maintained for a minimum of 4s following the shortening phase, to ensure that all transient force had subsided and a steady-state force had been reached. The difference between the isometric reference and shortening test force was defined as force depression. AEMG Iso ref, isometric reference average EMG; AEMG Iso short, isometric shortening average EMG; Raw EMG ref, reference raw EMG; Raw EMG short, isometric shortening raw EMG.

Example of thumb adduction force (A), raw (B), AEMG (C), and metacarpophalangeal joint angle (D) as a function of time during 30% of maximal voluntary contractions of human adductor pollicis for an isometric reference and an isometric-30° shortening-isometric test contraction. Muscle force was kept constant (force control) throughout the contractions (see Methods). Isometric reference EMG (AEMG Iso ref) was measured for a 2-s period in the final, steady-state part (labelled by the horizontal dotted arrow in C) for comparison with the corresponding values obtained from the isometric contractions following active shortening (AEMG Iso short). Contractions were maintained for a minimum of 4s following the shortening phase, to ensure that all transient EMG responses had subsided and a steady-state EMG response had been reached. The difference in activation between the isometric reference and shortening test force was defined as EMG increase. F Iso ref, isometric reference force; F Iso short, isometric force after shortening; Raw EMG ref, reference raw EMG; Raw EMG short, isometric shortening raw EMG.

Mean (SE) coefficients of variation of average EMG (white bars) and force (grey bars) in the activation control and the force control protocols of test 1 . Note that the variability of force and average EMG are similar (∼10%) for the activation and force control tests, respectively, while force can be controlled tighter than average EMG in the force control compared to the activation control protocol.

Mean (SE) isometric sub-maximal thumb adduction force (A) in the isometric reference and the 30° shortening test contractions , and mean (SE) average EMG (B) for isometric reference and 30° shortening test contractions . Force is significantly smaller in the shortening test contractions compared to the isometric reference contractions when activation is controlled, indicating that there is a force depression during sub-maximal voluntary contractions (*p<0.05). Conversely, average EMG is significantly greater for the test compared to the isometric reference contractions (*p<0.05) in the tests with force control.

Mean (SE) force depression for 10° (white bar), 20° (grey bar), and 30° (black bar) shortening amplitudes, for activation controlled testing . Force depression was significantly increased when shortening amplitude increased from 10° to 30° and from 20° to 30° (*p<0.05). The difference between 10° and 20° was not statistically significant.