IMUs Can Estimate Hip and Knee Range of Motion during Walking Tasks but Are Not Sensitive to Changes in Load or Grade.

The ability to estimate lower-extremity mechanics in real-world scenarios may untether biomechanics research from a laboratory environment. This is particularly important for military populations where outdoor ruck marches over variable terrain and the addition of external load are cited as leading causes of musculoskeletal injury As such, this study aimed to examine (1) the validity of a minimal IMU sensor system for quantifying lower-extremity kinematics during treadmill walking and running compared with optical motion capture (OMC) and (2) the sensitivity of this IMU system to kinematic changes induced by load, grade, or a combination of the two. The IMU system was able to estimate hip and knee range of motion (ROM) with moderate accuracy during walking but not running. However, SPM analyses revealed IMU and OMC kinematic waveforms were significantly different at most gait phases. The IMU system was capable of detecting kinematic differences in knee kinematic waveforms that occur with added load but was not sensitive to changes in grade that influence lower-extremity kinematics when measured with OMC. While IMUs may be able to identify hip and knee ROM during gait, they are not suitable for replicating lab-level kinematic waveforms.

A Minimal Sensor Inertial Measurement Unit System Is Replicable and Capable of Estimating Bilateral Lower-Limb Kinematics in a Stationary Bodyweight Squat and a Countermovement Jump.

This study aimed to validate a 7-sensor inertial measurement unit system against optical motion capture to estimate bilateral lower-limb kinematics. Hip, knee, and ankle sagittal plane peak angles and range of motion (ROM) were compared during bodyweight squats and countermovement jumps in 18 participants. In the bodyweight squats, left peak hip flexion (intraclass correlation coefficient [ICC] = .51), knee extension (ICC = .68) and ankle plantar flexion (ICC = .55), and hip (ICC = .63) and knee (ICC = .52) ROM had moderate agreement, and right knee ROM had good agreement (ICC = .77). Relatively higher agreement was observed in the countermovement jumps compared to the bodyweight squats, moderate to good agreement in right peak knee flexion (ICC = .73), and right (ICC = .75) and left (ICC = .83) knee ROM. Moderate agreement was observed for right ankle plantar flexion (ICC = .63) and ROM (ICC = .51). Moderate agreement (ICC > .50) was observed in all variables in the left limb except hip extension, knee flexion, and dorsiflexion. In general, there was poor agreement for peak flexion angles, and at least moderate agreement for joint ROM. Future work will aim to optimize methodologies to increase usability and confidence in data interpretation by minimizing variance in system-based differences and may also benefit from expanding planes of movement.

Measures of Lower Body Strength Associated With Injuries in Australian Special Forces Selection Candidates.

The diverse and grueling nature of activities undertaken during Special Forces selection makes it difficult to develop physical training to improve performance and reduce injury risk. It is generally accepted that increased strength is protective against injury, but it is unclear if this is evident in a Special Forces selection environment. This study investigated the effect of the rigors of a Special Forces selection course has on performance of the isometric mid-thigh pull, countermovement jump, squat jump, drop landing, elastic utilization ratio (EUR), and injury occurrence. Throughout the course, 26% of participants sustained a preventable lower limb injury, with 65% of these occurring at the knee. The uninjured had higher values of absolute strength as measured by isometric mid-thigh pull peak absolute force (3399 [371] N, 3146 [307] N; P = .022) and lower EUR (0.94 [0.08], 1.01 [0.09]; P = .025) compared to the injured. Preventable knee injury was significantly correlated with isometric mid-thigh pull (r = -.245, P = .031) and EUR (r = .227, P = .044). The selection course altered EUR for uninjured individuals only (P = .004). Findings indicate that individuals with higher strength levels may be at a lower risk of injury than their weaker counterparts.

Lower-Limb Biomechanics Differ Between Sexes During Maximal Loaded Countermovement Jumps.

