Validity of Inertial Measurement Units to Measure Lower-Limb Kinematics and Pelvic Orientation at Submaximal and Maximal Effort Running Speeds.

Inertial measurement units (IMUs) have been validated for measuring sagittal plane lower-limb kinematics during moderate-speed running, but their accuracy at maximal speeds remains less understood. This study aimed to assess IMU measurement accuracy during high-speed running and maximal effort sprinting on a curved non-motorized treadmill using discrete (Bland-Altman analysis) and continuous (root mean square error [RMSE], normalised RMSE, Pearson correlation, and statistical parametric mapping analysis [SPM]) metrics. The hip, knee, and ankle flexions and the pelvic orientation (tilt, obliquity, and rotation) were captured concurrently from both IMU and optical motion capture systems, as 20 participants ran steadily at 70%, 80%, 90%, and 100% of their maximal effort sprinting speed (5.36 ± 0.55, 6.02 ± 0.60, 6.66 ± 0.71, and 7.09 ± 0.73 m/s, respectively). Bland-Altman analysis indicated a systematic bias within ±1° for the peak pelvic tilt, rotation, and lower-limb kinematics and -3.3° to -4.1° for the pelvic obliquity. The SPM analysis demonstrated a good agreement in the hip and knee flexion angles for most phases of the stride cycle, albeit with significant differences noted around the ipsilateral toe-off. The RMSE ranged from 4.3° (pelvic obliquity at 70% speed) to 7.8° (hip flexion at 100% speed). Correlation coefficients ranged from 0.44 (pelvic tilt at 90%) to 0.99 (hip and knee flexions at all speeds). Running speed minimally but significantly affected the RMSE for the hip and ankle flexions. The present IMU system is effective for measuring lower-limb kinematics during sprinting, but the pelvic orientation estimation was less accurate.

Asymmetry in sprinting: An insight into sub-10 and sub-11 s men and women sprinters.

We assessed sprint mechanical asymmetry in world-class competitors and evaluated whether inter-limb sex-based differences in sprinting mechanics exist. The eight finalists in the men's and women's 100 m events at the 2017 IAAF World Championships were studied. Five high-speed cameras (150 Hz) were used to capture two consecutive steps of the whole body between 47.0 m and 55.5 m from the start, while four additional cameras (250 Hz) focussed on the lower extremities. A total of 33 spatio-temporal, touchdown and toe-off joint angles, and horizontal and vertical foot velocity parameters were extracted through three-dimensional analysis. Group mean asymmetry scores were assessed using the symmetry angle (SA) where scores of 0% and 100% represent perfect symmetry and perfect asymmetry, respectively. Although considered generally low (SA <3% for 22 out of 33 parameters), the magnitude of mechanical asymmetry varied widely between sprinters of the same sex. However, there was no mean SA scores difference between men and women for any stride mechanical parameters (all p ≥ 0.064). Asymmetry scores were inconsistent between parameters and phases (touchdown vs toe-off instants), and sprinting mechanics were generally not related to asymmetry magnitudes. In summary, low to moderate asymmetry is a natural phenomenon in elite sprinting. Asymmetry was inconsistent between parameters and competitors during near maximum velocity running, yet mean values for a given parameter generally did not differ between sexes. Sprinters' performances were not related to their SA scores.

Roller Massage Prior to Running Does Not Affect Gait Mechanics in Well-Trained Runners.

CONTEXT: Understanding if roller massage prior to a run can mitigate fatigue-related decrements in muscle force production during prolonged running is important because of the association between fatigue and running-related injury. OBJECTIVE: The authors investigated whether a bout of roller massage prior to running would (1) mitigate fatigue-related increases in vertical average load rate and free moment of the ground reaction force of running and (2) mitigate decreases in maximal countermovement jump height. DESIGN: Repeated-measures study. SETTING: Laboratory. PARTICIPANTS: A total of 14 recreational endurance athletes (11 men and 3 women) volunteered for the study. INTERVENTIONS: A 12.5-minute foam roller protocol for the lower extremities and a fatiguing 30-minute treadmill run. MAIN OUTCOME MEASURES: Vertical average load rate, free moment, and maximal jump height before (PRE) and after (POST) the fatiguing treadmill run on separate experimental days: once where participants sat quietly prior to the fatiguing run (REST) and another where the foam roller protocol was performed prior to the run (ROLL). RESULTS: A 2-way multiple analysis of variance found no significant differences in vertical average load rate, free moment, and jump height between PRE/POST times in both REST/ROLL conditions. CONCLUSIONS: The authors concluded that recreational endurance athletes maintain running mechanics and jump performance after a fatiguing run regardless of prerun roller massage and may not rely on prerun roller massage as a form of injury prevention.

