Trait-level predictors of human performance outcomes in personnel engaged in stressful laboratory and field tasks.

Introduction: Personnel performance under stress hinges on various factors, including individual traits, training, context, mental and physiological states, and task demands. This study explored the link between the traits of military personnel and their performance outcomes in five domains: move, shoot, communicate, navigate, and sustain. Methods: A total of 387 U.S. Army soldiers participated in this study, undergoing trait assessments covering physical, cognitive, social-emotional, demographic/lifestyle, and health domains. Performance was measured through lab and field events assessing a broad range of individual and team-level skills under conditions demanding resilience to acute cognitive and physical stress exposure. Analysis used feature selection and elastic net regression. Results: Analyses revealed complex associations between traits and performance, with physical, cognitive, health-related, social-emotional, and lifestyle traits playing roles in guiding and constraining performance. Measures of resilience, emotion regulation, grit, and mindfulness were identified as relevant predictors of several performance-related outcomes. Discussion: Results carry implications for the selection, training, and operational effectiveness of personnel in high-stakes occupations including military and first response. Further research is necessary to explore the mechanisms underlying these associations and inform targeted interventions to boost personnel effectiveness.

Metabolic adaptation and related biomechanics in an ankle-based exoskeleton system during six sessions of steady state walking.

Ankle-based exoskeletons have demonstrated metabolic benefits during steady-state walking; however, variability exists in individual adaptation timelines necessary to achieve those benefits. This study assessed timelines for metabolic and gait-related adaptation while wearing an ankle-based exoskeleton while powered (EXOP) compared to unpowered (EXNP) and no device worn (NOEX). Metabolic (VO2) and biomechanics data were collected while 14 participants walked on a treadmill at 1.3 m/s for six sessions. To better understand variability in responses to wearing exoskeletons, the cohort was divided based on the slope of the VO2 response of the first two sessions in the EXOP condition, and gait parameters were compared between subgroups. Repeated measures analyses of variance revealed a significant (p ≤ 0.001) 10% VO2 reduction for EXOP compared to EXNP and a non-significant 2.5% reduction for EXOP v NOEX. Lack of significant session-based comparisons indicated no additional VO2 adaptation; however, significant session-related results for peak knee flexion (interaction, p = 0.042) and step width (session main effect, p = 0.003) suggest gait-related adaptation continued during the sessions. Subgroup results indicated different response profiles to wearing exoskeletons; and implications of classifying initial responses based on metabolic response are discussed as an approach to understand what drives variation in responses to these devices.

After initial training, VO₂ reductions were observed with an ankle-based exoskeleton during the initial session and those reductions were maintained for the remaining sessions. Some gait-related variables continued to change over the remaining sessions. Exploratory work based on differences between early metabolic responses revealed potential adaptation strategy subgroups.

The intra- and interrater reliability of isometric muscle contractions using a novel fixation system for a hand-held dynamometer.

A novel fixation system for a hand-held dynamometer (HHD) was designed to enable isometric muscle measurements on various muscle groups of strong, healthy individuals in a field setting. The objective of this study was to evaluate the intra- and interrater reliability of the system and determine its suitability for use by multiple researchers within large-scale data collections during field activities. Four researchers tested eight healthy subjects, who each completed eight different maximal isometric muscle strength assessments using the HHD fixation system. Intraclass correlation coefficients (ICC) results were evaluated with a 95% confidence interval. ICC results for interrater reliability demonstrated excellent agreement of all eight measurements tested. ICC results for intrarater reliability demonstrated excellent agreement for six out of eight measurements. This system provides a new opportunity for several different high-quality maximal muscle strength measurements to be collected by multiple data collectors on large numbers of strong, healthy individuals in a field setting.

Characterizing Relationships Among the Cognitive, Physical, Social-emotional, and Health-related Traits of Military Personnel.

