Squat exercise biomechanics during short-radius centrifugation.

INTRODUCTION: Centrifuge-induced artificial gravity (AG) with exercise is a promising comprehensive countermeasure against the physiological de-conditioning that results from exposure to weightlessness. However, body movements onboard a rotating centrifuge are affected by both the gravity gradient and Coriolis accelerations. The effect of centrifugation on squat exercise biomechanics was investigated, and differences between AG and upright squat biomechanics were quantified. METHODS: There were 28 subjects (16 male) who participated in two separate experiments. Knee position, foot reaction forces, and motion sickness were recorded during the squats in a 1-G field while standing upright and while supine on a horizontally rotating 2 m radius centrifuge at 0, 23, or 30 rpm. RESULTS: No participants terminated the experiment due to motion sickness symptoms. Total mediolateral knee deflection increased by 1.0 to 2.0 cm during centrifugation, and did not result in any injuries. There was no evidence of an increased mediolateral knee travel "after-effect" during postrotation supine squats. Peak foot reaction forces increased with rotation rate up to approximately 200% bodyweight (iRED on ISS provides approximately 210% bodyweight resistance). The ratio of left-to-right foot force throughout the squat cycle on the centrifuge was nonconstant and approximately sinusoidal. Total foot reaction force versus knee flexion-extension angles differed between upright and AG squats due to centripetal acceleration on the centrifuge. DISCUSSION: A brief exercise protocol during centrifugation can be safely completed without significant after-effects in mediolateral knee position or motion sickness. Several recommendations are made for the design of future centrifuge-based exercise protocols for in-space applications.

The effect of head turn velocity on cross-coupled stimulation during centrifugation.

Short-radius centrifugation (SRC) provides a practical means of producing artificial gravity for long duration space flights, though perceptual side-effects could limit its operational feasibility. Head turns (HT) during SRC, other than those about the centrifugation axis, produce Cross-Coupled Stimulation (CCS), perceived as a tumbling sensation. CCS can be nauseagenic, though adaptation can minimize this detrimental effect over time. The force environment of CCS suggests that the head turn velocity plays a role in determining the stimulus magnitude, though its degree has not been characterized. Twenty-three subjects performed right quadrant head turns of 8 different velocities while spinning at 19 and 23 RPMs on the SRC over two consecutive days. The perceptual effects were characterized by subjective metrics, investigating the acute differences between velocities as well as the chronic effects on adaptation. It was found that the perceived CCS magnitude can be regulated by modulating HT velocity. Further, a threshold of HT velocity exists above which an asymptotic perceptual response is observed, and below which the perceptual response diminishes at an exponential rate relative to head turn velocity. Finally, the effects of HT velocity are independent of HT direction, though the differing head turn directions likely produce contextually specific stimuli. These results suggest that HT velocity modulation could provide a practical means of incremental adaptation to CCS during SRC.

Vestibular adaptation to centrifugation does not transfer across planes of head rotation.

Out-of-plane head movements performed during fast rotation produce non-compensatory nystagmus, sensations of illusory motion, and often motion sickness. Adaptation to this cross-coupled Coriolis stimulus has previously been demonstrated for head turns made in the yaw (transverse) plane of motion, during supine head-on-axis rotation. An open question, however, is if adaptation to head movements in one plane of motion transfers to head movements performed in a new, unpracticed plane of motion. Evidence of transfer would imply the brain builds up a generalized model of the vestibular sensory-motor system, instead of learning a variety of individual input/output relations separately. To investigate, over two days 9 subjects performed pitch head turns (sagittal plane) while rotating, before and after a series of yaw head turns while rotating. A Control Group of 10 subjects performed only the pitch movements. The vestibulo-ocular reflex (VOR) and sensations of illusory motion were recorded in the dark for all movements. Upon comparing the two groups we failed to find any evidence of transfer from the yaw plane to the pitch plane, suggesting that adaptation to cross-coupled stimuli is specific to the particular plane of head movement. The findings have applications for the use of centrifugation as a possible countermeasure for long duration spaceflight. Adapting astronauts to unconstrained head movements while rotating will likely require exposure to head movements in all planes and directions.

Incremental adaptation to yaw head turns during 30 RPM centrifugation.

A 3-day incremental protocol was conducted with the aim of adapting human subjects to make head movements comfortably during 30 RPM centrifugation. With motion sickness as a potentially limiting factor, the protocol was designed using a quantitative motion sickness model based upon the neural mismatch sensory conflict theory. Centrifuge velocity was incremented from 14 RPM on day 1, to 23 RPM on day 2, to 30 RPM on day 3, with subjects making a total of 42 head movements on each day. Twenty-four subjects completed the experiment with average motion sickness levels below five (out of 20). Four subjects aborted due to motion sickness. Adaptation of non-compensatory vertical nystagmus was observed through an 18% decrease in the vertical aVOR time constant over the 3 days. Subjective intensity ratings for the head movements decreased by approximately 40% over the 3 days, while illusory motion duration decreased by 18%. Feasibility of head movements during 30 RPM rotation was demonstrated with only 3 days of incremental training.

