Biomechanical Consequences of Using Passive and Active Back-Support Exoskeletons during Different Manual Handling Tasks.

The aim of this study was to assess, for both men and women, the consequences of using different back-support exoskeletons during various manual material tasks (MMH) on the activity of back muscles and trunk kinematics. Fifteen men and fourteen women performed MMH involving a 15 kg load (a static task, a symmetric lifting task, and an asymmetric lifting task). Four exoskeleton conditions were tested: without equipment (CON) and with three exoskeletons passive (P-EXO), and active (A-EXO1 and A-EXO2)). The electromyographic activity of the lower trapezius (TZ), latissimus dorsi (LD), erector spinae (ES), gluteus maximus (GM), and biceps femoris (BF) muscles was recorded. Trunk kinematics were evaluated to provide average thoracic, lumbar, and hip angles. The use of the P-EXO decreased the activity of LD, GM, and BF from -12 to -27% (p < 0.01) compared to CON, mostly during the static task. The A-EXO1 and A-EXO2 reduced the muscle activity of all studied muscles from -7 to -62% (p < 0.01) compared to CON and from -10 to -52% (p < 0.005) compared to the P-EXO, independently of the modalities of the experimental tasks. A statistical interaction between the sex and exoskeleton was only observed in a few rare conditions. Occupational back-support exoskeletons can reduce trunk extensor muscle activity compared to no equipment being used. However, these reductions were modulated by the exoskeleton technology (passive vs. active), design (weight and anthropomorphism), and the modalities of the task performed (static vs. dynamic). Our results also showed that the active exoskeletons could modify the trunk kinematics.

Effectiveness of Soft versus Rigid Back-Support Exoskeletons during a Lifting Task.

This study investigated the influence of passive back-support exoskeletons (EXOBK) 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 EXOBK 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 EXOBK. 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.

Physiological consequences of using an upper limb exoskeleton during manual handling tasks.

This study aimed to assess the physiological consequences of using an upper limb exoskeleton during manual handling task, as muscle activity, upper limb kinematics, postural balance and cardiac cost. Participants performed three tasks (load lifting (LIFT), carrying (WALK) and stacking-unstacking (STACK)) with (EXOS) and without (FREE) an exoskeleton. During LIFT and STACK, the activity of the deltoid anterior muscle was significantly lower for EXOS than for FREE. During LIFT, the activity of the triceps brachii (TB) and tibialis anterior muscles significantly increased for EXO. The TB muscle activity significantly decreased for EXOS during WALK. The cardiac cost tended to increase with the use of the exoskeleton during LIFT, compared to FREE. The upper limb kinematics significantly differed between the EXOS and FREE conditions for all tasks. The benefits of the upper limb exoskeleton to reduce shoulder flexor muscle activity has been demonstrated, while broader physiological consequences have also been evidenced as increased antagonist muscle activity, postural strains, cardiovascular demand, and modified kinematics.

Cycling Versus Uphill Walking: Impact on Locomotor Muscle Fatigue and Running Exercise.

PURPOSE: To describe the effects of uphill walking versus cycling exercises on knee-extensor (KE) neuromuscular properties and subsequent running exercise. METHODS: Nine athletes performed 4 different sessions (1 familiarization and 3 experimental sessions, visit 2-4). Visit 2 (cycling +10-km condition) consisted of the completion of 1-h cycling followed by a 10-km running time trial. Visit 3 consisted of the completion of 1-h uphill walking followed by a 10-km running exercise (RE). During the fourth visit, athletes only ran 10 km. Visits 3 and 4 were randomized. The uphill walking and cycling exercises were performed at the same intensity, and pacing of the RE was similar between conditions. Neuromuscular function of the KE was assessed before warm-up, after first exercise, and after RE. Heart rate and rating of perceived exertion (RPE) were recorded during all exercises. RESULTS: RPE during RE was greater following the 1-h cycling and uphill walking exercises than during RE alone. KE force (-21%), twitch torque (-20%), doublet torque (-16%), and twitch rate of force development (-13%) significantly decreased following cycling exercise and not after uphill walking exercise. Postactivation potentiation was observed after uphill walking and RE. KE force-production capacity partially recovered after running in the cycling +10-km condition. CONCLUSION: Uphill walking and running induced postactivation potentiation, limiting the decrease in KE force postexercise. Despite different alterations in force-production capacity induced by cycling and uphill walking, both exercises increased perception of effort during the subsequent RE.

