Footstrike angle cut-off values to classify footstrike pattern in runners.

Footstrike angle (FSA) has been widely used to classify footstrike pattern (FSP). However, inconsistent FSA cut-off values were adopted in previous studies. This study aimed to validate the FSA cut-off values in runners. Stride index, the gold standard to determine FSP, and FSA were obtained when 15 experienced runners, 14 novice runners and 14 untrained individuals performed 3-min run on an instrumented treadmill at their preferred running speeds in habitual, rearfoot, midfoot and forefoot strike patterns. According to the receiver operating characteristic curve associated with the Youden index, the optimal FSA cut-off values were -0.8° (i.e., cut-off angle for forefoot strike) -7.4° (i.e., cut-off angle for rearfoot strike) for runners. We observed minor differences in the FSA cut-off values across runners with various running experience and a wider cut-off range for midfoot strikers when a modified strike index was utilized. This validation study established cut-off footstrike angles for runners' FSP classification.

Ankle Muscle Activations during Different Foot-Strike Patterns in Running.

This study analysed the landing performance and muscle activity of athletes in forefoot strike (FFS) and rearfoot strike (RFS) patterns. Ten male college participants were asked to perform two foot strikes patterns, each at a running speed of 6 km/h. Three inertial sensors and five EMG sensors as well as one 24 G accelerometer were synchronised to acquire joint kinematics parameters as well as muscle activation, respectively. In both the FFS and RFS patterns, according to the intraclass correlation coefficient, excellent reliability was found for landing performance and muscle activation. Paired t tests indicated significantly higher ankle plantar flexion in the FFS pattern. Moreover, biceps femoris (BF) and gastrocnemius medialis (GM) activation increased in the pre-stance phase of the FFS compared with that of RFS. The FFS pattern had significantly decreased tibialis anterior (TA) muscle activity compared with the RFS pattern during the pre-stance phase. The results demonstrated that the ankle strategy focused on controlling the foot strike pattern. The influence of the FFS pattern on muscle activity likely indicates that an athlete can increase both BF and GM muscles activity. Altered landing strategy in cases of FFS pattern may contribute both to the running efficiency and muscle activation of the lower extremity. Therefore, neuromuscular training and education are required to enable activation in dynamic running tasks.

A new method applied for explaining the landing patterns: Interpretability analysis of machine learning.

As one of many fundamental sports techniques, the landing maneuver is also frequently used in clinical injury screening and diagnosis. However, the landing patterns are different under different constraints, which will cause great difficulties for clinical experts in clinical diagnosis. Machine learning (ML) have been very successful in solving a variety of clinical diagnosis tasks, but they all have the disadvantage of being black boxes and rarely provide and explain useful information about the reasons for making a particular decision. The current work validates the feasibility of applying an explainable ML (XML) model constructed by Layer-wise Relevance Propagation (LRP) for landing pattern recognition in clinical biomechanics. This study collected 560 groups landing data. By incorporating these landing data into the XML model as input signals, the prediction results were interpreted based on the relevance score (RS) derived from LRP. The interpretation obtained from XML was evaluated comprehensively from the statistical perspective based on Statistical Parametric Mapping (SPM) and Effect Size. The RS has excellent statistical characteristics in the interpretation of landing patterns between classes, and also conforms to the clinical characteristics of landing pattern recognition. The current work highlights the applicability of XML methods that can not only satisfy the traditional decision problem between classes, but also largely solve the lack of transparency in landing pattern recognition. We provide a feasible framework for realizing interpretability of ML decision results in landing analysis, providing a methodological reference and solid foundation for future clinical diagnosis and biomechanical analysis.

Strike index estimation using a convolutional neural network with a single, shoe-mounted inertial sensor.

Strike index is a measurement of the center of pressure position relative to the foot length, and it is regarded as a gold standard in classifying strike pattern in runners. However, strike index requires sophisticated laboratory equipment, e.g., force plates and optical motion capture. We present a method of estimating strike index using data from a shoe-mounted inertial measurement unit (IMU) analyzed by a participant-independent convolutional neural network (CNN), which consists of convolutional, max-pooling, and fully-connected layers. To promote data variability, 16 participants were required to land with three strike patterns (rearfoot, midfoot, and forefoot strike) while running on an instrumented treadmill in four conditions i.e., two footwear types and two running speeds. Using the proposed approach, strike index was estimated with a root mean square error of 6.9% and a R2 of 0.89. Training and testing the model with different variations of the data collected showed that the model was robust to changes in speed. The proposed approach enables accurate estimation of strike index outside of traditional gait laboratories. This solution potentially improves running performance and reduces injury risk in distance runners.

