P-041 - Post-Spaceflight Evaluation of Neck Force Control and Neck Muscle Activation in Astronauts

Abstract Title:
Post-Spaceflight Evaluation of Neck Force Control and Neck Muscle Activation in Astronauts



Background: Astronauts have an increased risk of cervical intervertebral disc herniation (IVD) post-spaceflight, yet the cause of this heightened risk remains elusive. People with neck pain commonly exhibit changes in neck muscle strength and altered neck muscle activity, which can contribute to the persistence of their symptoms. Whether astronauts experience similar alterations post-flight and if these are linked to the higher prevalence of IVD in this population is yet to be determined. The unique microgravity environment of space may lead to adaptations in the biomechanics of the cervical spine and changes in the morphology and motor control of neck musculature, potentially predisposing astronauts to cervical spinal disorders upon re-exposure to Earth's gravity. For example, prolonged exposure to zero gravity conditions could result in deconditioning of the neck musculature and alterations in the control of neck muscle force. Such alterations could contribute to the increased incidence of IVD among astronauts. Understanding these changes is crucial for developing effective countermeasures to safeguard astronaut spinal health during and after space missions.

Purpose/Objectives: We aimed to explore potential alterations in the behavior of the neck flexor and extensor muscles in astronauts following prolonged exposure (over 120 days) to microgravity conditions aboard the International Space Station. This study employed advanced high-density surface electromyography (HDsEMG) to capture muscle activity patterns and hand-held dynamometry to assess muscle strength quantitatively. By integrating these methodologies, we sought to understand the extent of neuromuscular adaptations that can occur in response to the microgravity environment.

Methods:
This study is ongoing, and to date, four astronauts have been measured at two measurement sessions (pre-flight and five days post-spaceflight). Neck flexor and extensor muscle isometric strength was recorded with a handheld dynamometer (NOD, OT Bioelettronica, Turin, Italy). Then, submaximal isometric neck flexor/extensor force steadiness was measured during force target-tracking contractions at 25% and 50% of their maximal voluntary contraction. Visual trapezoidal feedback of their force trace was provided on a monitor. HDsEMG signals were acquired bilaterally from the sternocleidomastoid (SCM) and splenius capitis (SCap) muscles in monopolar mode, using four grids of 64 equally spaced (4mm; GR04MM1305, OT Bioelettronica, Turin, Italy) electrodes. The monopolar signals were differentiated longitudinally in the direction of the muscle fibres for each muscle to form adjacent bipolar channels. Topographical HDsEMG amplitude maps were created to investigate regional changes in muscle activation across all electrode pairs, and modified entropy was calculated based on the root mean square values. Force steadiness (i.e., the ability of an individual to produce steady force output) was characterised by the force signal's standard deviation (SD) and coefficient of variation (CoV).



Results:
A consistent trend of reduced neck flexor and extensor muscle strength (15.07%), in addition to a decline in neck flexor and extensor muscle force steadiness (CoV: 56.16%; SD: 49.09%), was observed for all astronauts, five days after returning to Earth. During neck extension contractions, a notable shift of the center of SCap muscle activity was observed post-flight towards more caudal areas of the muscle. During the submaximal neck flexion contractions, alterations in the center of SCM activity were also observed, but the direction was not consistent. Similarly, changes in other HDsEMG variables were noted, such as in the level of neck flexor/extensor co-activation; however, due to the preliminary nature of the findings, it is difficult to ascertain a consistent trend.


Conclusions/Implications for future research and/or clinical care: This study offers preliminary insights into changes in neck flexor and extensor muscle behaviour immediately after spaceflight. The reduction in muscle strength and force control, together with changes in neck muscle activation, could be relevant factors in understanding the increased risk of IVD in astronauts. Given the preliminary nature of this work, these findings should be interpreted with caution.

References: 1) Belavy DL, Adams M, Brisby H, et al. Disc herniations in astronauts: What causes them, and what does it tell us about herniation on earth?. Eur Spine J. 2016;25(1):144-154.

2) Belavy DL, Armbrecht G, Albracht K, et al. Cervical spine and muscle adaptation after spaceflight and relationship to herniation risk: protocol from 'Cervical in Space' trial. BMC Musculoskelet Disord. 2022;23(1):772.

3) Muceli S, Farina D, Kirkesola G, Katch F, Falla D. Reduced force steadiness in women with neck pain and the effect of short term vibration. J Electromyogr Kinesiol. 2011;21(2):283-290.