INTRODUCTION: Personnel engaged in high-stakes occupations, such as military personnel, law enforcement, and emergency first responders, must sustain performance through a range of environmental stressors. To maximize the effectiveness of military personnel, an a priori understanding of traits can help predict their physical and cognitive performance under stress and adversity. This work developed and assessed a suite of measures that have the potential to predict performance during operational scenarios. These measures were designed to characterize four specific trait-based domains: cognitive, health, physical, and social-emotional. MATERIALS AND METHODS: One hundred and ninety-one active duty U.S. Army soldiers completed interleaved questionnaire-based, seated task-based, and physical task-based measures over a period of 3-5 days. Redundancy analysis, dimensionality reduction, and network analyses revealed several patterns of interest. RESULTS: First, unique variable analysis revealed a minimally redundant battery of instruments. Second, principal component analysis showed that metrics tended to cluster together in three to five components within each domain. Finally, analyses of cross-domain associations using network analysis illustrated that cognitive, health, physical, and social-emotional domains showed strong construct solidarity. CONCLUSIONS: The present battery of metrics presents a fieldable toolkit that may be used to predict operational performance that can be clustered into separate components or used independently. It will aid predictive algorithm development aimed to identify critical predictors of individual military personnel and small-unit performance outcomes.

Effects of Cognitive Over Postural Demands on Upright Standing Among Young Adults.

A growing body of research has shown that static stance control (e.g., body sway) is influenced by cognitive demands (CD), an effect that may be related to competition for limited central resources. Measures of stance control have also been impacted by postural demands (PD) (e.g., stable vs. unstable stances). However, less is known of any possible interactions between PD and CD on static stance control in populations with intact balance control and ample cognitive resources, like young healthy adults. In this study, among the same participants, we factorially compared the impact of PD with and without CD on static stance control. Thirty-four healthy young adults wore inertial measurement units (IMU) while completing static stance tasks for 30 seconds in three different PD positions: feet apart, feet together, and tandem feet. After completing these tasks alone, participants performed these tasks with CD by concurrently completing verbal serial seven subtractions from a randomly selected three-digit number. For two dependent measures, path length and jerk, there were main effects of CD and PD but no interaction effect between these factors. For all other stance control parameters, there was only a PD main effect. Thus, adding a cognitive demand to postural demands, while standing upright, may have an independent impact on stance control, but CD does not seem to interact with PD. These results suggest that young healthy adults may be less sensitive to simple PD and CD due to their greater inherent balance control and available cognitive resources. Future work might explore more complex PD and CD combinations to determine the boundaries under which young adults' resources are taxed.

Association Between Knee Anatomic Metrics and Biomechanics for Male Soldiers Landing With Load.

BACKGROUND: Anterior cruciate ligament (ACL) injury is a military occupational hazard that may be attributed to an individual's knee biomechanics and joint anatomy. This study sought to determine if greater flexion when landing with load resulted in knee biomechanics thought to decrease ACL injury risk and whether knee biomechanics during landing relate to knee anatomic metrics. HYPOTHESIS: Anatomic metrics regarding the slope and concavity of the tibial plateau will exhibit a significant relation to the increased anterior shear force on the knee and decreased knee flexion posture during landing with body-borne load. STUDY DESIGN: Descriptive laboratory study. METHODS: Twenty male military personnel completed a drop landing task with 3 load conditions: light (~6 kg), medium (15% body weight), and heavy (30% body weight). Participants were divided into groups based on knee flexion exhibited when landing with the heavy load (high- and low-Δflexion). Tibial slopes and depth were measured on weightbearing volumetric images of the knee obtained with a prototype cone beam computed tomography system. Knee biomechanics were submitted to a linear mixed model to evaluate the effect of landing group and load, with the anatomic metrics considered covariates. RESULTS: Load increased peak proximal anterior tibial shear force (P = .034), knee flexion angle (P = .024), and moment (P = .001) during landing. Only the high flexion group increased knee flexion (P < .001) during weighted landings with medium and heavy loads. The low flexion group used greater knee abduction angle (P = .030) and peak proximal anterior tibial shear force (P = .034) when landing with load. Anatomic metrics did not differ between groups, but ratio of medial-to-lateral tibial slope and medial tibial depth predicted peak proximal anterior tibial shear force (P = .009) and knee flexion (P = .034) during landing, respectively. CONCLUSION: Increasing knee flexion is an attainable strategy to mitigate risk of ACL injury, but certain individuals may be predisposed to knee forces and biomechanics that load the ACL during weighted landings. CLINICAL RELEVANCE: The ability to screen individuals for anatomic metrics that predict knee flexion may identify soldiers and athletes who require additional training to mitigate the risk of lower extremity injury.

