This study aimed to investigate mechanisms of neuromuscular fatigue during maximal concentric and isometric leg extensions inducing similar torque decrements. Nine physically active men performed two separate fatiguing sessions maintained until similar torque decreases were obtained. The first session, only conducted under isokinetic concentric conditions (CON), consisted of three series of 30 maximal voluntary concentric knee extensions (60°/s). The second session, exclusively isometric (ISO), mimicked the torque decreases registered during the CON session while performing three long-lasting ISO contractions. Maximal voluntary torque, activation level (twitch interpolation technique), electromyographic activity (root mean square and median frequency) of the vastus lateralis muscle, and electrically evoked doublet-twitch mechanical properties were measured before and at the end of each of the three series. After the three series, similar torque decrements were obtained for both fatiguing procedures. The total fatiguing contraction durations were not different among procedures. With equivalent voluntary torque decrements, the doublet-twitch amplitude reduction was significantly greater ( P < 0.01) during the two first series of the CON procedure compared with ISO. No difference was observed for the third series. Although no difference was recorded with fatigue for median frequency changes between CON and ISO, activation levels and root mean square values demonstrated greater reductions ( P < 0.05) for all three series during the ISO procedure compared with CON. Performing CON or ISO fatiguing exercises demonstrated different fatigue origins. With CON exercises, peripheral fatigue developed first, followed by central fatigue, whereas with ISO exercises the fatigue pattern was inverted.

Cises: FS is the fatigue carried out at short muscle length (S) (S = 40° of knee flexion) and FL is the fatigue at long muscle length (L) (L = 100°). Before and after each fatiguing exercise (i.e., three maximal isometric contractions maintained until 80, 60, and 40% of the initial maximal torque, respectively), activation level (AL, assessed by means of twitch interpolation technique), EMG activity (RMS), and peak doublet torque (Pd) were measured at the two lengths (S and L). Results: First, AL was greater (P < 0.05) at L compared with S before and after both exercises. Second, despite a similar decrease in maximal voluntary torque (~21% of the initial value) after the two exercises, AL and RMS were significantly reduced after FS (P < 0.05) but remained unchanged after FL, whereas the Pd decrease was more pronounced after FL than FS (P < 0.05). Nevertheless, after a given fatiguing exercise (i.e., FS or FL), AL, RMS, and Pd changes were similar at both postexercise test lengths (S and L). Conclusion: These results clearly demonstrate that i) the neural activation dependence on quadriceps muscle length is maintained with fatigue, and ii) neuromuscular fatigue after maximal isometric contractions is dependent on the muscle length at which the exercise is performed: short length preferentially induces neural activation impairment, whereas long length leads to higher contractile failure....

This study investigated the influence of passive back-support exoskeletons (EXO_BK) design, trunk sagittal inclination (TSI), and gender on the effectiveness of an exoskeleton to limit erector spinae muscle (ES) activation during a sagittal lifting/lowering task. Twenty-nine volunteers performed an experimental dynamic task with two exoskeletons (two different designs: soft (SUIT) and rigid (SKEL)), and without equipment (FREE). The ES activity was analyzed for eight parts of TSI, each corresponding to 25% of the range of motion (lifting: P1 to P4; lowering: P5 to P8). The impact of EXO_BK on ES activity depended on the interaction between exoskeleton design and TSI. With SKEL, ES muscle activity significantly increased for P8 (+36.8%) and tended to decrease for P3 (−7.2%, p = 0.06), compared to FREE. SUIT resulted in lower ES muscle activity for P2 (−9.6%), P3 (−8.7%, p = 0.06), and P7 (−11.1%), in comparison with FREE. Gender did not influence the effect of either back-support exoskeletons on ES muscle activity. These results point to the need for particular attention with regard to (1) exoskeleton design (rigid versus soft) and to (2) the range of trunk motion, when selecting an EXO_BK. In practice, the choice of a passive back-support exoskeleton, between rigid and soft design, requires an evaluation of human-exoskeleton interaction in real task conditions. The characterization of trunk kinematics and ranges of motion appears essential to identify the benefits and the negative effects to take into account with each exoskeleton design.