ABSTRACT: Fain, AC, Semore, KD, Lobb, NJ, and Brown, TN. Lower-limb biomechanics differ between sexes during maximal loaded countermovement jumps. J Strength Cond Res 35(2): 325-331, 2021-To improve military personnel's operational performance, this study determined the impact of heavy, military body-borne load on vertical jump performance. Twenty men and 17 women had lower-limb work and power quantified during a series of countermovement jumps with 4 body-borne loads (20, 25, 30, and 35 kg). For each jump, subjects stood in athletic position with feet shoulder-width apart, then squatted down and immediately performed a maximal-effort vertical jump. Subjects performed 3 successful jumps with each load. During each jump, limb and hip, knee and ankle work and power, each joint's contribution to limb work, as well as jump height and center of mass velocity were quantified. Each dependent measure was submitted to a 2-way repeated-meausres analysis of variance, with alpha level 0.05. Body-borne load reduced jump height (p = 0.001) but increased ankle work (p < 0.001). To jump higher (p < 0.001) with a greater center of mass velocity (p = 0.001), men produced more limb work (p < 0.001), hip (p = 0.001; p < 0.001), knee (p < 0.001; p < 0.001), and ankle (p < 0.001; p < 0.001) joint power and work. But, women produced a greater percentage of work at the ankle (p = 0.020) than men. Military practitioners may target different training adaptations to improve male and female personnel operational performance because lower-limb biomechanics differ between sexes during loaded vertical jumps.

Sex and Stride Impact Joint Stiffness During Loaded Running.

This study determined changes in lower limb joint stiffness when running with body-borne load, and whether they differ with stride or sex. Twenty males and 16 females had joint stiffness quantified when running (4.0 m/s) with body-borne load (20, 25, 30, and 35 kg) and 3 stride lengths (preferred or 15% longer and shorter). Lower limb joint stiffness, flexion range of motion (RoM), and peak flexion moment were submitted to a mixed-model analysis of variance. Knee and ankle stiffness increased 19% and 6% with load (P < .001, P = .049), but decreased 8% and 6% as stride lengthened (P = .004, P < .001). Decreased knee RoM (P < .001, 0.9°-2.7°) and increased knee (P = .007, up to 0.12 N.m/kg.m) and ankle (P = .013, up to 0.03 N.m/kg.m) flexion moment may stiffen joints with load. Greater knee (P < .001, 4.7°-5.4°) and ankle (P < .001, 2.6°-7.2°) flexion RoM may increase joint compliance with longer strides. Females exhibited 15% stiffer knee (P = .025) from larger reductions in knee RoM (4.3°-5.4°) with load than males (P < .004). Stiffer lower limb joints may elevate injury risk while running with load, especially for females.

Load increases IMU signal attenuation per step but reduces IMU signal attenuation per kilometre.

BACKGROUND: Despite deleterious biomechanics associated with injury, particularly as it pertains to load carriage, there is limited research on the association between physical demands and variables captured with wearable sensors. While inertial measurement units (IMUs) can be used as surrogate measures of ground reaction force (GRF) variables, it is unclear if these data are sensitive to military-specific task demands. RESEARCH QUESTION: Can wearable sensors characterise physical load and demands placed on individuals in different load, speed and grade conditions? METHODS: Data were collected on 20 individuals who were self-reportedly free from current injury, recreationally active, and capable of donning 23 kg in the form of a weighted vest. Each participant walked and ran on flat, uphill (+6 %) and downhill (-6 %) without and with load (23 kg). Data were collected synchronously from optical motion capture (OMC) and IMUs placed on the distal limb and the pelvis. Data from an 8-second window was used to generate a participant-based mean of OMC and IMU variables of interest. Repeated Measures ANOVA was used to measure main and interaction effects of load, speed, and grade. Simple linear regression was used to elucidate a relationship between OMC measures and estimated metabolic cost (EMC) to IMU measures. RESULTS: Load reduces foot and pelvic accelerations (p<0.001) but elevate signal attenuation per step (p=0.044). Conversely, attenuation per kilometre is lowered with the addition of load (p=0.017). Uphill had the lowest attenuation per step (p=0.003) and kilometre (p≤0.033) in walking, while downhill had the greatest attenuation per step (p≤0.002) and per kilometre (p≤0.004). Attenuation measures are inconsistently moderately related to limb negative work (R≤0.57). EMC is moderately positively related to unloaded running (R≥0.39), and moderately negatively related to walking with and without load (R≤-0.52). SIGNIFICANCE: While load reduces peak accelerations at both the pelvis and foot. However, it may increase demand on the lower extremity to attenuate the signal between the two sensors with each step, while attenuation over time reduces with load.

