The countermovement rebound jump assessment: validity, reliability, associations with athletic performance and sensitivity to change

Jumping is a common action in competitive sports, requiring precise coordination of the lower limbs. Among the various jump types, the countermovement jump (CMJ) and drop jump (DJ) are widely used to assess athletic performance. However, evaluating both slow and fast stretch-shortening cycle (SSC) capabilities through separate CMJ and DJ tests can be time-consuming if undertaking multiple trials for each test, and logistically challenging. To address this, the countermovement rebound jump (CMRJ), which combines characteristics of both the CMJ and DJ, has been proposed as a potential alternative. Despite its promise, the between-session reliability, and practical utility of the CMRJ remain underexplored, limiting its adoption as a comprehensive jump performance assessment tool. The primary aim of this thesis was to investigate the CMRJ as a valid, reliable, and practical alternative for assessing jump performance. Specifically, this thesis sought to: (i) review the literature relating to fast SSC mechanisms in rebound jumping and different methods that have been used to calculate jump height during CMJ and DJ tests; (ii) examine the between-session validity and reliability of the CMRJ in measuring kinetic and strategy-based metrics compared to the CMJ and DJ; (iii) evaluate the influence of external verbal cueing on CMRJ performance metrics; (iv) explore the associations between CMRJ outcomes with independent measures of athletic performance, including sprint speed, change-of-direction (COD) ability, and maximal force production; (v) and determine the sensitivity of CMRJ metrics to detect neuromuscular fatigue induced by a controlled repeated sprint intervention. The findings of this thesis provide novel insights into the applicability of the CMRJ as a streamlined and efficient tool for monitoring athletic performance, offering practical guidance for its use in both research and applied settings.

To better understand the underlying mechanisms of the SSC in jumping (particularly the fast SSC utilised during rebounding movements), a narrative review was conducted as part of this thesis. Findings from the narrative review highlighted the importance of selecting jump testing protocols that are familiar to the cohort of athletes being assessed, as this ensures that the testing outcomes accurately reflect the SSC function being assessed. Additionally, the selection of outcome metrics should prioritise quality over quantity, focusing on measures that effectively capture both the movement strategy and physical capabilities practitioners aim to evaluate through rebound jumping tests.

Jump height is one of the key metrics commonly used by practitioners to assess jump performance. However, the definition of jump height and the methods used to calculate it vary widely across the literature. To address this inconsistency, a systematic review was conducted using SPORTDiscus, Medline, CINAHL, and PubMed to identify the calculation methods employed to measure jump height during CMJ and DJ tests. A total of 21 studies were included for analysis, and five different calculation methods were compared: (i) flight time, (ii) impulse-momentum, (iii) double integration, (iv) jump-and-reach, and (v) three-dimensional (3D) motion capture systems. The results indicated that the choice of calculation method is largely influenced by the availability of equipment and the operational definition of jump height.

The primary objective of study 1a was to examine the between-session reliability of strategy-based metrics collected from the CMRJ compared to the CMJ and DJ. The secondary aim was to compare the differences in jump height, reactive strength index modified (RSImod), and reactive strength index (RSI) computed using four different jump height calculation methods. Results revealed that time to take-off (TTTO) in the CMJ was significantly longer than in the first jump of CMRJ (CMRJ1) across both sessions (effect size [ES] ≤ 0.49, p ≤ 0.027). Among all measured metrics, leg stiffness (Kleg) was not reliable, as indicated by a poor coefficient of variation (CV ≥ 15.29%) and wide 95% confidence intervals (CI). The longer TTTO alongside poor absolute reliability in Kleg suggested a potential change in jump strategy when subjects performed unfamiliar jump tasks. Thus, practitioners are advised to ensure sufficient habituation trials (≥ 6 repetitions for athletes with CMRJ test experience; ≥ 9 repetitions for athletes new to CMRJ test) are conducted before incorporating the CMRJ into routine testing. Furthermore, jump height, RSImod, and RSI did not differ significantly between calculation method (p ≥ 0.254). Building on the findings of study 1a, study 1b evaluated the validity and reliability of kinetic metrics measured during the CMRJ, CMJ, and DJ. Significant differences were observed in negative and positive hip and knee work, as well as positive ankle contributions during the CMRJ compared to the CMJ and DJ (p ≤ 0.048), with ES data indicating small differences (≤ 0.71) based on Hedges criteria. No significant differences were found in other kinetic variables across the three jump types (p ≥ 0.086). Joint work and joint contribution strategies remained consistent across jump types, with the ankle and knee joints contributing most to total positive work, particularly in the DJ and the second jump of the CMRJ (CMRJ2). Collectively for these two empirical investigations, findings suggest that the CMRJ is largely a viable alternative to the CMJ and DJ tests, providing practitioners with valuable insights into movement characteristics and training recommendations.