Rapid liquid chromatography tandem mass spectrometry method for targeted quantitation of human performance metabolites in saliva.

Saliva is increasingly being targeted for metabolic studies due to its non-invasive collection methods. Tracing levels of certain metabolites within biofluids can provide indications for a myriad of physiological conditions. This study was performed on a panel of eight analytes found in saliva that have shown associations with physiological conditions of human performance, such as stress, inflammation, and circadian rhythm. This dual polarity liquid chromatography tandem mass spectrometric (LCMS/MS) method was developed to accommodate a diverse group of analytes including steroids, alkaloids, and neurotransmitters. Samples collected during field exercises from soldiers were compared to those of civilians and baseline levels of each of these compounds was determined in saliva. Although most analytes showed no significant differences between the two populations, relative cortisol levels were higher for soldiers than for civilians. This developed dual polarity LCMS/MS method can be applied to very diverse groups of salivary analytes simultaneously.

Individuals with varus thrust do not increase knee adduction when running with body borne load.

Osteoarthritis (OA) is a common occupational hazard for service members. This study quantified how body borne load impacts knee biomechanics for participants who do and do not present varus thrust (range of knee adduction motion exhibited from heel strike to mid-stance (0-51%)) during over-ground running. Eighteen (9 varus thrust and 9 control) military personnel had knee biomechanics recorded when running with three load conditions (light, ∼6 kg, medium, 15% BW, and heavy, 30% BW). Subject-based means for knee biomechanics were calculated and submitted to a RM ANOVA to test the main effects and possible interactions between load and varus thrust group. The varus thrust group exhibited greater varus thrust (p = .001) and peak stance (PS, 0-100%) knee adduction (p = .009) posture compared to the control group with the light load, but not for the medium (p = .741 and p = .825) or heavy loads (p = .142 and p = .429). With the heavy load, varus thrust group reduced varus thrust (p = .023), whereas, the control group increased varus thrust (p = .037) compared to the light load, and increased PS knee adduction moment compared to light (p = .006) and medium loads (p = .031). The varus thrust group, however, exhibited no significant difference in knee adduction moment between any load (p = .174). With the addition of body borne load, varus thrust participants exhibited a significant reduction in knee biomechanics related to OA; whereas, control participants adopted knee biomechanics, including greater varus thrust and knee adduction moment, related to the development of OA.

Dual-task and anticipation impact lower limb biomechanics during a single-leg cut with body borne load.

This study quantified how a dual cognitive task impacts lower limb biomechanics during anticipated and unanticipated single-leg cuts with body borne load. Twenty-four males performed anticipated and unanticipated cuts with and without a dual cognitive task with three load conditions: no load (∼6 kg), medium load (15% of BW), and heavy load (30% of BW). Lower limb biomechanics were submitted to a repeated measures linear mixed model to test the main and interaction effects of load, anticipation, and dual task. With body borne load, participants increased peak stance (PS) hip flexion (p = .004) and hip internal rotation (p = .001) angle, and PS hip flexion (p = .001) and internal rotation (p = .018), and knee flexion (p = .016) and abduction (p = .001) moments. With the dual task, participants decreased PS knee flexion angle (p < .001) and hip flexion moment (p = .027), and increased PS knee external rotation angle (p = .034). During the unanticipated cut, participants increased PS hip (p = .040) and knee flexion angle (p < .001), and decreased PS hip adduction (p = .001), and knee abduction (p = .005) and external rotation (p = .026) moments. Adding body borne load produces lower limb biomechanical adaptations thought to increase risk of musculoskeletal injury, but neither anticipation nor dual task exaggerated those biomechanical adaptations. With a dual task, participants adopted biomechanics known to increase injury risk; whereas, participants used lower limb biomechanics thought to decrease injury risk during unanticipated cuts.