Association of clinically-measured and dynamic ankle dorsiflexion assessed by markerless motion capture during the drop-jump task on landing biomechanics and risk of ankle injury in military personnel undergoing 10 weeks of physical training.

OBJECTIVES: Determine the influence of clinically-measured maximum dorsiflexion, dynamic peak dorsiflexion and percent of clinically-measured maximum dorsiflexion used during a drop-jump task on landing biomechanics and risk of ankle injury in military personnel. DESIGN: Prospective cohort study. METHODS: 672 participants (122 women) enrolled. The weightbearing lunge test assessed clinically-measured maximum dorsiflexion averaged across limbs (degrees). Markerless motion capture and force plates collected lower extremity kinematic and kinetic data during a drop-jump task. Percent of clinically-measured maximum dorsiflexion used during landing was calculated as dynamic peak dorsiflexion divided by clinically-measured value, multiplied by 100 (%). De-identified injury data was derived from military physical therapists. Simple linear regression analysis determined the association between dorsiflexion measures and landing biomechanics. Simple binary logistic regression analyses identified predictors of ankle injuries. Statistical significance was set at α = 0.05. RESULTS: Eighteen participants sustained a traumatic ankle injury from a landing. All measures of dorsiflexion were associated with movement patterns that countered the stiff-legged landing strategy with dynamic measures showing a higher predictive value. Protective factors against ankle injury included height (odds ratio: 0.818, p = 0.006) and weight (odds ratio: 0.824, p = 0.023) for women. Relative braking impulse was a risk factor for men (odds ratio: 1.890, p = 0.001). CONCLUSIONS: Greater clinically-measured and dynamic measures of dorsiflexion were associated with movement patterns that countered the stiff-legged landing strategy but neither measure of dorsiflexion predicted ankle injury risk. Resultant biomechanics and anthropometrics influenced ankle injury risk to warrant recognition for injury prevention initiatives.

Back squat mechanics in persons with a unilateral transtibial amputation: A case study.

CASE DESCRIPTION: A 30-year-old person with a unilateral transtibial amputation (TTAmp) was assessed performing an Olympic-style back squat with an energy storage and return prosthetic foot. OBJECTIVE: Determine joint-level movement strategies of an individual with TTAmp while performing an Olympic-style back squat. STUDY DESIGN: Case study design. TREATMENT: Back squat mechanics are evaluated in an individual with TTAmp by comparing the contribution of the ankle, knee, and hip joint to total positive and negative amputated and sound limb work. OUTCOMES: The hip joint was the greatest contributor to total positive limb work compared with the knee and ankle, respectively, in the amputated (55.0% hip vs. 30.8% knee, P < 0.001; vs. 14.2% ankle, P = 0.001) and sound limbs (52.2% hip vs. 38.9% knee, P < 0.001; vs. 8.9% ankle, P < 0.001). The hip joint was the greatest contributor to total negative limb work compared with the ankle in both the amputated (51.0% hip vs. 15.9% ankle, P < 0.001) and sound limbs (47.3% hip vs. 10.0% ankle, P < 0.001) and the knee in the amputated limb (51.0% hip vs. 32.5% knee, P < 0.001). Ankle joint power demonstrated an atypical bimodal negative/positive/negative/positive pattern. CONCLUSION: The individual with TTAmp used a hip-dominant joint strategy in both the amputated and sound limbs while demonstrating more potential to optimize the prosthetic foot's energy storage and return capabilities when performing a back squat.

Unsupervised Clustering Techniques Identify Movement Strategies in the Countermovement Jump Associated With Musculoskeletal Injury Risk During US Marine Corps Officer Candidates School.

