Virtual fit assessment of U.S. army body armor using NASA spacesuit techniques.

Fit and accommodation are critical design goals for a body armor system to maximize Soldiers' protection, comfort, mobility, and performance. The aim of this study is to assess fit and accommodation of body armor plates for the US Army. A virtual fit assessment technique, developed, validated, and deployed by NASA for spacesuit design, was adopted for this work. Specifically, 3D manikins of the Soldier population were overlaid virtually with geometrically similar surrogates of the armor plates. Trained subject matter experts with the US Army and NASA manually assessed the fit of the armor plates to manikins using a computer visualization tool and selected the appropriate plate size and position. A prediction model was built from the assessment data to predict the plate size from an arbitrary body shape and the resultant patterns of body-to-plate contact were quantified. The outcome indicated a unique trend of the plate sizes covarying with anthropometry. More pronouncedly, when the overlap between the body tissue and armor plate was quantified, female Soldiers are likely to experience a 25 times larger body-to-plate contact volume and 6.5 times larger contact depth than males on average, due to sex-based anthropometric differences. Overall, the prediction model and contact patterns provided key metrics for virtual body armor fit assessments, of which the locations, patterns, and magnitudes can help to improve sizing and fit of body armor systems, as previously demonstrated for NASA spacesuit design.

Anthropometric Changes in Spaceflight.

OBJECTIVE: This study aims to identify the change in anthropometric measurements during spaceflight due to microgravity exposure. BACKGROUND: Comprehensive and accurate anthropometric measurements are crucial to assess body shape and size changes in microgravity. However, only limited anthropometric data have been available from the astronauts in spaceflight. METHODS: A new photogrammetry-based technique in combination with a tape-measure method was used for anthropometric measurements from nine crewmembers on the International Space Station. Measurements included circumference and height for body segments (chest, waist, bicep, thigh, calf). The time-dependent variations were also assessed across pre-, in-, and postflight conditions. RESULTS: Stature showed a biphasic change with up to 3% increase at the early flight phase, followed by a steady phase during the remaining flight. Postflight measurements returned to a similar level of the preflight. Other linear measurements, including acromion height, showed similar trends. The chest, hip, thigh, and calf circumferences show overall decrease during the flight up to 11%, then returned close to the preflight measurement at postflight. CONCLUSION: The measurements from this study provide critical information for the spacesuit and hardware design. The ground-based assessments for spacesuit fit needs to be revalidated and adjusted for in-flight extravehicular activities from this data. APPLICATION: These data can be useful for space suit design as well as habitat, vehicle, and additional microgravity activities such as exercise, where the body shape changes can affect fit, performance, and human factors of the overall design.

Spacesuit Center of Gravity Assessments for Partial Gravity EVA Simulation in an Underwater Environment.

OBJECTIVE: The objective is to analytically determine the expected CG and build hardware to measure and verify the suited subject's CG for lunar extravehicular activity (EVA) training in an underwater environment. BACKGROUND: For lunar EVAs, it is necessary for astronauts to train with a spacesuit in a simulated partial gravity environment. NASA's Neutral Buoyancy Laboratory (NBL) can provide these conditions by producing negative buoyancy for a submerged suited subject. However, it is critical that the center of gravity (CG) for the human-spacesuit system to be accurate for conditions expected during planetary EVAs. METHODS: An underwater force-transducer system and individualized human-spacesuit model was created to provide real-time measurement of CG, including recommendations for weight placement locations and quantity of weight needed on the spacesuit to achieve a realistic lunar spacesuit CG. This method was tested with four suited subjects. RESULTS: Across tested weighout configurations, it was observed that an aft and high CG location will have large postural differences when compared to low and fore CG locations, highlighting the importance of having a proper CG. The system had an accuracy of ±5lbs of the total lunar weight and within ± 15 cm for fore-aft and left-right CG directions of the model predictions. CONCLUSION: The developed method offers analytical verification of the suited subject's CG and improves simulation quality of lunar EVAs. Future suit design can also benefit by recommending hardware changes to create ideal CG locations that improve balance and mobility. APPLICATION: The developed methodology can be used to verify a proper CG location in future planetary EVA simulations such as different reduced gravity training analogs (e.g. active cable offloading systems).

Ergonomic Risk Identification for Spacesuit Movements Using Factorial Analysis.