Running mechanics and leg muscle activity patterns during early and late acceleration phases of repeated treadmill sprints in male recreational athletes.

PURPOSE: We determined whether running mechanics and leg muscle activity patterns for pre-activation (50 ms prior to foot contact) and loading (first half, second half and entire stance) phases vary between early, late and entire acceleration phases during repeated treadmill sprints. METHODS: Ten male athletes performed three sets of five 5-s sprint accelerations (25-s and 3-min recovery between sprints and sets, respectively) on an instrumented treadmill. Ground reaction forces and surface EMG data (root mean square values of vastus lateralis, rectus femoris, biceps femoris, gastrocnemius medialis, gastrocnemius lateralis and tibialis anterior muscles of the right leg) corresponding to early, late and entire acceleration (steps 2, 4 and 6; steps 8, 10 and 12; and all steps, respectively) have been compared. RESULTS: Independently of fatigue, vertical and horizontal forces, contact time, step length, and step frequency differed as running velocity increased over different sprint acceleration sections (all P < 0.05). For pre-activation, first half, second half and entire stance phases taken separately, each of the six studied muscles displayed specific main sprint number and analysis section effects (all P < 0.05). However, there was in general no significant interaction between sprint number and analysis section (all P > 0.27). CONCLUSION: During repeated treadmill sprints, ground reaction force variables and leg muscle activity patterns can vary between early, late and entire acceleration phases. Identification of neuro-mechanical adjustments across the gait cycle with fatigue, however, did not differ when considering all steps or only a few steps during the early or late acceleration phases.

Three-dimensional CoM energetics, pelvis and lower limbs joint kinematics of uphill treadmill running at high speed.

The purpose of this study was to investigate the effects of slope on three-dimensional running kinematics at high speed. Thirteen male sprinters ran at high speed (7.5 m/s) on a motorised treadmill in each a level and a 5.0% slope condition. Three-dimensional motion analysis was conducted to compare centre of mass (CoM) energetics, pelvis segment and lower limb joints kinematics. We found that contact time was not affected by the slope, whereas flight time and step length were significantly shorter in uphill compared to level running. Uphill running reduced negative CoM work and increased positive CoM work compared to level running. Ankle, knee and hip joints were more flexed at initial ground contact, but only the knee was more extended at the end of stance in uphill compared to level running. Additionally, the hip joint was more abducted, and the free leg side of the pelvis was more elevated at the end of stance in uphill running. Our results demonstrate that joint motion must be developed from a more flexed/adducted position at initial contact through a greater range of motion compared to level running in order to meet the greater positive CoM work requirements in uphill running at high speed.

Comprehensive mechanical power analysis in sprint running acceleration.

Sprint running is a common feature of many sport activities. The ability of an athlete to cover a distance in the shortest time relies on his/her power production. The aim of this study was to provide an exhaustive description of the mechanical determinants of power output in sprint running acceleration and to check whether a predictive equation for internal power designed for steady locomotion is applicable to sprint running acceleration. Eighteen subjects performed two 20 m sprints in a gym. A 35-camera motion capture system recorded the 3D motion of the body segments and the body center of mass (BCoM) trajectory was computed. The mechanical power to accelerate and rise BCoM (external power, Pext ) and to accelerate the segments with respect to BCoM (internal power, Pint ) was calculated. In a 20 m sprint, the power to accelerate the body forward accounts for 50% of total power; Pint accounts for 41% and the power to rise BCoM accounts for 9% of total power. All the components of total mechanical power increase linearly with mean sprint velocity. A published equation for Pint prediction in steady locomotion has been adapted (the compound factor q accounting for the limbs' inertia decreases as a function of the distance within the sprint, differently from steady locomotion) and is still able to predict experimental Pint in a 20 m sprint with a bias of 0.70 ± 0.93 W kg-1 . This equation can be used to include Pint also in other methods that estimate external horizontal power only.

