Background: Barbell squats are commonly used in strength training, but the anterior–posterior displacement of the Center of Mass (COM) may impair joint stability and increase injury risk. This study investigates the key factors influencing COM displacement during different squat modes.; Methods: This study recruited 15 male strength training enthusiasts, who performed 60% of their one-repetition maximum (1RM) in the Front Barbell Squat (FBS), High Bar Back Squat (HBBS), and Low Bar Back Squat (LBBS). Joint moments at both the hip, knee, and ankle were collected using a motion capture system and force plates, and a factor regression analysis was conducted using SPSS.; Results: In the FBS, primary factors influencing COM displacement included right knee adduction–abduction (38.59%), knee flexion–extension (31.08%), and hip internal–external rotation (29.83%). In the HBBS, they were right ankle internal–external rotation (19.13%), hip flexion–extension (−19.07%), and left knee flexion–extension (19.05%). In the LBBS, the key factors were left knee adduction–abduction (27.82%), right ankle internal–external rotation (27.59%), and left ankle internal–external rotation (26.12%).; Conclusion: The study identifies key factors affecting COM displacement across squat modes, with knee flexion–extension being dominant in the FBS and hip moments more significant in the HBBS and LBBS. These findings have implications for optimizing squat training and injury prevention strategies.

This investigation involved examining whether Rest-Redistribution Set (RR) and Traditional-Set (TS) resistance priming (RP) protocols could induce a Delayed Potentiation Effect (DPE). Additionally, it was analyzed whether the RR protocol demonstrated significant advantages over the TS protocol in various performance parameters across different testing time points following the RP. Twenty-four male collegiate Physical Education majors participated in this study, each completing two resistance protocols involving the back squat exercise: (1) a RR protocol incorporating a 30-s intra-set rest and a 200-s inter-set rest, and (2) a TS protocol without intra-set rest and with a 240-s inter-set rest. Both protocols comprised four sets of five repetitions performed at 85 % of one-repetition maximum (1RM). Performance outcomes, including countermovement jump (CMJ), sprint time, Y-balance test scores, change of direction (COD) ability, and agility, were evaluated at three time points: baseline (Pre), 6 h post-exercise (Post-6h), and 24 h post-exercise (Post-24h). Data were analyzed using a two-way repeated-measures analysis of variance (ANOVA) with factors for protocol (RR vs. TS) and time (Pre, Post-6h, Post-24h). When significant main effects or interaction effects were identified, Bonferroni-adjusted post hoc pairwise comparisons were conducted. (1) A significant interaction between RP protocols and testing time points was observed for CMJ, sprint, and agility performance (P < 0.05). Simple effects of time: At Post-6h, both RP protocols elicited significant DPE across most metrics. Specifically, the TS protocol improved sprint (P < 0.001) and agility (P < 0.001) compared to Pre, whereas the RR protocol enhanced CMJ (P = 0.003), sprint (P < 0.001), and agility (P < 0.001). By Post-24h, TS showed significant declines in CMJ (P = 0.007) and sprint (P < 0.001) relative to Pre, while RR maintained improvements in agility (P = 0.001). Simple effects of RP protocol: At Post-6h, RR outperformed TS in CMJ (P = 0.005), sprint (P = 0.013), and agility (P = 0.005). At Post-24h, RR demonstrated significantly superior CMJ performance versus TS (P = 0.014). (2) No significant interaction effects between RP and testing time points were observed in COD or Y-balance tests (P > 0.05). However, time-dependent effects emerged: COD improved at Post-6h compared to Pre (P = 0.005), while Y-balance performance showed enhancements at both Post-6h and Post-24h compared to Pre (P < 0.001), with Post-24h values exceeding Post-6h (P < 0.001). Both the RR and TS protocols induced DPE at the Post-6h mark. The RR protocol improved CMJ, sprint, and agility performance, while the TS protocol only enhanced sprint and agility metrics. In comparative terms, RR demonstrated a significantly greater DPE than TS across CMJ, sprint, and agility metrics. At the Post-24h mark, DPE diminished for most measures (with the exception of Y-balance), with RR maintaining CMJ and sprint performance levels close to baseline and demonstrating a significantly enhanced agility performance compared to baseline. Furthermore, RR outperformed TS in CMJ performance. The RP protocol also produced DPE in COD and Y-balance performance, with Y-balance achieving its highest recorded value at Post-24h. Although RR exhibited stronger overall efficacy, its effects were specific to certain performance parameters, indicating a need for further validation.

