Figure 1. An illustration of a possible mechanism leading to changes in force deficit under fatigue.

Total force (shown by the horizontal bars) is produced by small motor units (MUs; F₁), medium-size MUs (F₂) and large MUs (F₃). Force deficit during four-finger tests prior to fatigue (D_pre) is due to the lack of recruitment of the largest MUs (cf. the upper and lower left drawings). Fatigue leads to derecruitment of medium-size MUs (upper right drawing). Absolute force deficit is expected to be constant, whereas relative force deficit is expected to increase under fatigue (D_post; lower right drawing).