Impact Sound Across Rearfoot, Midfoot, and Forefoot Strike During Overground Running.

CONTEXT: Three foot-strike techniques are common in runners. If these techniques generate different sounds at the point of impact with the ground, lower limb kinetics may be influenced. No previous authors have determined whether such relationships exist. OBJECTIVES: To determine foot-ground impact sound characteristics and compare the impact-sound characteristics across foot-strike techniques and the relationships between impact-sound characteristics and vertical loading rates. DESIGN: Cross-sectional study. SETTING: Gait analysis laboratory. PATIENTS OR OTHER PARTICIPANTS: A total of 30 runners (15 women, 15 men; age = 23.5 ± 4.0 years, height = 1.67 ± 0.1 m, mass = 58.1 ± 8.2 kg) completed overground running trials with rearfoot-strike, midfoot-strike (MFS), and forefoot-strike (FFS) techniques in a gait analysis laboratory. MAIN OUTCOME MEASURE(S): Impact sound was measured using a shotgun microphone, and the peak sound amplitude, median frequency, and sound duration were analyzed. Separate linear regressions, clustering participants due to repeated measures, were used to compare the sound characteristics across foot-strike techniques. Kinetic data were collected from a force plate, and the vertical loading rates were calculated. Pearson correlation was used to determine the relationship between sound characteristics and kinetics. RESULTS: Landing with an MFS or FFS resulted in greater peak sound amplitude (P < .001) and shorter sound duration (P < .001) than a rearfoot strike. The MFS exhibited the highest median frequency among the 3 foot-strike patterns, followed by the FFS (P < .001). We did not find a significant relationship between vertical loading rates and any impact sound characteristics (P > .115). CONCLUSIONS: The results suggest that impact-sound characteristics may be used to differentiate foot-strike patterns in runners. However, these did not relate to lower limb kinetics. Therefore, clinicians should not solely rely on impact sound to infer impact loading.

Self-selected running gait modifications reduce acute impact loading, awkwardness, and effort.

Impact loading has been associated with running-related injuries, and gait retraining has been suggested as a means of reducing impact loading and lowering the risk of injury. However, gait retraining can lead to increased perceived awkwardness and effort. The influence of specifically trained and self-selected running gait modifications on acute impact loading, perceived awkwardness and effort is currently unclear. Sixteen habitual rearfoot/midfoot runners performed forefoot strike pattern, increased step rate, anterior trunk lean and self-selected running gait modifications on an instrumented treadmill based on real-time biofeedback. Impact loading, perceived awkwardness and effort scores were compared among the four gait retraining conditions. Self-selected gait modification reduced vertical average loading rate (VALR) by 25.3%, vertical instantaneous loading rate (VILR) by 27.0%, vertical impact peak (VIP) by 16.8% as compared with baseline. Forefoot strike pattern reduced VALR, VILR and peak tibial acceleration. Increased step rate reduced VALR. Anterior trunk lean did not reduce any impact loading. Self-selected gait modification was perceived as less awkward and require less effort than the specifically trained gait modification (p < 0.05). These findings suggest that self-selected gait modification could be a more natural and less effortful strategy than specifically trained gait modification to reduce acute impact loading, while the clinical significance remains unknown.

Effect of minimalist and maximalist shoes on impact loading and footstrike pattern in habitual rearfoot strike trail runners: An in-field study.

Running-related injuries among trail runners are very common and footwear selection may modulate the injury risk. However, most previous studies were conducted in a laboratory environment. The objective of this study was to examine the effects of two contrasting footwear designs, minimalist (MIN) and maximalist shoes (MAX), on the running biomechanics of trail runners during running on a natural trail. Eighteen habitual rearfoot strike trail runners completed level, uphill and downhill running at their preferred speeds in both shod conditions. Peak tibial acceleration, strike index and footstrike pattern were compared between the two footwear and slopes. Interactions of footwear and slope were not detected for all the selected variables. There was no significant effect from footwear (F = 1.23, p = 0.27) and slope (F = 2.49, p = 0.09) on peak tibial acceleration and there was no footwear effect on strike index (F = 3.82, p = 0.056). A significant main effect of slope on strike index (F = 13.24, p < 0.001) was found. Strike index during uphill running was significantly greater (i.e. landing with a more anterior foot strike) when compared with level (p < 0.001, Cohen's d = 1.72) or downhill running (p < 0.001, Cohen's d = 1.44) in either MIN or MAX. The majority of habitual rearfoot strike runners switched to midfoot strike during uphill running while maintaining a rearfoot strike pattern during level or downhill running. In summary, wearing either one of the two contrasting footwear (MIN or MAX) demonstrated no effect on impact loading and footstrike pattern in habitual rearfoot strike trail runners running on a natural trail with different slopes.