Cai, J, Sun, D, Li, F, Chen, D, Zhu, J, Song, Y, Fan, Y, Goda, TJ, and Gu, Y. Dose-dependent effects of heel elevation on kinetic chain reconstruction and lumbar stress during loaded squatting in men with restricted ankle dorsiflexion. J Strength Cond Res XX(X): 000–000, 2026—The aim of this study was to determine whether graded heel elevation modifies squat biomechanics, neuromuscular activity, and lumbar stress in men with restricted ankle dorsiflexion. Thirty healthy men performed loaded squats under 6 heel-elevation conditions (0, 1, 2, 3, 4, and 5 cm). Three-dimensional motion capture, force plates, and surface electromyography were used to assess lower-limb kinematics, net joint moments, center-of-pressure variables, muscle activity, and co-contraction. Finite element analysis was used to quantify peak von Mises stress in the L1–L5 segments. Discrete outcomes were analyzed using repeated-measures analysis of variance, and joint-moment waveforms were compared using one-dimensional statistical parametric mapping (SPM1D), with significance set at p ≤ 0.05. Increasing heel elevation reduced peak ankle dorsiflexion and trunk forward inclination, while increasing peak knee flexion, hip flexion, and tibial progression angle (all p < 0.001). Peak knee moment increased with heel elevation ( p < 0.001), whereas ankle and hip moments were not significantly changed. SPM1D revealed prolonged increases in knee moment during the middle phase of the squat cycle. Center-of-pressure path length increased at 2–3 cm and declined at higher elevations. Selected muscle activities and co-contraction indices increased, whereas peak von Mises stress in L3–L5 decreased overall, with the greatest reduction at L4. These findings show that heel elevation produces a dose-dependent redistribution of the lower-limb-to-trunk kinetic chain. Practitioners may use heel lifts to promote deeper, more upright squats, but excessive elevation may increase knee extensor demand.

Background: The open-stance forehand is a fundamental technique in tennis, playing a crucial role in competitive performance. Its execution depends heavily on lower limb coordination and neuromuscular control. Athletes of different skill levels often display distinct muscle activation strategies. This study employs non-negative matrix factorization (NMF) to analyze lower limb muscle synergy patterns during the forehand open stance across skill levels and explores their potential influence on stroke performance. Methods: A total of 30 tennis players, including 15 elite and 15 amateur athletes, participated in this study. Surface electromyography (sEMG) was used to record the activity of major lower limb muscles during the forehand open stance. Muscle synergy patterns were extracted using NMF, and K-means clustering was applied to classify synergy patterns. Independent sample t-tests were conducted to examine differences between muscle synergies. Results: Significant differences (p < 0.05) were observed in the spatial characteristics of each synergy component across different movement phases. However, temporal characteristics showed a significant difference only in Syn2 during the mid-phase of the backswing (BS) (56.2–60.4%) (p = 0.033). Conclusions: Elite athletes exhibited more optimized and stable muscle activation patterns, enabling more efficient coordination of major muscle groups. Based on sEMG decomposition and muscle synergy analysis, these activation patterns may contribute to improved stroke efficiency and energy transfer and potentially reduce the risk of sports-related injuries.

(1) Background: Changes in midsole hardness may affect lower-limb impact loading during forefoot strike (FFS) running in children, yet the biomechanical basis for discriminating elevated VALR-related loading remains unclear. (2) Methods: Fourteen school-aged boys performed FFS running tests in experimental shoes with four midsole hardness levels (37, 42, 47, and 52 Shore C). Lower-limb kinematics and surface electromyography (sEMG) data were collected during the dominant leg stance phase. After preprocessing, VALR was calculated from 336 valid trials, and 28 stance-phase biomechanical features were extracted, yielding a final machine-learning dataset of 324 trials after excluding incomplete feature data. VALR was used to compare loading changes and define trial-level elevated-loading labels based on the median VALR value. Classification models were evaluated under participant-level GroupKFold validation, and XGBoost was retained for exploratory SHAP analysis. (3) Results: VALR showed an upward trend with increasing hardness, but no statistically supported change point was identified. XGBoost achieved an accuracy of 75.93%, precision of 74.14%, recall of 79.63%, F1-value of 0.768, and pooled out-of-fold AUC of 0.738. SHAP analysis indicated that distal and non-sagittal kinematic features contributed most to model classification. (4) Conclusions: Elevated VALR-related loading during children’s FFS running may be characterized by a multi-feature model-based pattern rather than a fixed midsole hardness threshold.

The instep kick is a frequently performed, high-load soccer skill that requires precise intermuscular coordination around ball contact. This study compared time-frequency activation patterns between elite and amateur male soccer players across two kicking phases and identified the key time windows and dominant muscle contributions. Surface electromyography was recorded from seven muscles of the kicking leg in 15 elite and 15 amateur players: rectus femoris, vastus medialis, vastus lateralis, biceps femoris, tibialis anterior, medial gastrocnemius, and lateral gastrocnemius. The instep kick was divided into Phase 1, from maximum hip extension to ball contact, and Phase 2, from post-contact follow-through to maximum hip flexion. Morlet wavelet analysis was used to generate time-frequency intensity maps. Global principal component analysis was then performed to extract PC1 to PC6, and between-group differences in PC score trajectories were assessed using SPM1D independent-samples t tests, with significance evaluated under a Bonferroni adjusted threshold for the 12 PC phase comparisons. In Phase 1, PC3 showed a significant between-group difference at 54-75% of the normalized time (peak t = 7.62, p < 0.001, Hedges’ g = 2.96), with higher scores in the elite group. In Phase 2, PC1 differed significantly between groups at 60-70% of the normalized time (peak t = -9.88, p < 0.001, Hedges’ g = -2.03), with higher scores in the amateur group. Elite players showed a more temporally focused pre-contact activation pattern centered on mid-frequency components of the biceps femoris, whereas amateur players showed greater reliance on higher-frequency components of the medial gastrocnemius during the post-contact follow-through. These findings suggest that skill-level differences are expressed as phase-specific redistribution of muscle and frequency-band activity within critical time windows, rather than as a simple global difference in activation magnitude.