For electrically stimulated muscles, it has been observed that maximal muscle force during and after stretch is substantially greater than the corresponding isometric force. However, this observation has not been made for human voluntary contractions. We investigated the effects of active muscle stretch on muscle force production for in vivo human adductor pollicis (n= 12) during maximal voluntary contractions and electrically induced contractions. Peak forces during stretch, steady‐state isometric forces following stretch, and passive forces following muscle deactivation were compared to the corresponding isometric forces obtained at optimal muscle length. Contractions with different stretch magnitudes (10, 20, and 30 deg at a constant speed of 10 deg s−1) and different speeds (10, 20, and 60 deg s−1 over a range of 30 deg) were performed in triplicate in a random order, balanced design. We found three novel results: (i) there was steady‐state force enhancement following stretch in voluntarily contracted muscles; (ii) some force enhancement persisted following relaxation of the muscle and (iii) force enhancement, for some stretch conditions, exceeded the maximum isometric force at optimal muscle length. We conclude from these results that voluntary muscle contraction produces similar force enhancement to that observed in the past with electrically stimulated preparations. Therefore, steady‐state force enhancement may play a role in everyday movements. Furthermore, these results suggest that non‐uniformities in sarcomere length do not, at least not exclusively, account for the force enhancement following active muscle stretch, and that the stretch magnitude‐dependent passive force enhancement observed here may be responsible for the enhancement of force above the isometric reference force at optimal muscle length.

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The purpose of this study was to investigate steady-state force depression following active muscle shortening in human adductor pollicis during voluntary and electrically induced contractions. Subjects (n = 12; age 28 +/- 5 years; 7 males and 5 females) performed isometric reference contractions and isometric-shortening-isometric contractions, using maximal voluntary effort and near-maximal electrical stimulation. Force depression was assessed by comparing the steady-state isometric forces produced following active muscle shortening with the purely isometric reference forces obtained at the corresponding muscle length. In order to test for effects of the shortening conditions on the steady-state force depression, the amplitude and speed of shortening were changed systematically in a random order but balanced design. Thumb adduction force and carpometacarpal joint angle were continuously measured using a custom-designed dynamometer. During voluntary contractions, muscle activation was recorded using electromyography and the superimposed twitch technique. During electrically induced contractions, muscle stiffness was assessed using a quick-stretch method. Force depression during voluntary contractions, with a constant level of muscle activation, was similar to that obtained during electrically induced contractions. Force depression increased with increasing amplitudes of shortening (9.9 +/- 1.6%, 15.6 +/- 2.4% and 22.4 +/- 2.4% for 10, 20 and 30 deg of shortening, respectively) and decreased with increasing speeds of shortening (27.1 +/- 2.5%, 19.3 +/- 1.6% and 15.6 +/- 1.8% for 20, 60 and 300 deg s(-1) of shortening, respectively), regardless of the activation method. Muscle stiffness was significantly lower in the force-depressed state (5.9 +/- 0.2 N deg(-1)) compared with that of the isometric reference contractions (7.2 +/- 0.3 N deg(-1)), and decreased with increasing force depression (6.6 +/- 0.5, 6.0 +/- 0.5 and 5.3 +/- 0.4 N deg(-1) for the 10, 20 and 30 deg of shortening test contractions, respectively). Force depression appeared to be fully established at the end of the shortening phase. The results of this study suggest that steady-state force depression for voluntary movements is similar to that observed using electrical stimulation. Furthermore, it appears that force depression is established at the end of the shortening phase and is associated with a reduction in muscle stiffness and thus, presumably, a decrease in the proportion of attached cross-bridges.

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The purpose of this study was to investigate whether the isometric muscle force, redeveloped following maximal-effort voluntary shortening contractions in human skeletal muscle, is smaller than the purely isometric muscle force at the corresponding length. Isometric knee extensor moments, surface electromyographic (EMG) signals of quadriceps femoris, and interpolated twitch moments (ITMs) were measured while 10 subjects performed purely isometric knee extensor contractions at a 60 degrees knee angle and isometric knee extensor contractions at a 60 degrees knee angle preceded by maximal-effort voluntary shortening of the quadriceps muscles. It was found that the knee extensor moments were significantly decreased for the isometric-shortening-isometric contractions compared with the isometric contractions for the group as a whole, whereas the corresponding EMG and ITM values were the same. This study is the first to demonstrate force depression following muscle shortening for voluntary contractions. We concluded that force depression following muscle shortening is an actual property of skeletal muscle rather than a stimulation artifact and that force depression during voluntary contraction is not accompanied by systematic changes in muscle activation as evaluated by EMG and ITM.