Familiarisation

Proper administration of any performance assessment is fundamental to methodological reliability. To ensure robust and consistent data collection, participants should be adequately familiarised with the testing procedures.²⁵ Familiarisation sessions help minimise learning effects that may confound performance outcomes,^{5, 25, 27, 60} while also enhancing participant confidence and engagement.^{59, 64} For relatively simple tasks such as the CMJ, familiarisation requirements appear minimal in trained populations.^{23, 36} However, CMRJ is inherently more technically and physically demanding than the CMJ and DJ performed in isolation, the authors contend that careful consideration should be given to implementing an appropriate familiarisation process prior to formal testing. Building on this, it is also important to recognise that younger individuals, particularly those with limited training experience and lower relative strength levels, are likely to require a more extensive familiarisation period before confidence can be placed in the data obtained.^{71, 76} We advocate for the CMRJ to be broken down into the phases outlined in Figure 1 for the purposes of first familiarising an athlete with the movement.

To begin with, prioritisation should be given to ensuring that the athlete stands still and upright at the start and end of the CMRJ (i.e., stable weighing period pre-jump and post-landing).^{77, 83} If the athlete is indeed still and upright, this can be used as a zero-velocity reference; however, any amount of joint flexion during the weighting phase will violate this assumption and in turn have a knock-on effect towards phase identification and variable calculation.⁵⁰ Once this is achieved, it makes sense to encourage that the athlete executes the CMRJ1 in isolation with a consistent movement strategy, producing consistent jump heights while using their self-selected countermovement depth. Of note, other force-time metrics may vary, but these often reflect individual strategy changes rather than task error per se.⁸⁵

Once competency is demonstrated in the CMRJ1, emphasis should shift toward pre-ground contact kinematics and neuromuscular preparation prior to the rebound phase. As outlined in Figure 1, effective rebound performance is strongly influenced by posture and activation during the late flight phase of the CMRJ1.⁸⁵ Athletes should be familiarised with maintaining an upright trunk, forward gaze and appropriate centre of mass alignment, while adopting a pre-activated, stiff lower-limb position before touchdown. Passive descent, downward gaze or excessive dorsiflexion prior to contact may delay force application and compromise leg stiffness, resulting in prolonged ground contact times and reduced rebound performance.^{18, 38, 40, 82} Therefore, maximal-intent CMRJ trials should only be introduced once consistent and repeatable pre-contact mechanics are demonstrated.

It is important to note that the place to start and progress within and between each of the stages outlined in Figure 1 requires a degree of subjectivity. Nonetheless, this approach reduces cognitive overload and ensures that full-task execution reflects technical competency rather than compensatory strategies. Such an approach is particularly important in youth, novice or lower-strength populations, where the integration of fast SSC characteristics may require deliberate exposure and feedback before reliable data can be obtained.

INTERPRETING DATA FROM THE COUNTERMOVEMENT REBOUND JUMP

Much like data collection for any vertical jump assessment, the type and resolution of CMRJ metrics that can be reported may be determined by the equipment and processing workflow available. Where practitioners are limited to contact mats or mobile application-based jump systems, outcome metrics may be restricted to flight-time derived jump height, contact time and ratio values of these two (i.e., reactive strength index [modified]).^{28, 83} While these variables provide a practical overview of performance, they offer limited insight into the mechanical and neuromuscular processes underpinning task execution. In contrast, high-frequency dual force platforms enable phase-specific kinetic variables, centre-of-mass kinematics via numerical integration and limb-specific loading characteristics. This substantially expands the analytical scope of the CMRJ, with well over 100 potential metrics describing performance output, movement strategy, fatigue sensitivity and return-to-play readiness.^{10, 13} When force platforms are synchronised with two- or three-dimensional motion capture systems, further joint-level kinetic and kinematic variables (e.g., joint moments, power, work, angular velocity and displacement) can be derived. Despite this analytical richness, it is neither feasible nor time efficient for practitioners to interpret such an extensive set of metrics when evaluating a movement that unfolds within approximately 1-2 seconds. Most importantly, not all metrics are highly reliable across adjacent sessions and the indiscriminate reporting of large numbers of CMRJ metrics may obscure, rather than clarify, meaningful performance changes. Rather than proposing an exhaustive list, we recommend a tiered framework that matches the decision practitioners want to make with the minimum number of reliable, interpretable metrics.