A cross-sectional study of the effects of load carriage on running characteristics and tibial mechanical stress: implications for stress-fracture injuries in women.

BACKGROUND: Load carriage is associated with musculoskeletal injuries, such as stress fractures, during military basic combat training. By investigating the influence of load carriage during exercises on the kinematics and kinetics of the body and on the biomechanical responses of bones, such as the tibia, we can quantify the role of load carriage on bone health. METHODS: We conducted a cross-sectional study using an integrated musculoskeletal-finite-element model to analyze how the amount of load carriage in women affected the kinematics and kinetics of the body, as well as the tibial mechanical stress during running. We also compared the biomechanics of walking (studied previously) and running under various load-carriage conditions. RESULTS: We observed substantial changes in both hip kinematics and kinetics during running when subjects carried a load. Relative to those observed during running without load, the joint reaction forces at the hip increased by an average of 49.1% body weight when subjects carried a load that was 30% of their body weight (ankle, 4.8%; knee, 20.6%). These results indicate that the hip extensor muscles in women are the main power generators when running with load carriage. When comparing running with walking, finite element analysis revealed that the peak tibial stress during running (tension, 90.6 MPa; compression, 136.2 MPa) was more than three times as great as that during walking (tension, 24.1 MPa; compression, 40.3 MPa), whereas the cumulative stress within one stride did not differ substantially between running (15.2 MPa · s) and walking (13.6 MPa · s). CONCLUSIONS: Our findings highlight the critical role of hip extensor muscles and their potential injury in women when running with load carriage. More importantly, our results underscore the need to incorporate the cumulative effect of mechanical stress when evaluating injury risk under various exercise conditions. The results from our study help to elucidate the mechanisms of stress fracture in women.

Estimates of Running Ground Reaction Force Parameters from Motion Analysis.

We compared running mechanics parameters determined from ground reaction force (GRF) measurements with estimated forces obtained from double differentiation of kinematic (K) data from motion analysis in a broad spectrum of running speeds (1.94-5.56 m⋅s-1). Data were collected through a force-instrumented treadmill and compared at different sampling frequencies (900 and 300 Hz for GRF, 300 and 100 Hz for K). Vertical force peak, shape, and impulse were similar between K methods and GRF. Contact time, flight time, and vertical stiffness (kvert) obtained from K showed the same trend as GRF with differences < 5%, whereas leg stiffness (kleg) was not correctly computed by kinematics. The results revealed that the main vertical GRF parameters can be computed by the double differentiation of the body center of mass properly calculated by motion analysis. The present model provides an alternative accessible method for determining temporal and kinetic parameters of running without an instrumented treadmill.

Does "Live High-Train Low (and High)" Hypoxic Training Alter Running Mechanics In Elite Team-sport Players?

This study aimed to investigate if "Live High-Train Low (and High)" hypoxic training alters constant-velocity running mechanics. While residing under normobaric hypoxia (≥14 h·d-1; FiO2 14.5-14.2%) for 14 days, twenty field hockey players performed, in addition to their usual training in normoxia, six sessions (4 × 5 × 5-s maximal sprints; 25 s passive recovery; 5 min rest) under either normobaric hypoxia (FiO2 ~14.5%, n = 9) or normoxia (FiO2 20.9%, n = 11). Before and immediately after the intervention, their running pattern was assessed at 10 and 15 km·h-1 as well as during six 30-s runs at ~20 km·h-1 with 30-s passive recovery on an instrumented motorised treadmill. No clear changes in running kinematics and spring-mass parameters occurred globally either at 10, 15 or ~20 km·h-1, with also no significant time × condition interaction for any parameters (p > 0.14). Independently of the condition, heart rate (all p < 0.05) and ratings of perceived exertion decreased post-intervention (only at 15 km·h-1, p < 0.05). Despite indirect signs for improved psycho-physiological responses, no forthright change in stride mechanical pattern occurred after "Live High-Train Low (and High)" hypoxic training.

A Comparison of Stride Length and Lower Extremity Kinematics during Barefoot and Shod Running in Well Trained Distance Runners.

