Abstract
Gravity plays a crucial role in shaping patterned locomotor output to maintain dynamic stability during locomotion. The present study aimed to clarify the gravity-dependent regulation of modules that organize multiple muscle activities during walking in humans. Participants walked on a treadmill at seven speeds (1–6 km h−1 and a subject- and gravity-specific speed determined by the Froude number (Fr) corresponding to 0.25) while their body weight was partially supported by a lift to simulate walking with five levels of gravity conditions from 0.07 to 1 g. Modules, i.e., muscle-weighting vectors (spatial modules) and phase-dependent activation coefficients (temporal modules), were extracted from 12 lower-limb electromyographic (EMG) activities in each gravity (Fr ~ 0.25) using nonnegative matrix factorization. Additionally, a tensor decomposition model was fit to the EMG data to quantify variables depending on the gravity conditions and walking speed with prescribed spatial and temporal modules. The results demonstrated that muscle activity could be explained by four modules from 1 to 0.16 g and three modules at 0.07 g, and the modules were shared for both spatial and temporal components among the gravity conditions. The task-dependent variables of the modules acting on the supporting phase linearly decreased with decreasing gravity, whereas that of the module contributing to activation prior to foot contact showed nonlinear U-shaped modulation. Moreover, the profiles of the gravity-dependent modulation changed as a function of walking speed. In conclusion, reduced gravity walking was achieved by regulating the contribution of prescribed spatial and temporal coordination in muscle activities.
Highlights
Gravity plays a crucial role in shaping patterned locomotor output to maintain dynamic stability during locomotion
The amplitude of the EMGs in the plantar flexor muscles, including the medial head of the gastrocnemius muscle (MG), lateral head of the gastrocnemius muscle (LG) and soleus (Sol), hip abductor, gluteus medius (GMed), and the extensor muscle, and gluteus maximus (GMax), decreased with decreasing simulated gravity at a Froude number (Fr) of ~ 0.25 (Table 1), whereas the temporal profiles of the EMGs were similar among the different gravity levels (Fig. 2)
In a dorsiflexor muscle and the tibialis anterior (TA), the spatiotemporal EMG characteristics were similar from the gravity conditions of 1 g to 0.16 g but changed at 0.07 g
Summary
Gravity plays a crucial role in shaping patterned locomotor output to maintain dynamic stability during locomotion. Progress in space science has enabled the human race to challenge space exploration to the moon or Mars, which involves a sudden transition between different gravity conditions These challenges require rapid movement corrections, such as for preventing a fall during locomotion, in novel gravitational environments by recalibrating gravity-related sensorimotor transformation[4,5]. Previous studies have reported that a wide range of muscle activation in locomotor-like movements, such as walking, running and obstacle clearance, can be explained by shared spatial and temporal modules with several movement-dependent m odules[25,28,29,30]. A study in walking with body weight unloading demonstrated that the temporal modules shared by multiple muscles were similar across each simulated gravity condition between 1 and 0.05 g, whereas the spatial modules were considerably different among c onditions[31]. How the spatial and temporal locomotor modules are modulated as a function of gravity has not been systematically elucidated
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