Tier 1: Performance Outcomes

For applied testing, the primary question facing practitioners is whether the athlete can currently produce sufficient mechanical output. Tier 1 outcome-based metrics (including jump height, jump momentum, take-off velocity and flight-time) are therefore intended to provide a rapid snapshot of an athlete’s global neuromuscular capacity, answering a very fundamental performance question: “Can they jump and produce force at an expected level?” In practice, this judgement is typically made by benchmarking an athlete’s performance against normative data reported in the literature or against internal standards within a team or training group.² For example, recent work has begun to establish percentile-based normative values for the CMRJ, allowing practitioners to contextualise individual performance relative to population benchmarks using descriptors (e.g., poor to good) or normalised data such as z- or t-scores.⁹ Outcome-based metrics are well suited to this purpose because they describe the net result of force application to the ground, which are capabilities that underpin sprinting, change-of-direction and other complex sports-specific movements, rather than the underlying coordination strategy utilised to achieve them.^{13, 44} For practitioners managing large squads, congested competition schedules or limited testing windows, these variables are very intuitive and allow rapid decision making without the need for secondary data processing or extensive biomechanical interpretation.^{1, 9} Jump height represents the most familiar and widely reported indicator of vertical performance; however, reporting jump height alone does not provide insight into how that outcome was achieved.⁶¹ Specifically, while jump height is determined from take-off velocity via the impulse-momentum theory, it does not indicate the force-time characteristics underpinning that velocity. Accordingly, the inclusion of take-off velocity and jump momentum provides complementary information regarding mechanical output, with velocity reflecting the kinematic determinant of displacement and momentum providing a mass-related descriptor of the mechanical demand and impulse generated during propulsive phase.^{48, 54} From a mechanical perspective, jump momentum reflects the outcome of propulsive impulse through the impulse-momentum relationship.⁶¹ However, momentum may be easier for coaches and athletes to understand in practice, as it is a more common term and simply reflects how fast they moved their body mass at take-off, which may be more intuitive than interpreting the product of the summation of net force applied and the time during which the force was applied.⁵⁸ Importantly, Tier 1 metrics are among the most reliable variables reported across CMRJ, CMJ and rebound-jump literature when standardised cueing is applied, demonstrating good to excellent test-retest reliability across sessions, populations and competitive levels.^{3, 9, 31, 32, 71, 84, 85} The consideration of reliability is central to their applied value: when Tier 1 metrics change meaningfully (i.e., when the magnitude of change exceeds typical measurement error), practitioners can be reasonably confident that a true change in performance capacity has occurred, rather than a fluctuation driven by measurement noise or strategy variability.¹³

Another practical advantage of focusing on Tier 1 metrics is accessibility. Jump height, flight-time and take-off velocity can be obtained from a wide range of systems, including mobile applications and force platforms.⁴ In addition, body or system mass is typically known or easily measured. This allows the CMRJ to be integrated into existing monitoring systems without substantial methodological changes, making Tier 1 metrics transferable across environments and organisations, facilitating benchmarking, longitudinal tracking and communication between stakeholders. However, focusing on outcome metrics first is also a deliberate simplification.^{13, 15, 48} Athletes may achieve similar jump heights and take-off velocities via markedly different strategies, particularly during complex CMRJ task.⁸⁵ Tier 1 metrics therefore do not explain how performance was achieved or identify compensatory strategies that may hidden under fatigue or injury. Instead, they serve as an efficient ‘first-pass filter check’ – meaning that when mechanical output is preserved or declined (especially following a training intervention or congested competitions period), practitioners are encouraged to evaluate higher-tier metrics that provide better insights into fatigue, coordination and movement strategies.^{1, 13, 15, 48}

Tier 2: Movement Drivers and Strategy Metrics

Tier 2 metrics are intended to explain how or why an outcome has changed or been maintained by examining the mechanical drivers and movement strategies underlying CMRJ task execution. This tier becomes particularly informative when practitioners suspect changes in efficiency, coordination or neuromuscular fatigue that are not evident from outcome metrics alone. In practice, Tier 2 is trying to answer: “Is the athlete producing the same outcome with the same mechanical and temporal cost?”