Stride length, hip, knee and ankle angles were compared during barefoot and shod running on a treadmill at two speeds. Nine well-trained (1500m time: 3min:59.80s ± 14.7 s) male (22 ±3 years; 73 ±9 kg; 1.79 ±0.4 m) middle distance (800 m - 5,000 m) runners performed 2 minutes of running at 3.05 m·s-1 and 4.72 m·s-1 on an treadmill. This approach allowed continuous measurement of lower extremity kinematic data and calculation of stride length. Statistical analysis using a 2X2 factorial ANOVA revealed speed to have a main effect on stride length and hip angle and footwear to have a main effect on hip angle. There was a significant speed*footwear interaction for knee and ankle angles. Compared to shod running at the lower speed (3.05 m·s-1), well trained runners have greater hip, knee and ankle angles when running barefoot. Runners undertake a high volume (~75%) of training at lower intensities and therefore knowledge of how barefoot running alters running kinematics at low and high speeds may be useful to the runner.

The effect of a cadence retraining protocol on running biomechanics and efficiency: a pilot study.

Many studies have documented the association between mechanical deviations from normal and the presence or risk of injury. Some runners attempt to change mechanics by increasing running cadence. Previous work documented that increasing running cadence reduces deviations in mechanics tied to injury. The long-term effect of a cadence retraining intervention on running mechanics and energy expenditure is unknown. This study aimed to determine if increasing running cadence by 10% decreases running efficiency and changes kinematics and kinetics to make them less similar to those associated with injury. Additionally, this study aimed to determine if, after 6 weeks of cadence retraining, there would be carryover in kinematic and kinetic changes from an increased cadence state to a runner's preferred running cadence without decreased running efficiency. We measured oxygen uptake, kinematic and kinetic data on six uninjured participants before and after a 6-week intervention. Increasing cadence did not result in decreased running efficiency but did result in decreases in stride length, hip adduction angle and hip abductor moment. Carryover was observed in runners' post-intervention preferred running form as decreased hip adduction angle and vertical loading rate.

Does "Live High-Train Low and High" Hypoxic Training Alter Stride Mechanical Pattern During Repeated Sprints in Elite Team-Sport Players?

PURPOSE: We examined changes in stride temporal parameters and spring-mass model characteristics during repeated sprints following a 3-week period of "live high-train low and high" (LHTLH) altitude training in team-sport players. METHODS: While residing under normobaric hypoxia (≥14 h/d; inspired oxygen fraction [FiO2] 14.5%-14.2%) for 14 days, elite field hockey players performed, in addition to their regular field hockey practice in normoxia, 6 sessions (4 × 5 × 5-s maximal sprints; 25-s passive recovery; 5-min rest) under either normobaric hypoxia (LHTLH; FiO2 ∼14.5%, n = 11) or normoxia (live high-train low; FiO2 20.9%, n = 12). A control group (live low-train low; FiO2 ∼20.9%, n = 9) residing in normoxia without additional repeated-sprint training was included. Before (Pre) and a few days (Post-1) and 3 weeks (Post-2) after the intervention, stride mechanics were assessed during an overground repeated-sprint test (8 × 20 m, 20-s recovery). Two-way repeated-measures analysis of variance (time [Pre, Post-1, and Post-2] × condition [LHTLH, live high-train low, and live low-train low]) were conducted. RESULTS: Peak sprinting speed increased in LHTLH from Pre to Post-1 (+2.2% [2.0%]; P = .002) and Post-2 (+2.0% [2.4%]; P = .025), with no significant changes in live high-train low and live low-train low. There was no main effect of time (all P ≥ .062), condition (all P ≥ .771), or a significant time × condition interaction (all P ≥ .230) for any stride temporal variable (contact time, flight time, stride frequency, and stride length) or spring-mass model characteristics (vertical and leg stiffness). CONCLUSIONS: Peak sprinting speed improved in elite field hockey players following LHTLH altitude training, while stride mechanical adjustments to repeated overground sprints remained unchanged for at least 3 weeks postintervention.

The Determinants of Maximal Speed Adaptation During Preseason in Subelite Female Australian Rules Footballers.