Musculoskeletal injuries (MSKI) are a significant burden on the military healthcare system. Movement strategies, genetics, and fitness level have been identified as potential contributors to MSKI risk. Screening measures associated with MSKI risk are emerging, including novel technologies, such as markerless motion capture (mMoCap) and force plates (FP) and allow for field expedient measures in dynamic military settings. The aim of the current study was to evaluate movement strategies (i.e., describe variables) of the countermovement jump (CMJ) in Marine officer candidates (MOCs) via mMoCap and FP technology by clustering variables to create distinct movement strategies associated with MSKI sustained during Officer Candidates School (OCS). 728 MOCs were tested and 668 MOCs (Male MOCs = 547, Female MOCs = 121) were used for analysis. MOCs performed 3 maximal CMJs in a mMoCap space with FP embedded into the system. De-identified MSKI data was acquired from internal OCS reports for those who presented to the OCS Physical Therapy department for MSKI treatment during the 10 weeks of OCS training. Three distinct clusters were formed with variables relating to CMJ kinetics and kinematics from the mMoCap and FPs. Proportions of MOCs with a lower extremity and torso MSKI across clusters were significantly different (p < 0.001), with the high-risk cluster having the highest proportions (30.5%), followed by moderate-risk cluster (22.5%) and low-risk cluster (13.8%). Kinetics, including braking rate of force development (BRFD), braking net impulse and propulsive net impulse, were higher in low-risk cluster compared to the high-risk cluster (p < 0.001). Lesser degrees of flexion and shorter CMJ phase durations (braking phase and propulsive phase) were observed in low-risk cluster compared to both moderate-risk and high-risk clusters. Male MOCs were distributed equally across clusters while female MOCs were primarily distributed in the high-risk cluster. Movement strategies (i.e., clusters), as quantified by mMoCap and FPs, were successfully described with MOCs MSKI risk proportions between clusters. These results provide actionable thresholds of key performance indicators for practitioners to use for screening measures in classifying greater MSKI risk. These tools may add value in creating modifiable strength and conditioning training programs before or during military training.

Inclusion of a Military-specific, Virtual Reality-based Rehabilitation Intervention Improved Measured Function, but Not Perceived Function, in Individuals with Lower Limb Trauma.

INTRODUCTION: Lower extremity injury is common in the military and can lead to instability, pain, and decreased function. Military service also places high physical demands on service members (SMs). Standard treatment interventions often fail to align with these unique demands. Thus, the goal of the study was to evaluate the effectiveness of a military-specific virtual reality-based rehabilitation (VR) intervention supplemental to standard care (SC) in improving military performance in SMs with lower extremity injuries. MATERIALS AND METHODS: As part of an institutional review board-approved randomized control trial, SMs receiving care at an advanced rehabilitation center were randomized to receive either SC or VR in addition to SC (VR+SC). Participants were evaluated before treatment and ∼3 weeks later using a previously developed and validated military-specific assessment. Perceived improvement in physical function was measured using a Global Rating of Change (GROC) questionnaire. A repeated measures ANOVA was used to evaluate the effects of adding VR on the military-specific assessment measures. Linear regression was used to determine the relationship between perceived improvement, measured improvement, and VR volume. RESULTS: The VR+SC group was able to traverse a greater distance in the assessment following the VR intervention. There was no significant difference in GROC between groups. For the VR+SC group, change in distance completed was not correlated with GROC, but GROC was correlated with VR volume. CONCLUSION: VR improved the distance that participants were able to traverse in the assessment. However, the VR+SC group demonstrated a disconnect between their perceived functional improvement as measured by the GROC and functional improvement as measured by the change in the distance completed. Rather, the perceived improvement appears to be more correlated with the volume of VR received. The way in which the treatment progression is structured and communicated may influence how patients perceive their change in physical function.

Sex and limb impact biomechanics associated with risk of injury during drop landing with body borne load.

Increasing lower limb flexion may reduce risk of musculoskeletal injury for military personnel during landing. This study compared lower limb biomechanics between sexes and limbs when using normal and greater lower limb flexion to land with body borne load. Thirty-three participants (21 male, 12 female, age: 21.6±2.5 years, height: 1.7±0.1 m, weight: 74.5±9.0 kg) performed normal and flexed lower limb landings with four body borne loads: 20, 25, 30 and 35 kg. Hip and knee biomechanics, peak vertical ground reaction force (GRF), and the magnitude and direction of the GRF vector in frontal plane were submitted to two separate repeated measures ANOVAs to test the main and interaction effects of sex, load, and landing, as well as limb, load, and landing. Participants increased GRFs (between 5 and 10%) and hip and knee flexion moments when landing with body borne load, but decreased vertical GRF 19% and hip adduction and knee abduction joint range of motion and moments during the flexed landings. Both females and the non-dominant limb presented greater risk of musculoskeletal injury during landing. Females exhibited larger GRFs, increased hip adduction range of motion, and greater knee abduction moments compared to males. Whereas, the non-dominant limb increased knee abduction moments and exhibited a more laterally-directed frontal plane GRF vector compared to the dominant limb during the loaded landings. Yet, increasing lower limb flexion during landing does not appear to produce similar reductions in lower limb biomechanics related to injury risk for both females and the non-dominant limb during landing.