Movement drivers represent the force-time characteristics that directly underpin performance outcomes. In the CMRJ, these include phase-specific force, impulse and velocity, which collectively determine the magnitude of centre-of-mass deceleration and acceleration. Phase-specific kinetic variables derived from force platforms (e.g., braking and propulsive mean or peak force, net impulse) can therefore provide insight into whether changes in jump performance are driven by alterations in force magnitude, force application duration or both.^{46, 57} For example, examining braking and propulsive force characteristics, as well as the timing of peak force within each phase, allows practitioners to better understand how downward movement is coupled with subsequent upward propulsion and overall jump performance.^{55, 56} Recent evidence further highlights the importance of evaluating these mechanical drivers alongside outcome metrics, as alterations in jump height may arise through multiple mechanical pathways. For example, increases in jump height can result from longer propulsive duration with unchanged force application, leading to greater net impulse. It can also result from higher propulsive mean velocity, even in the absence of meaningful changes in metrics shown to produce greater jump heights, such as countermovement depth.²⁶ These findings emphasise that similar performance outcomes may be underpinned by distinct mechanical solutions, reinforcing the value of driver metrics for interpreting Tier 1 changes.

Movement strategy metrics describe the temporal and displacement-related cost of achieving a given outcome. In the CMRJ (or any jump type), this cost is commonly expressed through time-based variables (e.g., time to take-off, ground contact time, braking and propulsive phase durations) and ground-based displacement-based variables (e.g., countermovement depth and propulsive phase displacement). For the CMRJ1, time to take-off and phase durations offer complementary insight into rhythm and coordination, helping to identify whether preserved output is achieved through altered phase organisation (strategy) rather than unchanged propulsive impulse-generating capacity.^{7, 54, 70} Prolonged braking phases or a relative shift toward longer propulsive durations may indicate a deliberate strategy by allowing more time to develop force or by relying more heavily on concentric muscle action to generate impulse.^{3, 54} In the CMRJ2, ground contact time serves as a direct indicator of fast SSC behaviour, given that the CMRJ1 height is constant. Longer contact times (> 250 ms) reflect a greater temporal cost to sequentially slow down, stop and speed up the centre of mass, which may be associated with reduced reactive capacity, suboptimal utilisation of stored elastic energy or altered joint-dominant strategies (e.g., reduced ankle joint contribution).^{19, 84} Ratio-based metrics such as RSI and RSImod integrate performance outcomes with temporal constraints and are therefore commonly used to summarise SSC efficiency. While these indices provide a convenient overview, they are inherently composite and may change due to alterations in force, timing or both.^{10, 12, 13} Consequently, interpretations of RSI and RSImod should be supported by inspection of their constituent variables (e.g., jump height or flight time alongside timing elements) to avoid misattributing changes to neuromuscular capacity alone.

From a practical standpoint, Tier 2 metrics are valuable because they help explain how output is achieved; however, they should be interpreted with appropriate caution. Many commonly reported movement drivers and strategy-based metrics, such as countermovement depth, leg stiffness and braking-phase kinetic characteristics have demonstrated greater between-session variability and poorer test-retest reliability than Tier 1 metrics, particularly in youth and in-season contexts.^{3, 32, 71, 85} Importantly, this variability does not necessarily reflect measurement error alone, but may also indicate that performance can be achieved through multiple movement strategies, making it difficult to identify a single “dominant” solution. In addition, several frequently reported variables, including phase-specific peak or mean power and rate of force development, are not recommended for routine interpretation during CMRJ task. These metrics often exhibit low reliability, high signal-to- noise and limited added explanatory value beyond the Tier 2 metrics proposed above.⁴⁹ Consequently, observed changes in such variables are more likely to reflect adaptive or compensatory movement solutions rather than true alterations in neuromuscular capacity. Metrics in Tier 2 are best used as explanatory tools to contextualise changes (or stability) in Tier 1 performance, rather than as standalone indicators for routine monitoring when using CMRJ test.