PURPOSE: Maximal speed is an important physical-fitness attribute for female Australian footballers. The effects of sprint training, maximal strength, and technical training have been reported in laboratory studies. However, no study has determined the combined effect and relative contribution of these training modalities on maximal speed adaptation in situ. Therefore, the aim of this study was to determine the training factors affecting maximal speed adaptation during a preseason in subelite female Australian Rules footballers. METHODS: Maximal speed during field training, predicted 1-repetition maximum (1RM) for box squat and hip thrust, and sprint biomechanics were assessed during early and late preseason (∼9 wk apart) in 15 female subelite Australian Rules Football players (age 20 [3] y). On-field training volume and intensity (total distance, high-speed running, very-high-speed running, and maximal speed) were determined using a Global Positioning System. A multivariate regression model was used to determine the factors associated with changes in maximal speed across the preseason. RESULTS: The preseason training program had a small effect on maximal speed and large to very large effects on strength and sprint biomechanics. The multivariate regression with the greatest fit (P < .001, R2 = .939) included change in estimated 1RM box squat (ß = -0.63), total distance per week (ß = -0.55), and change in hip projection (ß = 0.16) as factors. Multivariate regression of biomechanical factors (P = .044, R2 = .717) and maximal strength factors (P = .003, R2 = .676) were also significant. CONCLUSION: The development of maximal speed across a preseason is dependent on (1) total distance per week, (2) maximal strength adaptation, and (3) sprint technique adaptation in female subelite Australian rules football players.

Constant low-to-moderate mechanical asymmetries during 800-m track running.

Introduction: Modifications in asymmetry in response to self-paced efforts have not been thoroughly documented, particularly regarding horizontally-derived ground reaction force variables. We determined the magnitude and range of gait asymmetries during 800 m track running. Methods: Eighteen physical education students completed an 800 m self-paced run on a 200 m indoor track. During the run, vertical and horizontal ground reaction forces were measured at a sampling frequency of 500 Hz using a 5 m-long force platform system, with data collected once per lap. The following mechanical variables were determined for two consecutive steps: contact time and duration of braking/push-off phases along with vertical/braking/push-off peak forces and impulses. The group mean asymmetry scores were evaluated using the "symmetry angle" (SA) formula, where scores of 0% and 100% correspond to perfect symmetry and perfect asymmetry, respectively. Results: There was no influence of distance interval on SA scores for any of the nine biomechanical variables (P ≥ 0.095). The SA scores were ∼1%-2% for contact time (1.3 ± 0.5%), peak vertical forces (1.8 ± 0.9%), and vertical impulse (1.7 ± 1.0%). The SA scores were ∼3%-8% for duration of braking (3.6 ± 1.1%) and push-off (3.2 ± 1.4%) phases, peak braking (5.0 ± 2.1%) and push-off (6.9 ± 3.1%) forces as well as braking (7.6 ± 2.3%) and push-off (7.7 ± 3.3%) impulses. The running velocity progressively decreased at 300 m and 500 m compared to that at 100 m but levelled off at 700 m (P < 0.001). Discussion: There were no modifications in gait asymmetries, as measured at 200-m distance intervals during 800-m track running in physical education students. The 800 m self-paced run did not impose greater mechanical constraints on one side of the body. Experimental procedures for characterizing the gait pattern during 800 m track running could be simplified by collecting leg mechanical data from only one side.

Relation between soft tissue energy dissipation and leg stiffness in running at different step frequencies.

This study aims to investigate the relationship between soft tissue energy dissipation and leg stiffness during running. Eight recreational healthy male runners (age: 22.2 ± 1.0 years; height: 1.84 ± 0.03 m; mass: 73.7 ± 5.7 kg) were asked to run at different speeds and step frequencies. Their soft tissue energy dissipation was estimated by the difference between the total mechanical work of the body, measured as the work done to move the body centre of mass relative to the surroundings plus the work to move the limbs relative to the body centre of mass, and lower-limb joint work. A mass-spring model with an actuator was used to analyse the force-length curve of the bouncing mechanism of running. In this way, the stiffness and damping coefficient were assessed at each speed and step frequency. Pearson's correlations were used to describe the relationship between the deviation from the spring-mass model and soft tissue energy fluctuations. The soft tissue dissipation was found to be significantly influenced by step frequency, with both positive and negative work phases decreasing when step frequency increases. Moreover, deviation from a spring-mass model was positively associated with the amount of soft tissue dissipation (r > 0.6). The findings emphasize the substantial role of soft tissues in dissipating or returning energy during running, behaving in a damped-elastic manner. Also, we introduce a novel approach for evaluating the elastic rebound of the body during running. The insights gained may have broad implications for assessing running mechanics, with potential applications in various contexts.