OCCUPATIONAL APPLICATIONSBiomechanical risk factors associated with spacesuit manual material handling tasks were evaluated using the singular value decomposition (SVD) technique. SVD analysis decomposed each lifting tasks into primitive motion patterns called eigenposture progression (EP) that contributed to the overall task. Biomechanical metrics, such as total joint displacement, were calculated for each EP. The first EP (a simultaneous knee, hip, and waist movement) had greater biomechanical demands than other EPs. Thus, tasks such as lifting from the floor were identified as "riskier" by having a greater composition of the first EP. The results of this work can be used to improve a task as well as spacesuit design by minimizing riskier movement patterns as shown in this case study. This methodology can be applied in civilian occupational settings to analyze open-ended tasks (e.g., complex product assembly and construction) for ergonomics assessments. Using this method, worker task strategies can be evaluated quantitatively, compared, and redesigned when necessary.

TECHNICAL ABSTRACTBackground Astronauts will perform manual materials handling tasks during future Lunar and Martian exploration missions. Wearing a spacesuit will change lifting kinematics, which could lead to increased musculoskeletal stresses. Thus, it is important to understand how suited motion patterns affect injury risk.Purpose The objective of this study was to use the singular value decomposition (SVD) technique to assess movement differences between lifting techniques in a spacesuit with respect to biomechanical risk factors.Methods Joint angles were derived from motion capture data of lifting tasks performed in the MK-III spacesuit. SVD was performed on the joint angles, extracting the common patterns ("eigenposture progressions") across each task and their weightings as a function of time. Biomechanical risk factors such as total joint displacement, moments at the low back waist joint, and stability metrics were calculated for each eigenposture progression (EP). These metrics were related back to each task and compared.Results The resulting EPs represented characteristic motions that composed each task. For example, the first eigenposture progression (EP1) was identified as waist, hip, and knee motions and the second eigenposture progression (EP2) was described as arm motions. EPs were coupled with different levels of biomechanical stresses, such that EP1 resulted in the greatest amount of joint displacement and low back moment compared to the other EPs. Tasks such as lifting from the floor were identified as "riskier" due to a higher composition of EP1. Differences in EP weightings were also observed across subjects with varying levels of suited experience.Conclusions The linear factorial analysis, combined with biomechanical stress variables, demonstrated an easy and consistent approach to assess injury risk by relating risk to derived EPs and motions. As shown in the lifting analysis and case study example, suited movement strategies or interventions that minimize "riskier" EPs and reduce injury risk were identified. With further development, a future analysis of relevant suited actions can inform mission and suit design.

Lumbar posture assessment with fabric strain sensors.

Astronauts are at risk for low back pain and injury during extravehicular activity because of the deconditioning of the lumbar region and biomechanical demands associated with wearing a spacesuit. To understand and mitigate injury risks, it is necessary to study the lumbar kinematics of astronauts inside their spacesuit. To expand on previous efforts, the purpose of this study was to develop and test a generalizable method to assess complex lumbar motion using 10 fabric strain sensors placed on the torso. Anatomical landmark positions and corresponding sensor measurements were collected from 12 male study participants performing 16 static lumbar postures. A multilayer principal component and regression-based model was constructed to estimate lumbar joint angles from the sensor measurements. Good lumbar joint angle estimation was observed (<9° mean error) from flexion and lateral bending joint angles, and lower accuracy (13.7° mean error) was observed from axial rotation joint angles. With continued development, this method can become a useful technique for measuring suited lumbar motion and could potentially be extrapolated to civilian work applications.

Negotiated control between the manual and visual systems for visually guided hand reaching movements.

BACKGROUND: Control of reaching movements for manual work, vehicle operation, or interactions with manual interfaces requires concurrent gaze control for visual guidance of the hand. We hypothesize that reaching movements are based on negotiated strategies to resolve possible conflicting demands placed on body segments shared by the visual (gaze) and manual (hand) control systems. Further, we hypothesize that a multiplicity of possible spatial configurations (redundancy) in a movement system enables a resolution of conflicting demands that does not require sacrificing the goals of the two systems. METHODS: The simultaneous control of manual reach and gaze during seated reaching movements was simulated by solving an inverse kinematics model wherein joint trajectories were estimated from a set of recorded hand and head movements. A secondary objective function, termed negotiation function, was introduced to describe a means for the manual reach and gaze directing systems to balance independent goals against (possibly competing) demands for shared resources, namely the torso movement. For both systems, the trade-off may be resolved without sacrificing goal achievement by taking advantage of redundant degrees of freedom. Estimated joint trajectories were then compared to joint movement recordings from ten participants. Joint angles were predicted with and without the negotiation function in place, and model accuracy was determined using the root-mean-square errors (RMSEs) and differences between estimated and recorded joint angles. RESULTS: The prediction accuracy was generally improved when negotiation was included: the negotiated control reduced RMSE by 16% and 30% on average when compared to the systems with only manual or visual control, respectively. Furthermore, the RMSE in the negotiated control system tended to improve with torso movement amplitude. CONCLUSIONS: The proposed model describes how multiple systems cooperate to perform goal-directed human movements when those movements draw upon shared resources. Allocation of shared resources can be undertaken by a negotiation process that is aware of redundancies and the existence of multiple solutions within the individual systems.

