Prolonged low-frequency force depression recovery response following an isometric triceps extension to volitional...

To investigate the recovery of prolonged low-frequency force depression (PLFFD) within the first hour post-volitional fatigue following a sustained isometric triceps contraction performed at high (70 % of maximum voluntary force (MVF)) and low (20 % of MVF) intensities.

High threshold and low threshold motor units contribute differently to force generation capability and fatigue at different rates. However, we know little about how high and low threshold motor units recover following fatiguing contractile activity.

Fourteen males and fourteen females sustained an isometric elbow extensor contraction until volitional fatigue at high (70 % MVF) and low (20 % MVF) intensities. The ratio of force generated by transcutaneous muscle stimulation of the triceps at 20 Hz and 100 Hz frequencies was measured at baseline and intermittently for 1 h post fatigue to characterize PLFFD.

The 20/100 Hz force ratio was initially (0-4 min) significantly greater (meaning less force depression) following the high intensity protocol. However, the 20/100 Hz ratio for both intensities was similar for the duration of recovery thereafter.

Initial differences in the 20/100 Hz ratio within the first 0-4 min are likely explained by post-activation potentiation following the higher intensity submaximal contraction. However, the similarity in the recovery of 20/100 Hz ratio thereafter supports the hypothesis that once PLFFD is present, the recovery rate may be independent from how fatigue and corresponding PLFFD developed.

The practical impact is that recovery of prolonged frequency fatigue may not depend on the intensity of the contraction that caused the fatigue.