Kinetic and Spatiotemporal Characteristics of Running During Regular Training Sessions for Collegiate Male Distance Runners Using Shoe-Based Wearable Sensors.

CONTEXT: Assessment of running mechanics has traditionally been conducted in laboratory settings; the advancement of wearable technology permits data collection during outdoor training sessions. Exploring changes in running mechanics across training-session types may assist runners, coaches, and sports medicine clinicians in improving performance and managing the injury risk. OBJECTIVE: To examine changes in running mechanics on the basis of routine training-session types. DESIGN: Descriptive observational study. SETTING: Field based, university. METHODS: Running mechanics data (ie, impact g, stride length, braking g, total shock g, cadence, and ground contact time) for National Collegiate Athletic Association Division I distance runners (n = 20 men) were collected using RunScribe sensors mounted to the laces during training sessions (long run [LR], interval run [IR], or recovery run [RR]) during a 1-week period. RESULTS: Repeated-measures analysis of covariance with Greenhouse-Geisser correction and training-session pace as a covariate indicated no statistically significant differences in spatiotemporal or kinetic measures across the 3 training-session types. Cadence and stride length were inversely related in all training sessions (LR: r = -0.673, P = .004; IR: r = -0.893, P < .001; RR: r = -0.549, P = .023). Strong positive correlations were seen between impact g and total shock in all training sessions (LR: r = 0.894, P < .001; IR: r = 0.782, P = < .001; RR: r = 0.922, P < .001). Ground contact time increased with stride length during LR training sessions (r = 0.551, P = .027) and decreased with braking g in IR training sessions (r = -0.574, P = .016) and cadence in RR training sessions (r = -0.487, P = .048). CONCLUSIONS: Running mechanics in collegiate distance runners were not statistically different among training-session types when training-session pace was controlled. The use of wearable technology provides a tool for obtaining necessary data during overland training to inform training and program design.

Preseason Vertical Center of Mass Displacement During Running and Bone Mineral Density Z-Score Are Risk Factors for Bone Stress Injury Risk in Collegiate Cross-country Runners.

OBJECTIVES: To (1) assess relationships between running biomechanics, bone health, and bone stress injuries (BSIs), and (2) determine which variables constitute the most parsimonious BSI risk model among collegiate cross-country runners. DESIGN: Prospective, observational cohort study. METHODS: Running gait and bone mineral density (BMD) data from healthy collegiate cross-country runners were collected at preseason over 6 seasons. A generalized estimating equation model with backward selection was used to develop the most parsimonious model for estimating BSI risk, controlling for sex, running speed, and prior BSI. The variables assessed were spatiotemporal, ground reaction force, and joint kinematics, based on previous literature. Quasi-likelihood under the independence model criterion values and R2 values were used to select the best-fitting model. RESULTS: Data from 103 runners were included in the analysis. The best-fitting model included vertical center of mass (COM) displacement and BMD z-score. Injury risk increased with greater vertical COM displacement (unit = 0.5 cm; relative risk [RR] = 1.14; 95% confidence interval [CI]: 1.01, 1.29; P = .04) and decreased with greater BMD z-score (unit = 0.5; RR = 0.83; 95% CI: 0.72, 0.95; P = .007). The model performed similarly when step rate was included instead of vertical COM displacement. CONCLUSION: Vertical COM displacement and BMD z-score contributed to the best model for estimating risk the risk of bone stress injury in cross-country runners. Step rate was also an important variable for assessing injury risk. J Orthop Sports Phys Ther 2023;53(12):1-8. Epub 20 October 2023. doi:10.2519/jospt.2023.11860.

Asymmetries during repeated treadmill sprints in elite female Rugby Sevens players.

