Using passive or active back-support exoskeletons during a repetitive lifting task: influence on cardiorespiratory parameters.

The objective of this laboratory study was to assess the cardiorespiratory consequences related to the use of different back-support exoskeletons during a repetitive lifting task. Fourteen women and thirteen men performed a dynamic stoop lifting task involving full flexion/extension of the trunk in the sagittal plane. This task was repeated for 5 min with a 10 kg load to handle. Four conditions were tested: with a passive exoskeleton (P-EXO), with two active exoskeletons (A-EXO1 and A-EXO2), as well as without exoskeleton (FREE). The oxygen consumption rate and cardiac costs were measured continuously. Results showed a significantly lower (p < 0.05) oxygen consumption rate for all exoskeletons as compared to FREE (12.6 ± 2.2 ml/kg/min). The values were also significantly lower (p < 0.001) for A-EXO1 (9.1 ± 1.8 ml/kg/min) compared to A-EXO2 (11.0 ± 1.8 ml/kg/min) and P-EXO (11.8 ± 2.4 ml/kg/min). Compared to FREE (59.7 ± 12.9 bpm), the cardiac cost was significantly reduced (p < 0.001) only for A-EXO1 (45.1 ± 11.5 bpm). Several factors can explain these differences on the cardiorespiratory parameters observed between exoskeletons: the technology used (passive vs active), the torque provided by the assistive device, the weight of the system, but also the level of anthropomorphism (related to the number of joints used by the exoskeleton). Our results also highlighted the lack of interaction between the exoskeleton and sex. Thereby, the three back-support exoskeletons tested appeared to reduce the overall physical workload associated with a repetitive lifting task both for men and women.

Evaluation of two upper-limb exoskeletons during overhead work: influence of exoskeleton design and load on muscular adaptations and balance regulation.

PURPOSE: Overhead works (OHW) are identified as a major risk factor for shoulder musculoskeletal disorders. The use of upper-limb exoskeletons (EXOUL) is emerging to address these challenges. This research tested the influence of EXOUL design and load on the upper-limb and postural muscles activity, and on the balance control, during OHW. METHODS: This study compared two passive EXOUL, notably differing by the level of assistive torque delivered. Both EXOUL was examined in two load conditions (2 vs. 8 kg). Twenty-nine volunteers performed a static OHW for each condition. RESULTS: Both EXOUL led to similar reductions in shoulder flexor muscle activity (12.3 ± 7.8% of RMSREF), compared to without equipment (29.0 ± 14.2% RMSREF). Both EXOUL resulted in a reduction in the activity of shoulder (3.6 ± 3.2% RMSREF) and wrist (2.4 ± 1.7% RMSREF) extensor muscles (4.9 ± 3.9 and 5.9 ± 6.1% RMSREF, respectively). The use of EXOUL led to reductions in back muscle activity, depending on the exoskeleton design (in % RMSREF, 12.9 ± 9.4 for EXO1, 22.8 ± 12.6 for EXO2 and 32.0 ± 18.4 without equipment). Wearing EXOUL induced changes in balance regulation, depending on both exoskeleton design and load condition. CONCLUSION: The increase of assistive torque was not associated with an increase in EXOUL performance. However, the exoskeleton design (mass, balance, and assistive torque) has to be suitable for the load handled during static OHW to optimize the effects of using an EXOUL on the postural muscles.

Neuromuscular fatigue following isometric contractions with similar torque time integral.

Torque time integral (TTI) is the combination of intensity and duration of a contraction. The aim of this study was to compare neuromuscular alterations following different isometric sub-maximal contractions of the knee extensor muscles but with similar TTI. Sixteen participants performed 3 sustained contractions at different intensities (25%, 50%, and 75% of Maximal Voluntary Contraction (MVC) torque) with different durations (68.5±33.4 s, 35.1±16.8 s and 24.8±12.9 s, respectively) but similar TTI value. MVC torque, maximal voluntary activation level (VAL), M-wave characteristics and potentiated doublet amplitude were assessed before and immediately after the sustained contractions. EMG activity of the vastus lateralis (VL) and -rectus femoris (RF) muscles was recorded during the sustained contractions. MVC torque reduction was similar in the 3 conditions after the exercise (-23.4±2.7%). VAL decreased significantly in a similar extent (-3.1±1.3%) after the 3 sustained contractions. Potentiated doublet amplitude was similarly reduced in the 3 conditions (-19.7±1.5%), but VL and RF M-wave amplitudes remained unchanged. EMG activity of VL and RF muscles increased in the same extent during the 3 contractions (VL: 54.5±40.4%; RF: 53.1±48.7%). These results suggest that central and peripheral alterations accounting for muscle fatigue are similar following isometric contractions with similar TTI. TTI should be considered in the exploration of muscle fatigue during sustained isometric contractions.

Effects of different pedalling techniques on muscle fatigue and mechanical efficiency during prolonged cycling.

