Force enhancement in the human vastus lateralis is muscle-length-dependent following stretch but not during stretch

Purpose Force enhancement is the phenomenon of increased forces during (transient force enhancement; tFE) and after (residual force enhancement; rFE) eccentric muscle actions compared with fixed-end contractions. Although tFE and rFE have been observed at short and long muscle lengths, whether both are length-dependent remains unclear in vivo. Methods We determined maximal-effort vastus lateralis (VL) force-angle relationships of eleven healthy males and selected one knee joint angle at a short and long muscle lengths where VL produced approximately the same force (85% of maximum). We then examined tFE and rFE at these two lengths during and following the same amount of knee joint rotation. Results We found tFE at both short (11.7%,P = 0.017) and long (15.2%,P = 0.001) muscle lengths. rFE was only observed at the long (10.6%,P < 0.001; short: 1.3%,P = 0.439) muscle length. Ultrasound imaging revealed that VL muscle fascicle stretch magnitude was greater at long compared with short muscle lengths (mean difference: (tFE) 1.7 mm, (rFE) 1.9 mm,P <= 0.046), despite similar isometric VL forces across lengths (P >= 0.923). Greater fascicle stretch magnitude was likely to be due to greater preload forces at the long compared with short muscle length (P <= 0.001). Conclusion At a similar isometric VL force capacity, tFE was not muscle-length-dependent at the lengths we tested, whereas rFE was greater at longer muscle length. We speculate that the in vivo mechanical factors affecting tFE and rFE are different and that greater stretch of a passive component is likely contributing more to rFE at longer muscle lengths.