Objectives: The purpose of this study was to identify the relationship between isometric force production symmetry and jumping performance in weighted and un-weighted static and countermovement jumps (SJ and CMJ). Design: Bivariate correlation between isometric force production symmetry and vertical jump performance variables. Methods: Collegiate athletes were evaluated for this study (n=36). Subjects performed SJ, CMJ, and isometric mid-thigh pulls (IMTP). Jumps were analyzed for jump height (JH) and peak power (PP). IMTP was analyzed for peak force (PF) for left and right sides, and values were calculated to produce a peak force symmetry index (PF-SI) score. Correlational statistics were performed examining the relationship between PF-SI and jump variables. Results: Moderate statistically significant negative correlations were observed between PF-SI and all jump variables, indicating that as asymmetry increases jump performance decreases. SJ correlations weakened in weighted conditions (JH r=-0.52 @ 0 kg/r=-0.39 @ 20 kg, PP r=-0.43 @ 0 kg/r=-0.34 @ 20 kg), but CMJ produced similar correlations for both conditions (JH r=-0.47 @ 0 kg/r=-0.49 @ 20 kg, PP r=-0.28 @ 0 kg/r=-0.34 @ 20 kg). Unlike the SJ, which only contains the propulsive or concentric portion of the jump, the CMJ also contains the eccentric portion and performance contributions of the stretch-shortening cycle (SSC). The addition of the SSC may play a role in the maintaining the magnitude of asymmetry in the CMJ weighted condition. Conclusions: The results indicate that force production asymmetry may be detrimental to bilateral vertical jumping performance. The findings should be considered for further investigation on sport-specific tasks.

Introduktion: Dynamic Strength Index är ett ratio som beskriver en individs förhållande mellan maximal och explosiv styrka. Post-activation potentiation (PAP) är ett fenomen där en förberedande aktivitet, exempelvis knäböj, används för att få en prestationsförbättring i en kommande aktivitet. Syftet med denna studie var att undersöka om (a) isometriska partiella knäböj (IPK) påverkar Dynamic Strength Index-ration (DSI), (b) IPK påverkar hopphöjd eller maximal kraft i CMJ, c) IPK påverkar maximal kraft i IMTP, (d) det finns någon signifikant korrelation mellan DSI-ratio och PAP-effekt. Metod: Fyra manliga och fyra kvinnliga beachvolleybollspelare (ålder 24.2 ± 3.8 år, vikt 72.7 ± 11.5 kg, längd 179.3 ± 9.2 cm) deltog i en counterbalanced measure design studie med två testtillfällen. Deltagarna utförde countermovement jump (CMJ) på kraftplattor och isometric mid-thigh-pull (IMTP) med en lastcell under båda testdagarna DSI och DSIPAP. Under DSIPAP utfördes även IPK som en förberedande aktivitet.                                                                                                                                Resultat: Resultaten visade ingen signifikant skillnad på DSI-ratio mellan testtillfällena DSI och DSIPAP (p=0.42). Resultaten för CMJ (cm) visade en lägre hopphöjd vid DSIPAP och en signifikant skillnad (p=0.042). Ingen signifikant skillnad för varken CMJ PF (p=0.96) eller IMTP PF (p=0.20) mellan DSI och DSIPAP hittades. Resultaten visade inte heller någon signifikant korrelation mellan DSI-ratio och PAP-effekt för CMJ peak power (r=-0.38) och CMJ hopphöjd (r=-0.21).                                                                                                                                                                       Konklusion: Maximala isometriska knäböj verkar varken påverka DSI-ratio, CMJ PF eller IMTP PF och hade en negativ påverkan på hopphöjd för beachvolleyspelare. Resultaten i den här studien stöder tidigare forskning att en möjlig negativ korrelation mellan styrka-effekt-ratio och PAP-effekt. Framtida studier behövs för att undersöka det närmare. / Introduction: Dynamic Strength Index (DSI) is a ratio which describes an individuals relationship between maximal and explosive strength. Post-activation potentiation (PAP) is a phenomenon where a conditioning activity, i.e. squats, is used to enhance a subsequent activity. The purpose of this study was to investigate if (a) isometric partial squats (IPS) could affect the DSI ratio, (b) IPS affects jump height or peak force in Countermovement jump (CMJ), (c) IPS will affect peak force in isometric mid thigh-pull (IMTP), (d) there is a significant correlation between DSI-ratio and PAP-effect. Methods: Four male and four female beach volleyball players (age 24.2 ± 3.8 years, weight 72.7 ± 11.5 kg, length 179.3 ± 9.2 cm) participated in a counterbalanced measure design study consisting of two experimental trials. The participants performed countermovement jump (CMJ) on force plates and isometric mid-thigh pull (IMTP) with a load cell during both trial days DSI and DSIPAP. During DSIPAP participants also performed IPS as a conditioning activity. Results: Results showed no significant difference on the DSI-ratio between the trials DSI and DSIPAP (p=0.42). Results for CMJ (cm) showed a decreased jump height at DSIPAP and a significant difference (p=0.042). No significant difference for neither CMJ PF (p=0.96) nor IMTP PF (p=0.20) between the trials were found. No significant correlation were found between DSI-ratio and PAP-effect neither for CMJ peak power (r=-0.38) nor CMJ jump height (r=-0.21). Conclusion: Maximal isometric squats does not seem to effect DSI-ratio, CMJ PF, CMJ PP nor IMTP PF but had a negative effect on jump height in CMJ for beach volleyball players. The findings in this study supports previous studies that a possible negative correlation exists between strength-power-ratio and PAP-effect. Future studies are required to investigate this further.

