Hypogravity reduces trunk admittance and lumbar muscle activation in response to external perturbations

Abstract

Reduced paraspinal muscle size and flattening of spinal curvatures have been documented after spaceflight. Assessment of trunk adaptations to hypogravity can contribute to development of specific countermeasures. In this study, parabolic flights were used to investigate spinal curvature and muscle responses to hypogravity. Data from five trials at 0.25 g, 0.50 g, and 0.75 g were recorded from six participants positioned in a kneeling-seated position. During the first two trials, participants maintained a normal, upright posture. In the last three trials, small-amplitude perturbations were delivered in the anterior direction at the T₁₀ level. Spinal curvature was estimated with motion capture cameras. Trunk displacement and contact force between the actuator and participant were recorded. Muscle activity responses were collected by intramuscular electromyography (iEMG) of the deep and superficial lumbar multifidus, iliocostalis lumborum, longissimus thoracis, quadratus lumborum, transversus abdominis, obliquus internus, and obliquus externus muscles. The root mean square iEMG and the average spinal angles were calculated. Trunk admittance and muscle responses to perturbations were calculated as closed-loop frequency-response functions. Compared with 0.75 g, 0.25 g resulted in lower activation of the longissimus thoracis (P = 0.002); lower responses of the superficial multifidus at low frequencies (P = 0.043); lower responses of the superficial multifidus (P = 0.029) and iliocostalis lumborum (P = 0.043); lower trunk admittance (P = 0.037) at intermediate frequencies; and stronger responses of the transversus abdominis at higher frequencies (P = 0.032). These findings indicate that exposure to hypogravity reduces trunk admittance, partially compensated by weaker stabilizing contributions of the paraspinal muscles and coinciding with an apparent increase of deep abdominal muscle activity.NEW & NOTEWORTHY This study presents for the first time novel insights into the adaptations to hypogravity of spinal curvatures, trunk stiffness, and paraspinal muscle activity. We showed that exposure to hypogravity reduces the displacement of the trunk by an applied perturbation, partially compensated by weaker stabilizing contributions of the paraspinal muscles and concomitant increase in abdominal muscle responses. These findings may have relevance for future recommendations for planetary surface explorations.

Keywords: intramuscular electromyography; low gravity; lumbar spine; parabolic flight; trunk stabilization.

Fig. 1.

Schematic depicting the different phases of parabolic flight profile at each gravitational level (0.25 g, 0.50 g, and 0.75 g).

Fig. 2.

Experimental setup, showing the participant position and the linear actuator applying a posterior-anterior force to the participant’s trunk at T₁₀. Note the 6 retroreflective markers positioned over the spinous processes of C₆, T₃, T₇, T₁₂, L₃, and S₁.

Fig. 3.

Raw traces recorded during perturbation from a representative participant and parabola (0.5 g). The target force (Force, N), position (Pos, mm), axial acceleration (Accel, m·s⁻²), and intramuscular electromyography (iEMG, mV) of 8 muscles are displayed in the time domain (seconds). The time window of interest (shaded area) was manually selected from the period when the axial acceleration was stable at the corresponding target gravity level. Trunk perturbations started with a 3-s ramp force increase to 60 N of preload during the pull-up phase and terminated at the end of the pull-out phase of each parabola. IO, internal oblique; TrA, transversus abdominis; IL, iliocostalis; QL, quadratus lumborum; LO, longissimus; sMF, superficial multifidus; dMF, deep multifidus; EO, external oblique.

Fig. 4.

A: time domain representation of the actuator’s input, showing the target force applied to the participant. B: Fourier transform of the actuator’s input, indicating the low frequency (LF), intermediate frequency (IF), and high frequency (HF) bands. C: Fourier transform of the target force, contact force, and actuator position calculated over a selected time window during perturbation. D: Fourier transform of the intramuscular electromyography (iEMG) of each muscle during the same time window. EO, external oblique; IO, internal oblique; TrA, transversus abdominis; QL, quadratus lumborum; LO, longissimus; IL, iliocostalis; MF, multifidus.

Fig. 5.

The average ± SE across subjects of the frequency-response functions, i.e., trunk admittance and EMG responses (top), as well as the corresponding coherence functions (bottom). The average values and the SE across subjects can be observed at each frequency measured, corresponding to the peak frequencies of the perturbation signal delivered by the actuator. MF, multifidus; IL, iliocostalis; QL, quadratus lumborum; LO, longissimus; TrA, transversus abdominis; IO, internal oblique; EO, external oblique.

Fig. 6.

Trunk admittance and EMG responses that were found to have significant differences between gravity levels. Values represent the group mean (bars) and standard deviation (error bars). MF, multifidus; IL, iliocostalis; TrA, transversus abdominis. *Post hoc analysis gravity effect (P < 0.05).

Fig. 7.

Each dot represents the root mean square electromyography (RMS EMG) of LO (longissimus) during rest and the axial acceleration (A) and the muscle responses at the intermediate frequencies (1.65–3.55 Hz) of superficial multifidus (MF) during trunk perturbation and the axial acceleration (B). Color identifies participant, and colored lines show repeated-measures correlation fits for each participant.