Metabolic Cost of a Proposed NMES Spaceflight Countermeasure Compared to Walking in Active Adults.

INTRODUCTION: Astronauts exercise to reduce microgravity-induced bone loss, but the resultant skeletal loading may not be sufficient to reduce fracture risk on an extended Mars mission. Adding additional exercise increases the risk of a negative caloric balance. Neuromuscular electrical stimulation (NMES) induces involuntary muscle contractions, which load the skeleton. The metabolic cost of NMES is not fully understood. On Earth, walking is a common source of skeletal loading. If the metabolic cost of NMES were equal to or less than walking, it could offer a low metabolic cost option for increasing skeletal loading.METHODS:We measured the oxygen consumed and carbon dioxide produced from 10 subjects during 5-min bouts of walking at 2 mph, 3 mph, and 2 mph on a 6° incline, and of NMES to the legs at duty cycles of 1 s on and 5 s, 4 s, or 3 s off. Metabolic cost was calculated using the Brockway equation and the percent increase above resting for each NMES bout was compared to walking.RESULTS: Metabolic cost increased 64.9 ± 52.8% from rest in the most intense NMES duty cycle (1 s/3 s) and 120.4 ± 26.5%, 189.3 ± 59.5%, 281.7 ± 66.8%, for the 2 mph, 3 mph, and incline walking, respectively. The metabolic cost did not differ significantly between the three NMES duty cycles.DISCUSSION: The increase in metabolic cost of the fastest NMES bout was less than that of the slowest walk, indicating that numerous NMES bouts offer a way to increase skeletal loading at a modest metabolic cost. This might allow for more daily skeletal loading cycles, potentially further reducing bone loss.Abitante TJ, Alemi MM, Newman DJ, Duda KR. Metabolic cost of a proposed NMES spaceflight countermeasure compared to walking in active adults. Aerosp Med Hum Perform. 2023; 94(7):523-531.

Longitudinal Impacts of Simulated Long-Duration Spaceflight Missions on Operationally Relevant Measures of Human Performance Using a Portable Simulation Platform.

OBJECTIVE: This project quantifies operationally relevant measures of flight performance and workload in a high-fidelity long-duration spaceflight analog, longitudinally across mission duration, using a portable simulation platform. BACKGROUND: Real-time performance measures allow for the objective assessment of task performance and the timely identification of performance degradations. METHODS: Measures of flight performance on a piloted lunar lander task were collected on 32 total crewmembers across 8 simulated space missions of 45 days each (623 total sessions). RESULTS: Mission duration demonstrated a significant effect on measures of flight performance across all campaigns. Flight measures showed a general pattern of peaking in accuracy during the middle-late quartiles of overall mission time, then degrading again towards baseline. On the workload measure, however, a general linear decrease in workload consistent with progressive task learning was observed in both campaigns. CONCLUSION: This investigation demonstrated the disruptive effect of time in mission on some, but not all, aspects of task performance. While mission interval differentially impacted measures of flight accuracy, workload, by contrast, seemed to steadily decrease with in-mission time. APPLICATION: While more work is needed, the observed discrepancy between progression of flight performance and workload assessment highlights the importance of sensitive and specific measurement tools for the tracking of distinct performance metrics.

Potential of Neuromuscular Electrical Stimulation as a Bone Loss Countermeasure in Microgravity.

INTRODUCTION: For future long-duration spaceflight missions, additional methods of loading the skeleton may be required to supplement exercise to minimize bone loss. Neuromuscular electrical stimulation (NMES) can elicit muscular contractions that create strain on bone. However, the potential effectiveness of NMES on the proximal femur during disuse is not known.METHODS: We measured the maximum isometric force of NMES-induced contractions of the rectus femoris and the hamstrings of 10 subjects (5 male, 5 female), sitting with the hips and knees at 90 degrees of flexion. We employed 2-D biomechanical models of the knee and hip to estimate the hip joint reaction forces, applied these forces to a generic femur finite element analysis model, and qualitatively compared the peak principal strains of the proximal femoral neck to the peak strains modeled in previous studies for other forms of exercise.RESULTS: The average peak tensile/compressive strains were 1380 ± 719 µÎµ/-2179 ± 1130 µÎµ and 573 ± 345 µÎµ/-900 ± 543 µÎµ for the male and female subjects, respectively. While results varied between studies, the strains achieved during NMES generally were comparable to those achieved during walking or stairs, with some individuals matching higher intensity activities.DISCUSSION: This study demonstrated that isometric NMES contractions of the thigh muscles can create strain in the proximal femoral neck similar to that achieved during low impact activities. While NMES alone will unlikely create a sufficient daily strain stimulus to prevent bone loss, it will likely improve the current spaceflight countermeasures by adding more frequent loading throughout the day.Abitante TJ, Bouxsein ML, Duda KR, Newman DJ. Potential of neuromuscular electrical stimulation as a bone loss countermeasure in microgravity. Aerosp Med Hum Perform. 2022; 93(11):774-782.

Effect of Athletic Training on Fatigue During Neuromuscular Electrical Stimulation.