The present study aimed to test the influence of the pedalling technique on the occurrence of muscular fatigue and on the energetic demand during prolonged constant-load cycling exercise. Subjects performed two prolonged (45 min) cycling sessions at constant intensity (75% of maximal aerobic power). In a random order, participants cycled either with their preferred technique (PT) during one session or were helped by a visual force-feedback to modify their pedalling pattern during the other one (FB). Index of pedalling effectiveness was significantly (P<0.05) improved during FB (41.4 ± 5.5%); compared with PT (36.6 ± 4.1%). Prolonged cycling induced a significant reduction of maximal power output, which was greater after PT (-15 ± 9%) than after FB (-7 ± 12%). During steady-state FB, vastus lateralis muscle activity was significantly (P<0.05) reduced, whereas biceps femoris muscles activities increased compared with PT. Gross efficiency (GE) did not significantly differ between the two sessions, except during the first 15 min of exercise (FB: 19.0 ± 1.9% vs PT: 20.2 ± 1.9%). Although changes in muscular coordination pattern with feedback did not seem to influence GE, it could be mainly responsible for the reduction of muscle fatigue after prolonged cycling.

Physiological and cognitive responses when riding an electrically assisted bicycle versus a classical bicycle.

The present study compared the physiological responses and the subsequent cognitive performance when riding an electrically assisted (EB) versus a classical (CB) bicycle. Oxygen uptake, heart rate and leg extensor muscles electromyographic (EMG) activity were recorded in 10 subjects during a 30-min intermittent cycling exercise performed with EB versus CB. Cognitive performance was evaluated by a mail sorting test, performed at rest and after each cycling session. Averaged oxygen uptake and heat rate were significantly (P < 0.05) lower during EB cycling than during CB cycling. The EMG activities of the vastus lateralis, rectus femoris and gastrocnemius medialis muscles were significantly (P < 0.001) greater during CB cycling than during EB cycling. The time to complete the mail sorting test was significantly (P < 0.05) shorter after EB cycling than after CB cycling. Because EB cycling reduced muscle strains and physiological stress, it might offer benefits for those using bicycles in their work, such as postal workers and police officers. STATEMENT OF RELEVANCE: This study compared physiological and cognitive responses when riding an electrically assisted versus a classical bicycle. The results showed that the electrically assisted bicycle led to reduced muscle strains and physiological stress and, therefore, might offer benefits for those using bicycles in their work, such as postal workers and police officers.

Neuromuscular fatigue following constant versus variable-intensity endurance cycling in triathletes.

The aim of this study was to determine whether or not variable power cycling produced greater neuromuscular fatigue of knee extensor muscles than constant power cycling at the same mean power output. Eight male triathletes (age: 33+/-5 years, mass: 74+/-4 kg, VO2max: 62+/-5 mL kg(-1) min(-1), maximal aerobic power: 392+/-17 W) performed two 30 min trials on a cycle ergometer in a random order. Cycling exercise was performed either at a constant power output (CP) corresponding to 75% of the maximal aerobic power (MAP) or a variable power output (VP) with alternating +/-15%, +/-5%, and +/-10% of 75% MAP approximately every 5 min. Maximal voluntary contraction (MVC) torque, maximal voluntary activation level and excitation-contraction coupling process of knee extensor muscles were evaluated before and immediately after the exercise using the technique of electrically evoked contractions (single and paired stimulations). Oxygen uptake, ventilation and heart rate were also measured at regular intervals during the exercise. Averaged metabolic variables were not significantly different between the two conditions. Similarly, reductions in MVC torque (approximately -11%, P<0.05) after cycling were not different (P>0.05) between CP and VP trials. The magnitude of central and peripheral fatigue was also similar at the end of the two cycling exercises. It is concluded that, following 30 min of endurance cycling, semi-elite triathletes experienced no additional neuromuscular fatigue by varying power (from +/-5% to 15%) compared with a protocol that involved a constant power.

Physiological stress monitoring of postmen during work.

The purpose of the present study was to characterize the physiological demands of postmen during a work day composed of mail sorting (inside work, IW) and cycling distribution (outside work, OW). Two groups of postal workers were constituted according to the geographical profile of their distribution courses: flat profile (FP; n = 7) vs hilly profile (HP; n = 7). Heart rate (HR) was recorded and energy expenditure (EE) was estimated during both IW and OW. EE was, on average, 1795 +/- 497 Kcal per workday. HR was higher (p < 0.001) during OW (104 +/- 14 bpm) than during IW (80 +/- 7 bpm). HR was highest during the cycling part (109 +/- 13 bpm). Average HR was greater (p < 0.001) for HP (112 +/- 9 bpm) than for FP route (95 +/- 8 bpm). On average, the workload of postmen could be considered as moderate. Nevertheless, the physiological demand greatly increased during the cycling part of OW, especially with hilly geographical profile. The greater physiological strains during the cycling should be factored into the improvement of work organization and delivery materials (e.g. bicycle) for postal workers.