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

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.

Effect of Weighted Vest at 0%, 5% and 10% of Body Mass on Gasometry Biomarkers and Performance during a Rectangular Test in Trained Trail Runners

Trail runners (TRs) must carry an extra load of equipment, food (bars and gels) and liquids, to delay the anticipation of fatigue and dehydration during their competitions. Therefore, we aimed to evaluate how an extra load can influence the metabolic level. Thirteen well-trained trail runners performed a randomized crossover study (total n = 39), completing three treadmill running sessions with a weighted vest of 0%, 5% and 10% of their body mass during a combined test (rectangular test + ramp test). In addition, biomarkers of oxygen metabolism, acid-base and electrolyte status pre-, during and post-test, as well as the rectangular from capillary blood of the finger and time to exhaustion, were analyzed. Repeated-measures ANOVA showed no significant difference between conditions for any of the analyzed biomarkers of blood gas. However, one-way ANOVA showed a significant difference in trial duration between conditions (p ≤ 0.001). Tukey's post hoc analysis observed a significant decrease in time to exhaustion in the weighted vest of 10% compared to 0% (p ≤ 0.001) and 5% (p ≤ 0.01) and 5% compared to 0% (p = 0.030). In addition, repeated-measures ANOVA detected a significant difference in pH in the group x time interaction (p = 0.035). Our results show that increasing the weighted vest (5% and 10%) anticipates fatigue in runners trained in TR. In addition, increasing the load decreased pH by a smaller magnitude at 10% compared to 0% and 5% at the end of the exercise protocol.

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.

Neuromechanical stabilisation of the centre of mass during running

BACKGROUND

Stabilisation of the centre of mass (COM) trajectory is thought to be important during running. There is emerging evidence of the importance of leg length and angle regulation during running, which could contribute to stability in the COM trajectory The present study aimed to understand if leg length and angle stabilises the vertical and anterior-posterior (AP) COM displacements, and if the stability alters with running speeds.

METHODS

Data for this study came from an open-source treadmill running dataset (n = 28). Leg length (m) was calculated by taking the resultant distance of the two-dimensional sagittal plane leg vector (from pelvis segment to centre of pressure). Leg angle was defined by the angle subtended between the leg vector and the horizontal surface. Leg length and angle were scaled to a standard deviation of one. Uncontrolled manifold analysis (UCM) was used to provide an index of motor abundance (IMA) in the stabilisation of the vertical and AP COM displacement.

RESULTS

IMAAP and IMAvertical were largely destabilising and always stabilising, respectively. As speed increased, the peak destabilising effect on IMAAP increased from -0.66(0.18) at 2.5 m/s to -1.12(0.18) at 4.5 m/s, and the peak stabilising effect on IMAvertical increased from 0.69 (0.19) at 2.5 m/s to 1.18 (0.18) at 4.5 m/s.

CONCLUSION

Two simple parameters from a simple spring-mass model, leg length and angle, can explain the control behind running. The variability in leg length and angle helped stabilise the vertical COM, whilst maintaining constant running speed may rely more on inter-limb variation to adjust the horizontal COM accelerations.

Differences in running biomechanics between young, healthy men and women carrying external loads