Soldier-relevant body borne loads increase knee joint contact force during a run-to-stop maneuver.

The purpose of this study was to understand the effects of load carriage on human performance, specifically during a run-to-stop (RTS) task. Using OpenSim analysis tools, knee joint contact force, grounds reaction force, leg stiffness and lower extremity joint angles and moments were determined for nine male military personnel performing a RTS under three load configurations (light, ~6kg, medium, ~20kg, and heavy, ~40kg). Subject-based means for each biomechanical variable were submitted to repeated measures ANOVA to test the effects of load. During the RTS, body borne load significantly increased peak knee joint contact force by 1.2 BW (p<0.001) and peak vertical (p<0.001) and anterior-posterior (p=0.002) ground reaction forces by 0.6 BW and 0.3 BW, respectively. Body borne load also had a significant effect on hip (p=0.026) posture with the medium load and knee (p=0.046) posture with the heavy load. With the heavy load, participants exhibited a substantial, albeit non-significant increase in leg stiffness (p=0.073 and d=0.615). Increases in joint contact force exhibited during the RTS were primarily due to greater GRFs that impact the soldier with each incremental addition of body borne load. The stiff leg, extended knee and large braking force the soldiers exhibited with the heavy load suggests their injury risk may be greatest with that specific load configuration. Further work is needed to determine if the biomechanical profile exhibited with the heavy load configuration translates to unsafe shear forces at the knee joint and consequently, a higher likelihood of injury.

Poststroke muscle architectural parameters of the tibialis anterior and the potential implications for rehabilitation of foot drop.

Poststroke dorsiflexor weakness and paretic limb foot drop increase the risk of stumbling and falling and decrease overall functional mobility. It is of interest whether dorsiflexor muscle weakness is primarily neurological in origin or whether morphological differences also contribute to the impairment. Ten poststroke hemiparetic individuals were imaged bilaterally using noninvasive medical imaging techniques. Magnetic resonance imaging was used to identify changes in tibialis anterior muscle volume and muscle belly length. Ultrasonography was used to measure fascicle length and pennation angle in a neutral position. We found no clinically meaningful bilateral differences in any architectural parameter across all subjects, which indicates that these subjects have the muscular capacity to dorsiflex their foot. Therefore, poststroke dorsiflexor weakness is primarily neural in origin and likely due to muscle activation failure or increased spasticity of the plantar flexors. The current finding suggests that electrical stimulation methods or additional neuromuscular retraining may be more beneficial than targeting muscle strength (i.e., increasing muscle mass).

Differences in Plantar Flexor Fascicle Length and Pennation Angle between Healthy and Poststroke Individuals and Implications for Poststroke Plantar Flexor Force Contributions.

Poststroke plantar flexor muscle weakness has been attributed to muscle atrophy and impaired activation, which cannot collectively explain the limitations in force-generating capability of the entire muscle group. It is of interest whether changes in poststroke plantar flexor muscle fascicle length and pennation angle influence the individual force-generating capability and whether plantar flexor weakness is due to uniform changes in individual muscle force contributions. Fascicle lengths and pennation angles for the soleus, medial, and lateral gastrocnemius were measured using ultrasound and compared between ten hemiparetic poststroke subjects and ten healthy controls. Physiological cross-sectional areas and force contributions to poststroke plantar flexor torque were estimated for each muscle. No statistical differences were observed for any muscle fascicle lengths or for the lateral gastrocnemius and soleus pennation angles between paretic, nonparetic, and healthy limbs. There was a significant decrease (P < 0.05) in the paretic medial gastrocnemius pennation angle compared to both nonparetic and healthy limbs. Physiological cross-sectional areas and force contributions were smaller on the paretic side. Additionally, bilateral muscle contributions to plantar flexor torque remained the same. While the architecture of each individual plantar flexor muscle is affected differently after stroke, the relative contribution of each muscle remains the same.

Muscle volume as a predictor of maximum force generating ability in the plantar flexors post-stroke.