Athletes may adopt varying movement strategies during the CMRJ test, which can lead to suboptimal performance. Given the limited research on the CMRJ, it remains unclear how athletes can be encouraged to maximise explosive strength, reactive strength or both simultaneously. Study 2 examined the impact of external verbal cues on CMRJ performance across both strategy-based and kinetic metrics. As the use of external cues may initially alter movement strategies, it is important to consider that introducing new cueing conditions could require additional time for athletes to habituate. The combined cue, designed to maximise rapid force production and reactive strength, significantly reduced contact time and increased Kleg compared to the height cue, which focused solely on maximising jump height (p ≤ 0.038). Furthermore, the combined cue increased jump height compared to the velocity cue, which aimed to minimise contact time (p ≤ 0.005). For kinetic metrics, the combined cue enhanced positive ankle contributions compared to the height cue (p = 0.020) and increased positive hip and negative knee joint work compared to the velocity cue during the CMRJ2 (p ≤ 0.040). These findings suggest that practitioners can utilise specific verbal cues to tailor specific performance outcomes: the height cue to maximise jump height, the velocity cue to minimise contact times, and the combined cue to optimise RSImod and RSI.

Study 3 built upon the findings of previous studies by exploring the relationship between CMRJ performance and various independent measures of athletic performance, including the isometric mid-thigh pull (IMTP), 20-m linear sprint, and 505 COD speed tests. Significant positive relationships were observed between IMTP peak force and force at 300 milliseconds (ms) with the CMRJ1 height (r = 0.54 to 0.55, p ≤ 0.002). Additionally, inverse relationships were identified between RSImod and RSI with 20-m sprint total and split times (r = -0.55 to -0.66, p ≤ 0.001). The 10-m and total sprint times were also significantly correlated with the CMRJ1 height (r = -0.54, p = 0.003), suggesting greater rapid vertical force production and reactive strength are associated with faster sprint performance. Finally, significant inverse relationships were observed between CMRJ metrics (two jump height values and RSImod) and 505 speed times on both sides (r = -0.51 to -0.68, p ≤ 0.006). These results highlight the value of CMRJ performance metrics as indicators of lower-limb rapid force production ability.

The final experimental chapter (Study 6) investigated the impact of a controlled repeated sprint protocol on CMRJ performance measures, with retests conducted after each sprint set and at varying recovery time points (e.g., 0.5-hours, 1-hour, and-24 hours). These time points were chosen to provide a holistic approach as to the acute effects of fatigue and the subsequent recovery dynamics, offering demonstrations of how CMRJ performance evolves over a period of time when jump data was likely to be most affected. Results revealed significant reductions in CMRJ heights (in both jumps), RSImod, and RSI immediately following the sprints (ES = 0.27 to 1.78, p ≤ 0.040), indicating acute decrements in fast and slow SSC performance. In contrast, strategy-based metrics such as Kleg and contact time remained stable (p ≥ 0.079), suggesting limited sensitivity to fatigue. These findings highlight that while strategy-based metrics can provide valuable insights into individual athlete responses, outcome metrics remain the most practical and actionable measures for field-based monitoring and direct application.

In conclusion, the findings of this thesis provide a comprehensive understanding of CMRJ performance and offer practical insights for coaches, scientists, and practitioners in the field of sports performance. The results indicate that the CMRJ test produces comparable kinetic and strategy-based metrics to the CMJ and DJ tests, establishing its validity as an alternative jump performance assessment tool. Additionally, the influence of external verbal cues on jump strategies was evident, emphasising the need to select specific cues to optimise targeted CMRJ performance. This thesis also demonstrated significant relationships between CMRJ performance and other athletic performance metrics, such as IMTP strength, sprinting speed, and COD ability, underscoring the relevance of CMRJ metrics as proxy measures for an athlete’s overall performance capabilities. Furthermore, when using CMRJ test metrics to assess neuromuscular fatigue, outcome metrics were shown to be more reliable and actionable than strategy-based metrics. The inclusion of controlled fatigue interventions confirmed the robustness of CMRJ metrics across varying levels of fatigue, supporting their utility in both refreshed and fatigued states. Overall, these findings highlight the CMRJ as a reliable and versatile test that effectively assesses various aspects of athletic performance, including neuromuscular coordination, explosive power, and reactive strength. The CMRJ test, therefore, represents a valuable tool for practitioners seeking comprehensive insights into an athlete’s jump performance and broader physical capabilities.