Adaptation of torso movement strategies in persons with spinal cord injury or low back pain.

STUDY DESIGN: Controlled laboratory study. Statistical regression and between-group comparisons. OBJECTIVE: To characterize functional limitation and adaptive strategies in seated manual transport tasks for spinal cord injury (SCI), low back pain (LBP), and control participants. SUMMARY OF BACKGROUND DATA: People with SCI are known to have adapted electromyographic activities and slow hand movement velocity, while those with LBP have reduced range of motion and lumbar joint contribution. However, their resultant outcome in torso movements has not been systematically quantified. METHODS: Seated participants performed either 2- or 1-handed loaded transports to 1 of 6 targets 49 cm above the hip-point, at 0 degrees, 45 degrees, and 90 degrees azimuths, at close and far distance. Three-dimensional torso movements were modeled by combinations of B-spine base functions. RESULTS: The SCI and LBP participants exhibit smaller torso flexion and axial rotation than control participants. The SCI participants tend to move the torso away from the target to maintain upper body balance. These differences among groups are significantly reduced in the 1-handed transport condition and/or transports to the frontal target. CONCLUSION: The movement patterns suggest that SCI participants may have adapted torso movement strategies to compensate for the limited control of upper body balance, while LBP participants may limit torso motion to avoid pain.

A model of head movement contribution for gaze transitions.

Head posture has been associated with work-related neck symptoms and discomfort, but its relationship with visual tasks has received much less attention. Head movement amplitude is normally a fraction of the angular distance to a visual target, as gaze transition is usually achieved through the combination of both head and eye movement. In this study, the proportion of head orientation vs. target orientation, named head movement contribution ratio (HMCR), was quantified and modelled as a function of target location. Head movements were measured on subjects orienting and maintaining gaze for 2 s at randomly presented visual targets distributed along an arc placed horizontally or vertically. Bootstrap regression models showed that the horizontal HMCR was approximately 69% of target azimuth. The vertical HMCR was bilinear and corresponded to 52% and 8% and of target elevation for targets above and below the horizontal plane, respectively. The data also demonstrated that head orientation is affected by the kinematic coupling between horizontal and vertical components of head movement. STATEMENT OF RELEVANCE: Awkward head and neck posture is a risk factor for work-related musculoskeletal disorders. This study investigated the influence of visual target location on head orientation over a large range of target eccentricity, as an attempt to predict the head and neck posture required for visual target detection and identification.

Head movement control in visually guided tasks: postural goal and optimality.

This work investigates the control of horizontal head movements in the context of unconstrained visually guided head and arm/finger aiming tasks. In a first experiment, the head was free to move while gaze was directed at randomly presented eccentric targets distributed horizontally (0 degrees-120 degrees) at eye level. In a second experiment, the horizontal head orientation was constrained to predetermined positions (0 degrees, 15 degrees, 30 degrees, 45 degrees or 60 degrees rightward) while the right index finger aimed at targets with the arm fully extended. Kinematics of head movements in gaze displacements exhibits an initial component weakly correlated with target position, followed by multiple corrections. Since the eyes are assumed to already be aimed at the target when the corrections occur, it is suggested that one goal of head movement control is to achieve a desired final orientation (posture). This hypothesis is supported by results from the second experiment that reveal an association between eye/head orientation angles and errors exhibited in the visuo-spatial representation of the environment. The minimization of error then underlies the control of head movement as a postural response optimized for a given target and task condition.

Modelling of shoulder and torso perception of effort in manual transfer tasks.

The aim of the present study was to develop statistical models of perceived effort at the shoulder and torso levels associated with manual load transfer tasks. The motions were directed from a home location toward one of twenty-two target shelves distributed in the right hemisphere. A total of 2149 ratings were obtained from 31 subjects for effort perception at the selected joints, using a ten-point modified Borg scale. Regression models, developed for the perception associated with each body part, included target locations (azimuth, height and distance), posture constraints (standing or sitting), task types (one or two handed transfer conditions), and demographic and anthropometric measures (stature, body weight, gender, and age) as parameters. The models provide a prediction of effort perception with adjusted r-square coefficients of 0.41 and 0.50 for the shoulder and torso, respectively. The results indicate that space and posture interact in a complex way to affect the rating of perceived effort, and are in agreement with the hypothesis that the 'sense of effort' is primarily associated with the efference copy of the descending motor command. Since a level of effort is not associated with a unique pattern of motor command, it is proposed that effort perception is likely to result from a summation of the components of the motor command. The models can be applied to optimize the spatial organization of the work environment in an attempt to reduce the risk of musculoskeletal injury.