INTRODUCTION: Post-stroke muscle weakness is commonly thought to be the result of a combination of decreased voluntary activation and decreased maximum force generating ability (MFGA). We assessed the ability of muscle volumes obtained using MRI to estimate the MFGA of the plantar flexor muscle group in individuals post-stroke. METHODS: MRI was used to measure muscle volume of the plantar flexor muscle group in 17 individuals with post-stroke hemiparesis. A modified burst superimposition test was used to measure force of volitional contraction and predict the MFGA of the plantar flexors. RESULTS: While muscle volume obtained by means of MRI provided information on the overall size of muscle, it overestimated the force generating ability of the paretic plantar flexors. CONCLUSIONS: Results suggest that MRI-derived muscle volume underestimates the functional impairment in individuals post-stroke. Interestingly, the central activation ratio had a strong relationship with the maximum volitional force of contraction.

Hybrid models of the neuromusculoskeletal system improve subject-specificity.

Muscle-actuated simulations of pathological gait have the capacity to identify muscle impairments and compensatory strategies, but the lack of subject-specific solutions prevents the prescription of personalized therapies. Conversely, electromyographic-driven models are limited to muscles for which data are available but can capture the true neural drive initiated by an individual subject. In order to improve subject-specificity and enforce physiological constraints on muscle activity, we propose a hybrid strategy for the optimization of subject-specific muscle patterns that involves forward dynamic simulation of whole body movement coupled with electromyographic-driven models of muscle subsets. In this paper we apply the hybrid approach to an example of post-stroke gait and demonstrate its unique ability to account for the unusual muscle activation patterns and muscle properties in patients with neuromuscular impairments.

Paretic muscle atrophy and non-contractile tissue content in individual muscles of the post-stroke lower extremity.

Muscle atrophy is one of many factors contributing to post-stroke hemiparetic weakness. Since muscle force is a function of muscle size, the amount of muscle atrophy an individual muscle undergoes has implications for its overall force-generating capability post-stroke. In this study, post-stroke atrophy was determined bilaterally in fifteen leg muscles with volumes quantified using magnetic resonance imaging (MRI). All muscle volumes were adjusted to exclude non-contractile tissue content, and muscle atrophy was quantified by comparing the volumes between paretic and non-paretic sides. Non-contractile tissue or intramuscular fat was calculated by determining the amount of tissue excluded from the muscle volume measurement. With the exception of the gracilis, all individual paretic muscles examined had smaller volumes in the non-paretic side. The average decrease in volume for these paretic muscles was 23%. The gracilis volume, on the other hand, was approximately 11% larger on the paretic side. The amount of non-contractile tissue was higher in all paretic muscles except the gracilis, where no difference was observed between sides. To compensate for paretic plantar flexor weakness, one idea might be that use of the paretic gracilis actually causes the muscle to increase in size and not develop intramuscular fat. By eliminating non-contractile tissue from our volume calculations, we have presented volume data that more appropriately represents force-generating muscle tissue. Non-uniform muscle atrophy was observed across muscles and may provide important clues when assessing the effect of muscle atrophy on post-stroke gait.

Muscle moment arm and normalized moment contributions as reference data for musculoskeletal elbow and wrist joint models.

A geometric musculoskeletal model of the elbow and wrist joints was developed to calculate muscle moment arms throughout elbow flexion/extension, forearm pronation/supination, wrist flexion/extension and radial/ulnar deviation. Model moment arms were verified with data from cadaver specimen studies and geometric models available in the literature. Coefficients of polynomial equations were calculated for all moment arms as functions of joint angle, with special consideration to coupled muscles as a function of two joint angles. Additionally, a "normalized potential moment (NPM)" contribution index for each muscle across the elbow and wrist joints in four degrees-of-freedom was determined using each muscle's normalized physiological cross-sectional area (PCSA) and peak moment arm (MA). We hypothesize that (a) a geometric model of the elbow and wrist joints can represent the major attributes of MA versus joint angle from many literature sources of cadaver and model data and (b) an index can represent each muscle's normalized moment contribution to each degree-of-freedom at the elbow and wrist. We believe these data serve as a simple, yet comprehensive, reference for how the primary 16 muscles across the elbow and wrist contribute to joint moment and overall joint performance.