Tier 3: Bilateral Asymmetry and Movement Competency

Tier 3 metrics are primarily intended to support assessments of movement competency, where the focus shifts from overall performance capacity to how safely and symmetrically mechanical load is distributed between limbs.^{11, 39} This information may help practitioners identify residual deficits following injury or periods of rehabilitation. As mentioned, asymmetries in force application and attenuation or temporal characteristics can persist and may not be captured by Tier 1 and Tier 2 metrics alone.⁸² At this stage, the practitioner’s question transitions to “How is that performance being shared between limbs?” Metrics in this tier typically include peak rebound landing force, inter-limb asymmetry in braking and propulsive force or impulse across both jumps of the CMRJ. Phase-duration and contact time asymmetry may also be informative in cases where athletes adopt premature or asynchronous take-off strategies, such as when athletes prioritise movement velocity, leading to one limb taking off earlier despite similar magnitude of peak phase-specific force outputs.^{11, 39}

Although asymmetry in CMRJ metrics has received limited attention in the literature, available evidence suggests that asymmetry measures, particularly those derived from the rebound landing and barking phases, exhibit substantial variability. Fahey et al.³¹ reported poor-to-moderate absolute and relative reliability for rebound landing force variables and landing stiffness, while Xu et al.⁸² observed low agreement for net braking impulse asymmetry during the rebound phase (Kappa coefficient = 0.19), with approximately 33% of participants switching the dominant braking limb between sessions. It is also important to recognise that the asymmetry during the CMRJ may be influenced by pre-ground contact kinematics,^{38, 39} including limb positioning or joint configuration at touchdown, which in turn, can affect how load is distributed during the braking phase and upon rebound landing. Variability in these pre-contact conditions may therefore contribute to the between-session inconsistency commonly reported for asymmetry metrics, even in the absence of meaningful changes in global output or movement strategy.^{38, 39}

From an applied perspective, Tier 3 metrics reflect not only inter-limb loading characteristics, but also task execution and entry conditions. As such, asymmetry magnitude may be reduced following adequate task familiarisation and targeted verbal cueing designed to encourage symmetrical landing strategies. However, whether explicit cueing to promote symmetrical landings can meaningfully reduce asymmetry in the CMRJ remains unclear and warrants further investigation.¹³ Given their variability and strong dependence on execution context, Tier 3 metrics are best applied in targeted assessment scenarios, such as pre-season assessment, late-stage rehabilitation, return-to-play progression or post-injury reconditioning, where the practitioner’s focus has shifted from performance capacity to load distribution and movement safety.^{8, 14, 42} In these situations, asymmetry metrics should be interpreted on an individual basis and in conjunction with clinical information and movement observations, rather than as isolated indicators of injury risk.^{13, 15}

Practical Applications

The CMRJ provides practitioners with a time-efficient and integrated assessment of both slow and fast SSC functions within a single task. When implemented appropriately, it can reduce the need for separate CMJ and DJ testing while still offering meaningful insight into neuromuscular performance. The value of the CMRJ lies not in the number of metrics it can generate, but in how the test is coached, familiarised and interpreted. Clear and consistent verbal instructions, adequate familiarisation, stable execution conditions and prior quality check are essential to ensure that collected data reflect neuromuscular function rather than task variability (Figure 1). To facilitate applied interpretation of CMRJ data, Figure 3 provides a tiered framework that categorises outcome measures according to their practical purpose, underlying mechanical interpretation, equipment requirements and typical reliability characteristics. Global performance outcome measures (Tier 1) are well suited for routine monitoring establishing meaningful changes over time, while more movement driver and strategy-based variables (Tier 2) should be used selectively to explain changes in performance rather than tracked indiscriminately. Asymmetry and movement competency metrics should be applied in targeted scenarios, where understanding load distribution and movement safety is prioritised over maximal output. Collectively, practitioners should recognise that changes in CMRJ performance may arise from multiple mechanical solutions. By adopting a tiered approach to data interpretation and prioritising execution quality alongside metric selection, practitioners can use the CMRJ not only to quantify performance, but to better understand how that performance is achieved and whether it is expressed safely and consistently.

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A review of sport-specific performance testing in squash

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