Reactive strength index-modified (RSImod) is a measure of lower body explosiveness calculated by dividing jump height by time to takeoff. RSImod is different between stronger and weaker athletes and between males and females. The purpose of this study was to evaluate differences in RSImod between males and females while controlling for maximal strength and lower body explosiveness. Forty-three female and fifty-eight male Division-I athletes performed countermovement jumps on a force plate during unloaded (0kg) and loaded (20kg) conditions. We used an ANCOVA to test whether RSImod is different between sexes conditioning on relative maximum strength (PFa) and average RFD 0-200ms (RFD200) measured during the isometric midthigh pull (IMTP). Differences of 0.087 (95% CI: 0.040 - 0.134; p = 0.0005) and 0.075 (95% CI: 0.040 - 0.109, p < 0.0001) were observed for RSImod between sexes in unloaded and loaded conditions, respectively. A male with PFa of 186 (grand mean of the sample) and RFD200 of 6602 N/s (grand mean of the sample) is predicted to have 28% greater RSImod than a female of similar PFa and RFD200. Maximum strength development should be a primary aim of training in female athletes, in addition to other trainable factors, such as stiffness and RFD.

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No description available.

The purpose of this study is to investigate the use of isometric mid-thigh pulls to determine lower-limb asymmetrical strength differences in NCAA D-I athletes. Sixty-six subjects (40 males and 26 females) performed 2 maximal effort isometric pulls over two force plates sampling at 1000 Hz each. Peak force was scaled for body weight, and rate of force development was examined from 0-200ms. Results of the study show subjects' produced significantly greater scaled force with the left leg as compared to the right leg; however, no significant differences existed for rate of force development (RFD). Men exhibited significant differences between both scaled peak force and RFD, while women only showed significant differences in scaled peak force. Of the 66 subjects tested, 6 subjects (5 men and 1 woman) exhibited percent differences of 15% or greater asymmetry for scaled peak force. The results indicate that isometric mid-thigh pulls are a way to show the presence of asymmetries in D-I athletes.

Acute exposure to vibration has been suggested to produce transient increases in muscular strength (1,2,8), vertical jump displacement (4,8), and power output (2,6,7) recorded while performing various tasks. It has been hypothesized that the reported acute vibration induced increases in performance occur as a result of alterations in neuromuscular stimulation (1,3,4). Specifically, most studies have ascribed the observed improvements to the likeliness of Whole Body Vibration (WBV) in producing a “tonic vibration reflex” (TVR) in which the primary nerve endings of the Ia afferents of the muscle spindle are activated. This is thought to result in the excitation of the alpha-motor neurons and activation of the extrafusal fibers (4) which likely leads to a greater synchronization of motor units as a result of homonymous motor unit contraction. However, not all investigations report improvements in muscular strength (4), vertical jump (7), and power production in response to acute vibration (4). While the current body of scientific knowledge offers conflicting evidence on the effectiveness of WBV in augmenting neuromuscular performance it is possible that WBV may result in alterations to specific aspects of the force-time curve during the performance of a maximal isometric contraction. Therefore, the primary purpose of this investigation was to examine the effects of WBV performed using 30 Hz frequency and 2-4 mm amplitude on the force-time curves of an isometric mid-thigh pull.

Abstract available in the Medicine and Science in Sports and Exercise.

Currently, multi-joint, position-specific isometric tests (MJIT) are commonly used as part of the monitoring of an athlete’s progress. Strong associations between isometric force-time curve parameters have been found. Perhaps the most commonly used test is the isometric mid-thigh pull (IMTP): the IMTP was created in the early 1990s and its use has grown considerably since that time. One sport in which it has been used extensively from the early 1990s to the present, particularly by the authors, is weightlifting. The relationships between weightlifting performance and IMTP force-time characteristics are quite strong, particularly for RFD. The IMTP is not only useful in assessing a weightlifter’s performance but can also be used for monitoring fatigue.

The purposes of this study were to examine the relationships between dynamic strength index (DSI) and other strength-power performance characteristics and to contextualize DSI scores using case study comparisons. 88 male and 67 female NCAA division I collegiate athletes performed countermovement jumps (CMJ) and isometric mid-thigh pulls (IMTP) during a pre-season testing session as part of a long-term athlete monitoring program. Spearman's correlations were used to assess the relationships between DSI and CMJ peak force, height, modified reactive strength index, peak power and IMTP peak force and rate of force development (RFD). Very large relationships existed between DSI and IMTP peak force (r = -0.848 and -0.746), while small-moderate relationships existed between DSI and CMJ peak force (r = 0.297 and 0.313), height (r = 0.108 and 0.167), modified reactive strength index (r = 0.174 and 0.274), and IMTP RFD (r = -0.341 and -0.338) for men and women, respectively. Finally, relationships between DSI and CMJ peak power were trivial-small for male (r = 0.008) and female athletes (r = 0.191). Case study analyses revealed that despite similar DSI scores, each athlete's percentile rankings for each variable and CMJ force-time characteristics were unique, which may suggest different training emphases are needed. Based on the explained variance, an athlete's IMTP performance may have a larger influence on their DSI score compared to the CMJ. DSI scores should be contextualized using additional performance data to ensure each individual athlete receives the appropriate training stimulus during different training phases throughout the year.