Residual force depression is not related to positive muscle fascicle work during submaximal voluntary dorsiflexion contractions in humans

Residual force depression (rFD) following active muscle shortening is commonly assumed to strongly and linearly increase with increasing muscle work, but this has not been systematically tested during voluntary contractions in humans. Using dynamometry, we compared steady-state ankle joint torques ( N =16) following tibialis anterior (TA) muscle-tendon unit (MTU) lengthening and shortening to the torque during submaximal voluntary fixed-end dorsiflexion reference contractions (REF) at a matched MTU length and EMG amplitude. B-mode ultrasound revealed that TA fascicle shortening amplitudes were significantly reduced ( p <0.001) during MTU lengthening with no preload over small (LEN small ) and medium (LEN medium ) amplitudes, respectively, relative to REF. MTU lengthening with a preload over a large (LEN largeP ) amplitude significantly ( p <0.001) increased fascicle shortening relative to REF, as well as stretch amplitudes relative to LEN small and LEN medium ( p ≤0.001), but the significant ( p ≤0.028) steady-state fascicle force enhancement relative to REF was similar to LEN small and LEN medium (3-5%). MTU shortening with and without a preload over small (SHO smallP /SHO small ) and large (SHO largeP/ SHO large ) amplitudes significantly ( p <0.001) increased positive fascicle and MTU work relative to REF, but significant ( p ≤0.006) rFD was observed in SHO smallP and SHO largeP (7-10%) only. rFD was linearly related to positive MTU work ( r rm (47)=0.48, p <0.001), but not positive fascicle work ( r rm (47)=0.16, p =0.277). Our findings indicate that MTU lengthening without substantial fascicle stretch enhances steady-state force output, which might be due to less shortening-induced rFD. Our findings also indicate that different amounts of positive fascicle and MTU work induce similar rFD, which cautions against using work to predict rFD during submaximal voluntary contractions.