Quantifying the effectiveness of a passive trunk-support exosuit at reducing erector spinae muscle fatigue during a quasi-static posture maintenance task

Creep deformation of viscoelastic lumbar tissue and its implication in biomechanical modeling of the lumbar spine

Creep in the viscoelastic tissues of the lumbar spine reduces the force-producing capability of these tissues. This study aimed to explore the impact of passive tissue creep on lumbar biomechanics. Sixteen participants performed controlled sagittally symmetric trunk flexion motions after a 30-minute protocol consisting of 12 min of full trunk flexion and 18 min of upright standing. Trunk kinematics and EMG activities of trunk muscles were captured as input variables in three biomechanical models: a) EMG-assisted model with no passive tissue (Active), b) EMG-assisted model with time-invariant passive tissue (No-Creep), and c) EMG-assisted model with time-variant passive tissue components (Creep). The mean absolute error (MAE) between the external moment and the estimated internal moment was calculated as a function of model type and trunk flexion. Results revealed no significant difference in MAE between the three models at 0-30° trunk flexion but as the angle exceeded 30°, the MAE of the No-Creep and Creep models were significantly smaller than that of the Active model. Beyond thetrunk flexion angle of flexion-relaxation of erector spinae muscles, the MAE of the Creep model was significantly smaller than that of the No-Creep model (21.8 Nm vs. 40.3 Nm), leading to reduced compression and shear forces of the L4/L5 disc by 784.7 N (31.7 %) and 280.6 N (21.6 %) at full flexion. These results indicate the modulation of the time-dependent stiffness of passive tissues led to amore accurate prediction of the net internal moment at near full flexion postures, preventing overestimation of spinal loads.

Effects of a Passive Back-Support Exosuit on Objective and Subjective Measures of Human Performance During a Simulated Bush-Crop Harvesting Task

OBJECTIVE

Interest in wearable passive back-supports (exoskeletons/suits) has grown rapidly as a tool to reduce the risk of low back injury by reducing lumbar extensor muscle loading. Previous studies have shown the effectiveness of passive back-support exoskeleton/suit at reducing low back muscle activity/fatigue in a variety of tasks, but it is unclear whether an exoskeleton/exosuit intervention would be effective in agricultural harvesting tasks that require complex three-dimensional dynamic motions, long duration stooped postures, and variable engagement of the lower extremities. The objective of the current study was to evaluate the effects of a passive lumbar support exosuit on muscle fatigue and comfort/mobility during a simulated harvesting task in a controlled laboratory setting.

METHODS

Sixteen participants were asked to perform a continuous work task that simulated a 3-min bout of harvesting from a bush crop. Participants harvested at a rate of 1 unit per 2 s and were permitted to assume any effective harvesting position except a full kneeling posture. Test contractions and subjective assessments were performed before and after each 3-min bout of a simulated harvesting task to allow for an assessment of the 1) changes in objective measures of erector spinae muscle fatigue development (both time domain and frequency domain measures of muscle fatigue) and 2) subjective measures of physical fatigue, and 3) exosuit comfort and mobility constraints.

RESULTS

The exosuit significantly mitigated the increases in EMG amplitude in the time domain (p = .015; Cohen's d = 0.46) indicating a mitigation of muscle fatigue, but there was no significant exosuit effect on median frequency (p = .145) or perceived fatigue in the low back (p = .289). In addition, the exosuit use was associated with significant increases in perceived movement restriction (p < .001; d = 0.82) which were also manifested in terms of significant effects of the exosuit on the magnitude of the 90th percentile of the trunk flexion (p = .027, d = 0.29).

CONCLUSION

The results of this study demonstrated moderate muscle fatigue reduction effects at the cost of a negative impact on objective and subjective measures of trunk motion restrictions and comfort.

Effects of a Passive Back-Support Exosuit on Postural Control and Cognitive Performance During a Fatigue-Inducing Posture Maintenance Task

OBJECTIVE

To evaluate the effectiveness of passive back-support exosuit on postural control and cognitive performance during a fatigue-inducing posture maintenance task.

BACKGROUND

Wearable support systems (exoskeletons/exosuits) reduce physical demands but may also influence postural control and cognitive performance by reducing muscular fatigue.

METHOD

Eighteen participants visited on two different days to test an exosuit system and performed dual-task cognitive assessments based on human information processing (information acquisition, information integration, and action implementation) while maintaining a 35° trunk flexion posture for 16 minutes. Center-of-pressure (CoP), cognitive performance, and perceived workload were recorded, while erector spinae muscle activity was captured to quantify muscle fatigue.

RESULTS

The exosuit was effective in reducing erector spinae muscle fatigue during the static posture maintenance task (61% less in Δmedian frequency: -9.5 Hz (EXO-Off) versus -3.7 Hz (EXO-On)). The fatigue-inducing task increased CoP velocity as a function of time (29% greater: 9.3 mm/sec (pre) versus 12.0 mm/sec (post)), and exosuit use decreased CoP velocity (23% less: 12.1 mm/sec (EXO-Off) versus 9.4 mm/sec (EXO-On)). The exosuit was also effective at mitigating cognitive degradation, as evidenced by a higher hit-to-signal ratio (8% greater: 81.3 (EXO-Off) versus 87.9 (EXO-On)) in the information integration task and reducing perceived workload in all stages of human information processing.

CONCLUSION

Exosuit provided benefits of postural control and information integration processing during a 16-min static posture maintenance task.

APPLICATION

Torso exoskeletons/suits can have positive implications for occupations with concurrent physical and cognitive demands.