ECSS Paris 2023: OP-BM06
INTRODUCTION: Squat is one of the most commonly performed exercises in strength training, conditioning, and rehabilitation [1,2]. While “ideal” forms are often instructed, factors such as skeletal ratios and hip morphology can substantially affect movement strategies [3,4]. Previous studies have primarily focused on local constraints or final posture, with limited attention to effects of kinematic strategies during descent to maximal flexion. The purpose of this study was to examine how differences in initial movement instruction influence subsequent kinematics and joint load distribution throughout the squat. METHODS: Twenty-one healthy male adults (age 24.0 ± 4.0 years; body mass 61.9 ± 7.8 kg) performed squats under three conditions: Normal, Knee-initiated, and Hip-initiated. Each trial consisted of a 3-s descent, 3-s hold, and 3-s ascent. Three-dimensional motion capture, force plates, and surface electromyography (sEMG) were used to collect data. Repeated measures ANOVA and Friedman tests were applied to time-series data. RESULTS: Movement instructions successfully changed descent-phase kinematics, with significantly increased hip or knee flexion contributions during the first 0–10% of descent for their respective conditions. In the descent phase, Hip-initiated condition delayed knee and ankle flexion, whereas Knee-initiated condition delayed hip flexion but accelerated ankle flexion. During the hold phase, Knee-initiated condition showed significantly larger Center of Pressure (CoP) sway (95% ellipse area and anterior-posterior [AP] standard deviation) compared to Normal. By the end of descent, joint angles in Hip- and Knee-initiated conditions tended to converge toward Normal, but cumulative joint moments (integrated over time) established at the start persisted throughout the descent. Ascent-phase kinematics showed no notable differences between conditions. CONCLUSION: Initial movement strategy substantially influences descent and hold-phase kinematics and joint load distribution, while ascent-phase kinematics are largely unaffected. Even when final posture appears similar, internal states may differ depending on early movement patterns. Increased CoP sway in Knee-initiated movements suggests that initial strategy is critical for both performance and injury prevention. These findings highlight the importance of considering early movement processes, in addition to final posture, when defining individually optimized squat strategies. REFERENCES: 1) Schoenfeld BJ. J Strength Cond Res. 2010 2) Bashir et al. PLOS ONE. 2022 3) Myer GD et al. Strength Cond J. 2014 4) Song K et al. J Orthop Res. 2022
Read CV Yuichi SakamotoECSS Paris 2023: OP-BM06
INTRODUCTION: The Dynamic Strength Index (DSI) is widely used to guide strength–power training prescription by expressing maximal ballistic force relative to maximal isometric force. Although the ballistic peak force is commonly derived from the countermovement jump (CMJ), the drop jump (DJ) may better reflect reactive stretch–shortening cycle demands. The aim of this study was to determine if the choice of ballistic task (CMJ vs DJ) influences DSI values and athlete classification, and to examine relationships between DSI and jump performance. METHODS: Twenty-nine elite female netballers completed a maximum effort isometric squat push (ISP), CMJ, and DJ using force-plate systems (VALD Performance). Jump height was determined using the impulse–momentum method. DSI derived from CMJ (DSI_CMJ) was obtained directly from the software, while DSI derived from DJ (DSI_DJ) was calculated as DJ peak concentric force divided by ISP peak force. Athletes were categorised using established DSI thresholds (<0.60 ballistic emphasis, 0.60–0.80 concurrent, >0.80 maximal force). Descriptive statistics were calculated, normality assessed, and Pearson or Spearman correlations performed. Agreement between DSI methods was examined using Bland–Altman analysis. One-way ANOVA was used for group comparisons (p<0.05). RESULTS: Median CMJ and DJ heights were 28.1 cm (IQR 4.9) and 