Effects of string stiffness on muscle fatigue after a simulated tennis match.

We tested the influence of string stiffness on the occurrence of forearm muscle fatigue during a tennis match. Sixteen tennis players performed two prolonged simulated tennis matches with low-stiffness or high-stiffness string. Before and immediately after exercise, muscle fatigability was evaluated on the forearm muscles during a maximal intermittent gripping task. Groundstroke ball speeds and the profile of acceleration of the racquet frame at collision were recorded during each match. The peak-to-peak amplitude of acceleration and the resonant frequency of the frame were significantly greater with high- (5060 ± 1892 m/s(2) and 204 ± 29 Hz, respectively) than with low-stiffness string (4704 ± 1671 m/s(2) and 191 ± 16 Hz, respectively). The maximal and the averaged gripping forces developed during the gripping task were significantly reduced after the tennis match with high- (-15 ± 14%, and -22 ± 14%, respectively), but not with low-stiffness string. The decrease of ball speed during the simulated matches tended to be greater with high- than with low-stiffness string (P = .06). Hence, playing tennis with high-stiffness string promotes forearm muscle fatigue development, which could partly contribute to the groundstroke ball speed decrement during the game.

Neuromuscular fatigue is greater following highly variable versus constant intensity endurance cycling.

The present study compared neuromuscular fatigue of the knee extensor muscles following highly variable versus constant power output cycling. Ten subjects performed two 33-min cycling trials of the same average power output, in a random order. Cycling exercise was performed either at constant (CST) power output, corresponding to 70% of the maximal aerobic power (MAP), or at variable (VAR) power output with alternating high (200, 150 and 100% of MAP during 10, 15 and 20 s, respectively) and moderate (50% of MAP) power output periods. Neuromuscular tests were performed before and immediately after the two trials. Heart rate (HR) was measured during exercise and blood lactate concentration ([La]) at the end of both trials. Reductions in maximal voluntary contraction torque, voluntary activation level and peak doublet were significantly greater after VAR than after CST. HR and [La] were significantly higher during VAR than during CST. Cycling at a varying power output in comparison to constant power resulted in additional muscular fatigue that may be explained by greater anaerobic contribution and muscle solicitation during the highly variable power output protocol.

Differences in cardiorespiratory and neuromuscular responses between voluntary and stimulated contractions of the quadriceps femoris muscle.

The aim of this study was to compare respiratory gas exchange variables and muscle fatigue between equal-intensity (i.e., same force output) electrostimulated and voluntary contractions of the quadriceps muscle (46+/-10% of maximal voluntary force). Twelve healthy men served as volunteers. Oxygen consumption, ventilation and respiratory exchange ratio were recorded during the exercise bouts. Muscle fatigue was quantified as the exercise-induced reduction in maximal voluntary force. The average oxygen consumption (11+/-3 versus 8+/-2 mL min(-1)kg(-1)), ventilation (23+/-4 versus 16+/-2 L min(-1)) and respiratory exchange ratio (0.96+/-0.02 versus 0.85+/-0.01) were significantly higher during electrostimulation compared with voluntary exercise (P<0.05-0.001). Maximal voluntary force decreased significantly after electrostimulation (-21+/-10%; P<0.001), while no changes were observed following voluntary exercise. Electrostimulation-resistance exercise of the quadriceps muscle elicited greater cardiorespiratory demand and muscle fatigue compared with voluntary contractions of the same intensity. These findings probably reflect differences in the patterns of motor unit recruitment between stimulated and voluntary contractions, despite equal force productions.