Microgravity and Hypergravity Induced by Parabolic Flight Differently Affect Lumbar Spinal Stiffness

Abstract

The objective of this study was to determine the response of the lumbar spinal motor control in different gravitational conditions. This was accomplished by measuring indicators of lumbar motor control, specifically lumbar spinal stiffness, activity of lumbar extensor and flexor muscles and lumbar curvature, in hypergravity and microgravity during parabolic flights. Three female and five male subjects participated in this study. The mean age was 35.5 years (standard deviation: 8.5 years). Spinal stiffness of the L3 vertebra was measured using impulse response; activity of the erector spinae, multifidi, transversus abdominis, and psoas muscles was recorded using surface electromyography; and lumbar curvature was measured using distance sensors mounted on the back-plate of a full-body harness. An effect of gravity condition on spinal stiffness, activity of all muscles assessed and lumbar curvature ( p ’s < 0.007) was observed (Friedman tests). Post hoc analysis showed a significant reduction in stiffness during hypergravity ( p < 0.001) and an increase in stiffness during microgravity ( p < 0.001). Activity in all muscles significantly increased during hypergravity ( p ’s < 0.001). During microgravity, the multifidi ( p < 0.002) and transversus abdominis ( p < 0.001) increased significantly in muscle activity while no significant difference was found for the psoas ( p = 0.850) and erector spinae muscles ( p = 0.813). Lumbar curvature flattened in hypergravity as well as microgravity, albeit in different ways: during hypergravity, the distance to the skin decreased for the upper ( p = 0.016) and the lower sensor ( p = 0.036). During microgravity, the upper sensor showed a significant increase ( p = 0.016), and the lower showed a decrease ( p = 0.005) in distance. This study emphasizes the role of spinal motor control adaptations in changing gravity conditions. Both hypergravity and microgravity lead to changes in spinal motor control. The decrease in spinal stiffness during hypergravity is interpreted as a shift of the axial load from the spine to the pelvis and thoracic cage. In microgravity, activity of the multifidi and of the psoas muscles seems to ensure the integrity of the spine. Swiss (BASEC-NR: 2018-00051)/French “EST-III” (Nr-ID-RCB: 2018-A011294-51/Nr-CPP: 18.06.09).

In vivo measurements of spinal stiffness according to a stepwise increase of axial load

“…Decreased lumbar spinal stiffness with higher axial loads might be the result of the changes in the lumbar motor control strategy (Swanenburg et al 2020 ). It has been previously observed that hypergravity causes a decrease in spinal stiffness, increase in lumbar muscle activity, and a flattening of the lumbar curvature (Swanenburg et al 2020 ).…”

“…Decreased lumbar spinal stiffness with higher axial loads might be the result of the changes in the lumbar motor control strategy (Swanenburg et al 2020 ). It has been previously observed that hypergravity causes a decrease in spinal stiffness, increase in lumbar muscle activity, and a flattening of the lumbar curvature (Swanenburg et al 2020 ). Thus, it seems that an increased activation of the global muscle system dominates over the increased activation of the local muscle system and the flattening of the lumbar spine in hypergravity.…”

“…Despite the previous findings regarding the in vivo spinal stiffness decreases with one large axial load or hypergravity (Hausler et al 2020;Swanenburg et al 2020, the behavior of spinal motor control with smaller levels of additional loads remains to be elucidated.…”

“…Such in vitro studies test the passive structures, including bones and ligaments, but they do not include muscle activity or motor control of the spine (Stokes and Gardner-Morse 2003 ; Gardner-Morse and Stokes 2003 ; Zhang et al 2020 ). Both the active and passive subsystems can be assessed by measuring spinal stiffness (Hausler et al 2020 ; Swanenburg et al 2018 , 2020 ). Spinal stiffness can be seen as a proxy for the resistance to deformation of all subsystems together in vivo (muscles, joints, and ligaments) to the energy infused by the impulse.…”

“…Spinal stiffness has been observed to decrease in response to hypergravity conditions (1.8 g) induced by parabolic flights (Swanenburg et al 2018 , 2020 ). These studies observed an increase in lumbar flexor and extensor muscle activity, and the decrease in lumbar curvature during hypergravity, which lead to the interpretation that the load shifts from the spine to the pelvis and thorax with the increased axial pressure during hypergravity (Swanenburg et al 2018 , 2020 ; Bergmark 1989 ). This finding was confirmed in a study with 100 healthy young participants, which showed significantly decreased spinal stiffness during standing with an additional axial load of 50% of the participant’s body weight (Hausler et al 2020 ).…”