During U.S. Army basic combat training (BCT), women are more prone to lower-extremity musculoskeletal injuries, including stress fracture (SF) of the tibia, with injury rates two to four times higher than those in men. There is evidence to suggest that the different injury rates are, in part, due to sex-specific differences in running biomechanics, including lower-extremity joint kinematics and kinetics, which are not fully understood, particularly when running with external load. To address this knowledge gap, we collected computed tomography images and motion-capture data from 41 young, healthy adults (20 women and 21 men) running on an instrumented treadmill at 3.0 m/s with loads of 0.0 kg, 11.3 kg, or 22.7 kg. Using individualized computational models, we quantified the running biomechanics and estimated tibial SF risk over 10 weeks of BCT, for each load condition. Across all load conditions, compared to men, women had a significantly smaller flexion angle at the trunk (16.9%-24.6%) but larger flexion angles at the ankle (14.0%-14.7%). Under load-carriage conditions, women had a larger flexion angle at the hip (17.7%-23.5%). In addition, women had a significantly smaller hip extension moment (11.8%-20.0%) and ankle plantarflexion moment (10.2%-14.3%), but larger joint reaction forces (JRFs) at the hip (16.1%-22.0%), knee (9.1%-14.2%), and ankle (8.2%-12.9%). Consequently, we found that women had a greater increase in tibial strain and SF risk than men as load increases, indicating higher susceptibility to injuries. When load carriage increased from 0.0 kg to 22.7 kg, SF risk increased by about 250% in women but only 133% in men. These results provide quantitative evidence to support the Army's new training and testing doctrine, as it shifts to a more personalized approach that shall account for sex and individual differences.

Effect of stride length on the running biomechanics of healthy women of different statures

BACKGROUND

Tibial stress fracture is a debilitating musculoskeletal injury that diminishes the physical performance of individuals who engage in high-volume running, including Service members during basic combat training (BCT) and recreational athletes. While several studies have shown that reducing stride length decreases musculoskeletal loads and the potential risk of tibial injury, we do not know whether stride-length reduction affects individuals of varying stature differently.

METHODS

We investigated the effects of reducing the running stride length on the biomechanics of the lower extremity of young, healthy women of different statures. Using individualized musculoskeletal and finite-element models of women of short (N = 6), medium (N = 7), and tall (N = 7) statures, we computed the joint kinematics and kinetics at the lower extremity and tibial strain for each participant as they ran on a treadmill at 3.0 m/s with their preferred stride length and with a stride length reduced by 10%. Using a probabilistic model, we estimated the stress-fracture risk for running regimens representative of U.S. Army Soldiers during BCT and recreational athletes training for a marathon.

RESULTS

When study participants reduced their stride length by 10%, the joint kinetics, kinematics, tibial strain, and stress-fracture risk were not significantly different among the three stature groups. Compared to the preferred stride length, a 10% reduction in stride length significantly decreased peak hip (p = 0.002) and knee (p < 0.001) flexion angles during the stance phase. In addition, it significantly decreased the peak hip adduction (p = 0.013), hip internal rotation (p = 0.004), knee extension (p = 0.012), and ankle plantar flexion (p = 0.026) moments, as well as the hip, knee, and ankle joint reaction forces (p < 0.001) and tibial strain (p < 0.001). Finally, for the simulated regimens, reducing the stride length decreased the relative risk of stress fracture by as much as 96%.

CONCLUSIONS

Our results show that reducing stride length by 10% decreases musculoskeletal loads, tibial strain, and stress-fracture risk, regardless of stature. We also observed large between-subject variability, which supports the development of individualized training strategies to decrease the incidence of stress fracture.

Training Specificity in Trail Running: A Single-Arm Trial on the Influence of Weighted Vest on Power and Kinematics in Trained Trail Runners

Participants in trail running races must carry their equipment throughout the race. This additional load modifies running biomechanics. Novel running powermeters allow further analyses of key running metrics. This study aims to determine the acute effects of running with extra weights on running power generation and running kinematics at submaximal speed. Fifteen male amateur trail runners completed three treadmill running sessions with a weighted vest of 0-, 5-, or 10% of their body mass (BM), at 8, 10, 12, and 14 km·h^-1. Mean power output (MPO), leg spring stiffness (LSS), ground contact time (GCT), flight time (FT), step frequency (SF), step length (SL), vertical oscillation (VO), and duty factor (DF) were estimated with the Stryd wearable system. The one-way ANOVA revealed higher GCT and MPO and lower DF, VO, and FT for the +10% BM compared to the two other conditions (p < 0.001) for the running speeds evaluated (ES: 0.2-7.0). After post-hoc testing, LSS resulted to be higher for +5% BM than for the +10% and +0% BM conditions (ES: 0.2 and 0.4). Running with lighter loads (i.e., +5% BM) takes the principle of specificity in trail running one step further, enhancing running power generation and LSS.