This study describes asymmetry in the main running mechanical variables during repeated treadmill sprints in elite female athletes and examines whether inter-limb differences in sprinting mechanics increase with fatigue. Eighteen elite female players (French national Rugby Sevens team) performed 8 × 5-s sprints (25-s rest) on an instrumented motorised sprint treadmill. The group mean 'symmetry angle' (SA) scores were ~1-2% for contact time (1.6 ± 0.6%), aerial time (2.1 ± 0.8%), step frequency (1.3 ± 0.5%) and step length (1.6 ± 0.6%). Mean vertical and horizontal forces, vertical and leg stiffness presented SA values of 1.7 ± 1.5%, 2.4 ± 1.2%, 2.6 ± 0.2% and 2.5 ± 0.2%, respectively. The SA scores were ~2-8% for duration of braking (6.9 ± 5.0%) and propulsive (6.5 ± 4.4%) phases, peak braking (6.5 ± 2.5%) and propulsive (1.6 ± 0.9%) forces as well as net (5.8 ± 5.6%), braking (7.7 ± 5.3%) and propulsive (2.7 ± 1.6%) impulses. However, there was no influence of sprint repetition number on SA scores for tested variables (P > 0.05). In elite female Rugby Sevens players, there was no noticeable difference in asymmetries for the great majority of stride mechanical variables during repeated treadmill sprints.

Muscle inertial contributions to ankle kinetics during the swing phase of running.

Skeletal muscles have inertia that leads to inertial forces acting around joints. Although these inertial muscle forces contribute to joint kinetics, they are not typically accounted for in musculoskeletal models used for human movement biomechanics research. Ignoring inertial forces can lead to errors in joint kinetics, but how large these errors are in inverse dynamics calculations of common movements is yet unclear. We, therefore, examined the role of shank muscle inertia on ankle joint moments during the swing phase of running at different speeds. A custom musculoskeletal modelling and simulation platform was used to perform inverse dynamics with a model that either combined muscle mass in the total shank mass, or considered the gastrocnemius lateralis/medialis, soleus, and tibialis anterior muscles as separate masses from the shank. Ankle moments were considerably affected when muscles were modelled as separate masses, with a general shift towards reduced dorsiflexion and higher plantarflexion moments. Differences between both modelling conditions increased with running speed and ranged between 0.8 and 1.6 Nm (ankle moment profile root mean square error), 8-18 % (peak dorsiflexion moment difference) and 24-42 % (peak plantarflexion moment difference). Moreover, we observed a complex combination of inertial forces, especially those due to rotation and translation of the shank, in which the direction of inertial force changed during the swing phase. These results show that ignoring muscle inertia in musculoskeletal models can lead to under- or overestimations of structure-specific loads and thus erroneous study conclusions. Our results suggest that muscle inertial forces should be carefully considered when using musculoskeletal models.

Mechanical asymmetries remain low-to-moderate during 30 min of self-paced treadmill running.

Introduction: We characterized the magnitude and range of gait asymmetry during self-paced treadmill running. Methods: On an instrumented treadmill, twelve trained runners (11 males, 1 female) completed a 30-min self-paced run, during which participants were instructed to cover the most distance possible. Ground reaction force recordings at a constant velocity corresponding to 70% of their maximal aerobic velocity (13.3 ± 0.8 km.h-1) allowed for the measurement of running kinetics and kinematics, as well as the calculation of spring-mass characteristics at the beginning, middle, and end of the run (minutes 1, 14, and 29, respectively). Group mean asymmetry scores were assessed using the "symmetry angle" (SA) formulae, where scores of 0% and 100% represent perfect symmetry and perfect asymmetry, respectively. Results: There was no time effect on SA scores for any of the 13 biomechanical variables (p ≥ 0.128). Mean SA scores were <2.5% for contact time (0.8% ± 0.7%), flight time (1.4% ± 0.6%), step frequency (0.7% ± 0.3%), duty factor (0.7% ± 0.3%), duration of braking (1.3% ± 0.7%) and push-off phases (0.9% ± 0.8%), as well as peak braking (2.3% ± 1.3%) and push-off forces (1.4% ± 0.9%). Mean SA scores were ≥2.5% for peak vertical loading rate (3.1% ± 1.7%), mean vertical loading rate (3.4% ± 2.1%), peak vertical forces (2.9% ± 2.2%), as well as vertical stiffness (5.2% ± 3.5%) and leg stiffness (2.5% ± 1.5%). Conclusion: Throughout a 30-min running time trial, there were consistently low-to-moderate mechanical asymmetries for spatiotemporal variables, kinetics, and spring-mass model characteristics. This suggests that trained runners maintained relatively even strides during the self-paced treadmill run, with lower extremities behaving similarly when controlling for velocity.