Sex and stride length impact leg stiffness and ground reaction forces when running with body borne load.

This study quantified leg stiffness and vGRF measures for males and females using different stride lengths to run with four body borne loads (20, 25, 30, and 35 kg). Thirty-six participants (20 males and 16 females) ran at 4.0 m/s using either: their preferred stride length (PSL), or strides 15% longer (LSL) and shorter (SSL) than PSL. Leg stiffness and vGRF measures, including peak vGRF, impact peak and loading rate, were submitted to a RM ANOVA to test the main effect and interactions of load, stride length, and sex. Leg stiffness was greater with the 30 kg (p = 0.016) and 35 kg (p < 0.001) compared to the 20 kg load, but decreased as stride lengthened from SSL to PSL (p < 0.001) and PSL to LSL (p < 0.001). Males exhibited greater leg stiffness than females with SSL (p = 0.029). Yet, males decreased leg stiffness with each increase in stride length (p < 0.001; p < 0.001), while females only decreased leg stiffness between PSL and LSL (p = 0.014). Peak vGRF was greater with the addition of body borne load (p < 0.001) and increase in stride length (p < 0.001). Both impact peak and loading rate were greater with the 30 kg (p = 0.034; p = 0.043) and 35 kg (p = 0.004; p = 0.015) compared to the 20 kg load, and increased as stride lengthened from SSL to PSL (p = 0.001; p = 0.004) and PSL to LSL (p < 0.001; p < 0.001). Running with body borne load may elevate injury risk by increasing leg stiffness and vGRFs. Injury risk may further increase when using longer strides to run with body borne load.

Sex and limb differences during a single-leg cut with body borne load.

BACKGROUND: Military personnel don body borne loads that produce maladaptive lower limb biomechanics, increasing risk of musculoskeletal injury during common training tasks. Female personnel have over twice the injury risk as males, but it is unknown if a sex dimorphism in lower limb biomechanics exists during common training-related tasks. RESEARCH QUESTION: To determine whether lower limb biomechanics exhibited during a single-leg cut with military body borne loads differ between sexes. METHODS: Sixteen females and 20 males had lower limb biomechanics quantified during five single-leg cuts off each limb with four loads (20, 25, 30 and 35 kg). Each cut required participants run 4 m/s, before planting their foot on a force platform and cut 45° towards the opposite limb. Lower limb biomechanics related to musculoskeletal injury were submitted to a repeated measures ANOVA to test for main and interaction effects of load, sex, and limb. RESULTS: During the cut, load increased peak proximal anterior tibial shear force (p < 0.001) and peak hip flexion (p = 0.010) and knee abduction (p = 0.045) moments, but decreased peak knee flexion angle (p = 0.032). Females exhibited greater peak proximal anterior tibial shear (p = 0.014), and peak hip adduction (p < 0.001) and knee external rotation (p = 0.001) moment than males. Dominant limb exhibited larger peak hip adduction (p = 0.002); whereas, the non-dominant limb exhibited greater peak hip internal (p = 0.002) and knee external (p = 0.007) rotation moments. Only the non-dominant limb increased peak knee abduction moment (p = 0.001) with additional load. SIGNIFICANCE: During the cut, adding body borne load produced maladaptive biomechanics that may increase knee musculoskeletal injury risk. Load increased peak proximal tibial shear and potential strain of knee's soft-tissues. Females exhibited a sex dimorphism in lower limb biomechanics that may further elevate their injury risk. Both limbs exhibited biomechanics that may increase injury risk, but only the non-dominant limb further increased injury risk with load.