Biomechanical and physiological effects of female soldier load carriage: A scoping review

Loads carried by military populations can affect those of smaller stature, such as the average female, due to the higher percentage of body weight the loads represent. Despite this, most load carriage research is performed on males. Peer reviewed articles were collected from four databases to summarize available research on biomechanical and physiological effects of load carriage on females in the military. Extraction and thematic analysis were performed on 18 articles. 39% looked at biomechanical differences between loads in females, 61% looked at how the same load affected males and females, 44% looked at sex-by-load interaction effects, and 72% discussed impacts of load on females. The research revealed that military load carriage affects the biomechanics and physiology differently in females and to a greater extent than in males. Several gaps in available literature were found. Very few studies used military participants, military equipment, and/or employed occupationally relevant data collection methodologies.

Tibial compression during sustained walking with body borne load

This study determined if sustained walking with body borne load increases tibial compression, and whether increases in tibial compression are related to vertical GRFs. Thirteen participants had tibial compression and vertical GRF measures quantified while walking at 1.3 m/s for 60 min with body borne load. Each tibial compression (maximum and impulse) and GRF measure (peak, impulse, impact peak and loading rate) were submitted to a RM ANOVA to test the main effect and interaction between load (0, 15, and 30 kg) and time (minute 0, 30 and 60), and correlation analyses determined the relation between tibial compression and vertical GRF measures for each load and time. Each tibial compression and GRF measure increased with the addition of body borne load (all: p < 0.001). Time impacted impact peak (p = 0.034) and loading rate (p = 0.017), but no other GRF or tibial compression measure (p > 0.05). Although both tibial compression and vertical GRFs increased with load, vertical GRF measures exhibited negligible to weak (r: -0.37 to 0.35), and weak to moderate (r: -0.62 to 0.59) relation with maximum and impulse of tibial compression with each body borne load. At each time point, GRF measures exhibited negligible to weak (r: -0.39 to 0.27), and weak to moderate (r: -0.53 to 0.65) relation with maximum and impulse of tibial compression, respectively. Walking with body borne load increased tibial compression, and may place compressive forces on the tibia that lead to stress fracture. But, increases in tibial compression may not stem from concurrent increases in vertical GRFs.

Weighted vests in CrossFit increase physiological stress during walking and running without changes in spatiotemporal gait parameters

This study quantified the physiological and biomechanical effects of the 20 lb (9.07 kg, males) and 14 lb (6.35 kg, females) weighted vest used in CrossFit, and whether they were predisposed to injury. Twenty subjects (10 males, 10 females) undertook walking (0%, 5% and 10% gradient) and running trials in two randomised study visits (weighted vest/no weighted vest). Physiological demand during walking was increased with the vest at 10% but not 5% or 0% with no change in gait variables. In the running trial, the weighted vest increased oxygen uptake (males; females) (+0.22L/min, p < 0.01; +0.07 L/min, p < 0.05), heart rate (+11bpm, p < 0.01; +11bpm, p < 0.05), carbohydrate oxidation (+0.6 g/min, p < 0.001; +0.2 g/min, p < 0.01), and energy expenditure (+3.8 kJ/min, p < 0.001; +1.5 kJ/min, p < 0.05) whilst blood lactate was increased only in males (+0.6 mmol/L, p < 0.05). There was no change in stride length or frequency. Weighted vest training increases physiological stress and carbohydrate oxidation without affecting measured gait parameters. Practitioner summary: We examined the effect of weighted vest training prescribed in CrossFit (20 lb/9.07 kg, males and 14 lb/6.35 kg, females) in a randomised controlled trial. We found that physiological stress is increased in both sexes, although three-fold greater in males, but with no change in biomechanical gait that predisposes to lower-limb injury.