The effects of midfoot strike gait retraining on impact loading and joint stiffness.

OBJECTIVE: To assess the biomechanical changes following a systematic gait retraining to modify footstrike patterns from rearfoot strike (RFS) to midfoot strike (MFS). DESIGN: Pre-post interventional study. All participants underwent a gait retraining program designed to modify footstrike pattern to MFS. SETTING: Research laboratory. PARTICIPANTS: Twenty habitual RFS male runners participated. MAIN OUTCOME MEASURES: Gait evaluations were conducted before and after the training. Footstrike pattern, vertical loading rates, ankle and knee joint stiffness were compared. RESULTS: Participants' footstrike angle was reduced (p < 0.001, Cohen's d = 1.65) and knee joint stiffness was increased (p = 0.003, Cohen's d = 0.69). No significant difference was found in the vertical loading rates (p > 0.155). Further subgroup analyses were conducted on the respondents (n = 8, 40% of participants) who exhibited MFS for over 80% of their footfalls during the post-training evaluation. Apart from the increased knee joint stiffness (p = 0.005, Cohen's d = 1.14), respondents exhibited a significant reduction in the ankle joint stiffness (p = 0.019, Cohen's d = 1.17) when running with MFS. CONCLUSIONS: Gait retraining to promote MFS was effective in reducing runners' footstrike angle, but only 40% of participants responded to this training program. The inconsistent training effect on impact loading suggests a need to develop new training protocols in an effort to prevent running injuries.

Foot strike pattern, step rate, and trunk posture combined gait modifications to reduce impact loading during running.

Elevated impact loading can be detrimental to runners as it has been linked to the increased risk of tibial stress fracture and plantar fasciitis. The objective of this study was to investigate the combined effects of foot strike pattern, step rate, and anterior trunk lean gait modifications on impact loading in runners. Nineteen healthy runners performed 12 separate gait modification trials involving: three foot strike patterns (rearfoot, midfoot, and forefoot strike), two step rates (natural and 10% increased), and two anterior trunk lean postures (natural and 10-degree increased flexion). Overall, forefoot strike combined with increased step rate led to the lowest impact loading rates, and rearfoot strike combined with anterior trunk lean led to the highest impact loading rates. In addition, there were interaction effects between foot strike pattern and step rate on awkwardness and effort, such that it was both more natural and easier to transition to a combined gait modification involving forefoot strike and increased step rate than to an isolated gait modification involving either forefoot strike or increased step rate. These findings could help to inform gait modifications for runners to reduce impact loading and associated injury risks.

The effect of foot landing position on biomechanical risk factors associated with anterior cruciate ligament injury.

BACKGROUND: Identification of biomechanical risk factors associated with anterior cruciate ligament (ACL) injury can facilitate injury prevention. The purpose of this study is to investigate the effects of three foot landing positions, "toe-in", "toe-out" and "neutral", on biomechanical risk factors for ACL injury in males and females. The authors hypothesize that 1) relative to neutral, the toe-in position increases the biomechanical risk factors for ACL injury, 2) the toe-out position decreases these biomechanical risk factors, and 3) compared to males, females demonstrate greater changes in lower extremity biomechanics with changes in foot landing position. METHODS: Motion capture data on ten male and ten female volunteers aged 20-30 years (26.4 ± 2.50) were collected during double-leg jump landing activities. Subjects were asked to land on force plates and target one of three pre-templated foot landing positions: 0° ("neutral"), 30° internal rotation ("toe-in"), and 30° external rotation ("toe-out") along the axis of the anatomical sagittal plane. A mixed-effects ANOVA and pairwise Tukey post-hoc comparison were used to detect differences in kinematic and kinetic variables associated with biomechanical risk factors of ACL injury between the three foot landing positions. RESULTS: Relative to neutral, landing in the toe-in position increased peak hip adduction, knee internal rotation angles and moments (p < 0.01), and peak knee abduction angle (p < 0.001). Landing in the toe-in position also decreased peak hip flexion angle (p < 0.001) and knee flexion angle (p = 0.023). Landing in the toe-out position decreased peak hip adduction, knee abduction, and knee internal rotation angles (all p < 0.001). Male sex was associated with a smaller increase in hip adduction moment (p = 0.043) and knee internal rotation moment (p = 0.032) with toe-in landing position compared with female sex. CONCLUSIONS: Toe-in landing position exacerbates biomechanical risk factors associated with ACL injury, while toe-out landing position decreases these factors.