25.6 cm (IQR 5.4), respectively. Mean DJ peak concentric force (3169 ± 886 N) exceeded ISP force (2091 ± 370 N). Using DSI_CMJ, 31% of athletes were classified <0.60, 59% between 0.60–0.80, and 10% >0.80. In contrast, DSI_DJ classified 100% of athletes >0.80. Mean CMJ heights across DSI_CMJ categories were 27.0 cm (<0.60), 29.9 cm (0.60–0.80), and 26.7 cm (>0.80), while DJ heights were 26.1 cm, 29.6 cm, and 23.1 cm, respectively. No significant (p>0.05) differences in CMJ or DJ height were observed between categories. No significant correlations were found between DSI_CMJ and CMJ height (r=0.21; 95%CI: -0.15-0.55), DJ height (r=0.12; 95%CI:-0.42-0.31), or DJ:CMJ height ratio (r=–0.29; 95%CI:-0.66-0.09). DSI_DJ also showed no meaningful associations with jump height. Agreement between DSI_CMJ and DSI_DJ was poor (r=-0.23; 95%CI:-0.67-0.09). CONCLUSION: DSI values and athlete classification were highly sensitive to the ballistic task used, with DJ-derived DSI uniformly categorising athletes as requiring maximal force training. Importantly, neither DSI approach was associated with jump performance during stretch–shortening cycle–challenging tasks. Practitioners should therefore exercise caution when using DSI for exercise prescription, as DSI may not reflect functional explosive performance. A more task-specific, multi-metric approach incorporating jump kinetics and performance outcomes may provide superior insight for individualized strength–power profiling and training design. These findings question the interchangeability and practical validity of DSI-based decision-making in applied settings.
Read CV Candice MacMillanECSS Paris 2023: OP-BM06
INTRODUCTION: Countermovement jump (CMJ) is widely used to assess lower-limb neuromuscular function and stretch–shortening cycle (SSC) efficiency.(Philipp et al., 2023)However, CMJ outcomes are sensitive to verbal instructions, which may alter movement strategies and mechanical outputs, ultimately compromising test reliability. (Khuu et al., 2015) This study investigated the effects of varying verbal instructions (“Jump high” or “Jump fast” ) on the mechanical properties in elite pole vaulters. METHODS: Forty-one national pole vaulters (23 males, 18 females, status = ± cm, body mass = kg, age = years) performed three CMJs on a three-dimensional force platform (KWYP-FP6050, Kunwei; 1000 Hz). Athletes were randomly assigned to either a height-focused instruction (“Jump as high as possible”, High) or a speed-focused instruction (“Jump as fast as possible”, Fast). The CMJ mechanical properties (i.e., jump height [JH], ground contact time [GCT], braking phase duration [T Bra], propulsive phase duration [T Pro], countermovement depth [CMD], braking peak force [PF Bra], propulsive peak force [PF Pro], and modified reactive strength index [RSImod]) were calculated. RESULTS: Independent samples t-test indicates that the High condition produced greater JH (0.36 ± 0.07 m vs. 0.33 ± 0.06 m; p = 0.02, d = 0.42). The Fast condition significantly reduced GCT (0.59 ± 0.10 s vs. 0.68 ± 0.11 s; p < 0.001, d = 0.90), primarily due to shorter T Bra (0.13 ± 0.03 s vs. 0.16 ± 0.04 s; p < 0.001, d =0.9 ). PF Bra was greater in the Fast condition (1524.57 ± 345.39 N vs. 1273.03 ± 320.75 N; p < 0.001, d = 0.75), as was PF Pro(0.21±0.04 s vs. 0.24±0.04 s; p < 0.012，d =0.46). RSImod was also higher under the speed instruction (0.58 ± 0.14 vs. 0.54 ± 0.11; p = 0.05, d = 0.36). CONCLUSION: Different verbal instructions substantially modify CMJ biomechanics in pole vaulters. Height-focused instructions promote greater JH through prolonged force application, whereas speed-focused instructions increase PF Bra, reduce GCT, and enhance the RSImod. These findings demonstrate that CMJ outcomes are task-dependent and sensitive to verbal constraints. For performance monitoring in pole vaulting, speed-focused instructions may better reflect rapid force production and neuromuscular readiness, whereas height-focused instructions are more suitable for assessing maximal explosive capacity.
Read CV Weiyi XuECSS Paris 2023: OP-BM06