A stair-stepper for exercising on a short-radius centrifuge.

INTRODUCTION: One requirement for long-duration spaceflight is provisions for exercise to prevent deconditioning. We evaluated the feasibility of using a stair-stepper on a short-radius centrifuge for this purpose. METHODS: A stair-stepper was implemented on a centrifuge with a 2-m radius. There were 13 subjects who performed stepping exercise in a supine horizontal position while spinning at 0, 12.5, 23, and 30 rpm. They were instructed to step as fast and hard as possible during each 2-min session. We measured the forces on the feet, the heart rate, BP, stepping cadence, and medial-lateral deflections of the knees due to Coriolis forces. RESULTS: Subjects completed the 2-min sessions successfully. Voluntary cadence of exercise and foot forces increased as rotation rate increased (average of 68 steps x min(-1) at 0 rpm and 91 steps x min(-1) at 30 rpm). Foot forces during exercise increased from an average of 43% bodyweight at 0 rpm to 84% bodyweight at 30 rpm. Heart rate and systolic BP increased with exercise compared with rest at each rotation rate, but the change was smaller as rotation rate increased (average of 134 bpm at 0 rpm and 128 bpm at 30 rpm). Medial-lateral deflections of the knee during exercise while spinning were significantly greater than when not spinning in some cases. DISCUSSION: Presumably heart rate and BP were higher during exercise on a static centrifuge due to the muscular work required to pull with one foot while stepping with the other in a supine position. Subjects can sustain greater ground reaction forces when exercising than when lying still on the centrifuge (in some cases, greater than the full bodyweight). Medial-lateral knee deflections are a potential problem and should be monitored in future rotation studies.

Threshold-based vestibular adaptation to cross-coupled canal stimulation.

Prior experiments have demonstrated that people are able to adapt to cross-coupled accelerations associated with head movements while spinning at high rotation rates (e.g., 23 rpm or 138 degrees/s). However, while adapting, subjects commonly experience serious side effects, such as motion sickness, non-compensatory eye movements, and strong and potentially disorienting illusory body tilt or tumbling sensations. In the present study, we investigated the feasibility of adaptation using a threshold-based method, which ensured that the illusory tilt sensations remained imperceptible or just barely noticeable. This was achieved by incrementally increasing the angular velocity of the horizontal centrifuge while supine subjects made repeated consistent yaw head turns. Incremental adaptation phases started at centrifugation speeds of 3 rpm. Centrifuge speed was slowly increased in steps of 1.5 rpm until a light illusory tilt was experienced. At the end of the incremental procedure, subjects were able to make head turns while rotating 14 rpm without experiencing illusory tilt. Moreover, motion sickness symptoms could be avoided and a limited carry over of the adaptive state to stronger stimulation at 23 rpm was found. The results are compared to prior studies which adapted subjects to super-threshold stimuli.

Adaptation of VOR to Coriolis stimulation.

The vestibulo-ocular reflex (VOR) is normally characterized by the gain and phase of slow-phase velocity (SPV) relative to the stimulus velocity. Although this is perfectly satisfactory for steady-state sinusoidal oscillations about a single axis, it is less useful when applied to transient responses. The well-known decay of nystagmus following a step change of head velocity approximately follows a double exponential, with an initial amplitude (A), a long time constant (tau), and an adaptation time constant (tau(a)). We have developed a means of representing the transient response for a complex head velocity stimulus as experienced during high-speed artificial gravity (AG) experiments. When a subject, lying supine on a rotating horizontal platform, makes a yaw head movement of amplitude theta, the vertical semicircular canals experience a step in angular velocity. The pitch stimulus is equal to the change in the component of the centrifuge angular velocity (omega(c)) aligned with the interaural axis, and gives rise to a vertical VOR. The magnitude of the step change is omega(c) sin theta. The SPV is approximated by an exponential decay of amplitude A and single time constant tau, and then normalized relative to this stimulus step. MATLAB scripts filter the raw eye position data to remove noise, blinks, and saccades, differentiate the signal, and remove fast phases to obtain SPV. The amplitude of the fitted SPV exponential is divided by omega(c) sin theta to obtain the normalized SPV. A and tau are shown to behave differently as subjects adapt to repeated head movements of different amplitudes.