The purpose of this study was to explore the effect an individual's exercise training type will have on muscle fatigability during repetitive contractions induced by Neuromuscular Electrical Stimulation (NMES). Thirty-four subjects comprising of competitive athletes and controls were recruited into three cohorts: Endurance (runners/cyclists) n = 13; nine male, four female; 27 ± 8 years old, Explosive (Lifters/Sprinters) n = 11; nine male, two female; 30 ± 7 years old, and controls n = 10, six male, four female, 26 ± 4 years old. Subjects were placed in a custom-made leg extension rig, and received NMES against a fixed resistance (NMES-FR), to the Vastus Medialis muscle resulting in isometric leg extensions, at a duty cycle of 1 s on/3 s rest, for 20 min. The force of the isometric contractions was recorded using a Hogan MicroFet2 dynamometer, and three separate fatigue metrics were calculated to compare the different cohorts, sports within each cohort, and gender within each cohort. For every fatigue metric, the endurance group fatigued significantly less than both the explosive and control cohorts, with no difference observed between the explosive and the controls. Within each cohort, no significant difference was observed in any fatigue metric between sport or gender, but these comparisons lacked power. The results show that only high capacity endurance activity will have any effect on reducing one's fatigability during repetitive NMES. The implications of this conclusion can aid in the development of NMES regimens for use in healthy populations, such as athletic training or astronaut musculoskeletal countermeasures, as well as clinical applications when fatigue is to be minimized.

A Closed-Loop Model of Operator Visual Attention, Situation Awareness, and Performance Across Automation Mode Transitions.

OBJECTIVE: This article describes a closed-loop, integrated human-vehicle model designed to help understand the underlying cognitive processes that influenced changes in subject visual attention, mental workload, and situation awareness across control mode transitions in a simulated human-in-the-loop lunar landing experiment. BACKGROUND: Control mode transitions from autopilot to manual flight may cause total attentional demands to exceed operator capacity. Attentional resources must be reallocated and reprioritized, which can increase the average uncertainty in the operator's estimates of low-priority system states. We define this increase in uncertainty as a reduction in situation awareness. METHOD: We present a model built upon the optimal control model for state estimation, the crossover model for manual control, and the SEEV (salience, effort, expectancy, value) model for visual attention. We modify the SEEV attention executive to direct visual attention based, in part, on the uncertainty in the operator's estimates of system states. RESULTS: The model was validated using the simulated lunar landing experimental data, demonstrating an average difference in the percentage of attention ≤3.6% for all simulator instruments. The model's predictions of mental workload and situation awareness, measured by task performance and system state uncertainty, also mimicked the experimental data. CONCLUSION: Our model supports the hypothesis that visual attention is influenced by the uncertainty in system state estimates. APPLICATION: Conceptualizing situation awareness around the metric of system state uncertainty is a valuable way for system designers to understand and predict how reallocations in the operator's visual attention during control mode transitions can produce reallocations in situation awareness of certain states.

The Variable Vector Countermeasure Suit (V2Suit) for space habitation and exploration.

The "Variable Vector Countermeasure Suit (V2Suit) for Space Habitation and Exploration" is a novel system concept that provides a platform for integrating sensors and actuators with daily astronaut intravehicular activities to improve health and performance, while reducing the mass and volume of the physiologic adaptation countermeasure systems, as well as the required exercise time during long-duration space exploration missions. The V2Suit system leverages wearable kinematic monitoring technology and uses inertial measurement units (IMUs) and control moment gyroscopes (CMGs) within miniaturized modules placed on body segments to provide a "viscous resistance" during movements against a specified direction of "down"-initially as a countermeasure to the sensorimotor adaptation performance decrements that manifest themselves while living and working in microgravity and during gravitational transitions during long-duration spaceflight, including post-flight recovery and rehabilitation. Several aspects of the V2Suit system concept were explored and simulated prior to developing a brassboard prototype for technology demonstration. This included a system architecture for identifying the key components and their interconnects, initial identification of key human-system integration challenges, development of a simulation architecture for CMG selection and parameter sizing, and the detailed mechanical design and fabrication of a module. The brassboard prototype demonstrates closed-loop control from "down" initialization through CMG actuation, and provides a research platform for human performance evaluations to mitigate sensorimotor adaptation, as well as a tool for determining the performance requirements when used as a musculoskeletal deconditioning countermeasure. This type of countermeasure system also has Earth benefits, particularly in gait or movement stabilization and rehabilitation.

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.

Motion sickness: a cholinomimetic agent hypothesis.

Motion sickness has been defined as a set of physiological signs and symptoms produced as a result of prolonged sensory conflict in central nervous system vestibular centers. It has long been noted that the particular pattern of motion sickness signs and symptoms does not fit the conventional "fight or flight vs. rest and digest" autonomic synergy. We argue that most of the progression of symptoms is consistent with a new etiologic hypothesis: that an as-yet-unidentified ganglionic cholinomimetic agent is slowly released in proportion to sensory conflict. The agent accumulates systemically and stimulates the peripheral sympathetic and parasympathetic ganglia, the adrenal medulla, and potentiates the response of central cholinergic emetic pathways to the same conflict stimulus. The predominant effects of ganglionic stimulation on each autonomic organ, determined by resting tone, are selectively enhanced or inhibited by adrenal catecholamine release, producing the atypical pattern of autonomic changes seen in motion sickness. The adrenergic response may eventually also counter the central emetic drive. The hypothesis could be experimentally pursued via human and animal experiments employing a nicotinic antagonist that has both central and peripheral ganglionic actions such as mecamylamine.