Military load carriage effects on the gait of military personnel: A systematic review

Carrying heavy loads results in biomechanical changes to gait and to an increased risk of injury in soldiers. The aim of this review is to examine the effects of military specific load carriage on the gait of soldiers. The Web of Science, PubMed and CINAHL databases were searched, a total of 1239 records were screened and 20 papers were included in the review. Participant, load and task characteristics and a summary of key findings were extracted. Due to heterogeneity in the reviewed studies, analysis was restricted to qualitative synthesis. There were limited effects on spatio-temporal variables but consistently reported increased trunk, hip and knee flexion and increased hip and knee extension moments. Muscle activation of lower limb and trunk muscles were also increased with loads. However, there were some conflicting findings for most parameters reviewed and apart from spatio-temporal parameters the findings of this review were in line with previous reviews of combined military and civilian populations.

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.

Trunk-pelvis coordination during load carriage running

Understanding the influence of load carriage on trunk-pelvis coordination and its variability has important functional implications for athletes who need to run with load. The aim of this study was to examine the influence of load carriage on trunk-pelvis coordination in running. Thirty healthy adults performed running while wearing a 20% bodyweight backpack, and without load. Vector coding was used to quantify trunk-pelvis segmental coordination and its variability during the stance phase of running. The four coordination patterns were: 1) anti-phase (segments moving in opposite directions), in-phase (segments moving in same directions), trunk-only phase (only trunk movement), and pelvic-only phase (only pelvic movement). For each plane, the percentage of stance phase spent in a specific coordination pattern was quantified. Coordination variability for each plane was averaged over the stance phase. Mixed effects models were used to analyse the effects of load, adjusted for the covariate of sex, on coordination and its variability. Running with load increased trunk-only coordination in the sagittal plane (P < 0.001), increased anti-phase coordination in the frontal plane (P < 0.001), reduced trunk-only phase coordination in axial rotation (P < 0.001), and increased coordination variability in all three planes (Flexion-Extension: P < 0.001; Lateral flexion: P = 0.03; Axial rotation: P < 0.001). Future studies would benefit from investigating how trunk-pelvis coordination and its variability alters candidate end-point variability indices (e.g. COM displacement), and its functional implications in load carriage running.

Effects of load carriage on biomechanical variables associated with tibial stress fractures in running

BACKGROUND

Military personnel are required to run while carrying heavy body-borne loads, which is suggested to increase their risk of tibial stress fracture. Research has retrospectively identified biomechanical variables associated with a history of tibial stress fracture in runners, however, the effect that load carriage has on these variables remains unknown.

RESEARCH QUESTION

What are the effects of load carriage on running biomechanical variables associated with a history of tibial stress fracture?

METHODS

Twenty-one women ran at 3.0 m/s on an instrumented treadmill in four load carriage conditions: 0, 4.5, 11.3, and 22.7 kg. Motion capture and ground reaction force data were collected. Dependent variables included average loading rate, peak absolute free moment, peak hip adduction, peak rearfoot eversion, and stride frequency. Linear mixed models were used to asses the effect of load carriage and body mass on dependent variables.

RESULTS

A load x body mass interaction was observed for stride frequency only (p = 0.017). Stride frequency increased with load carriage of 22.7-kg, but lighter participants illustrated a greater change than heavier participants. Average loading rate (p < 0.001) and peak free moment (p = 0.015) were greater in the 22.7-kg condition, while peak rearfoot eversion (p ≤ 0.023) was greater in the 11.3- and 22.7-kg conditions, compared to the unloaded condition. Load carriage did not affect peak hip adduction (p = 0.67).

SIGNIFICANCE

Participants adapted to heavy load carriage by increasing stride frequency. This was especially evident in lighter participants who increased stride frequency to a greater extent than heavier participants. Despite this adaptation, running with load carriage of ≥11.3-kg increased variables associated with a history of tibial stress fracture, which may be indicative of elevated stress fracture risk. However, the lack of concomitant change amongst variables as a function of load carriage may highlight the difficulty in assessing injury risk from a single measure of running biomechanics.