Effect of Acute Fatigue and Training Adaptation on Countermovement Jump Performance in Elite Snowboard Cross Athletes

Abstract

Gathercole, RJ, Stellingwerff, T, and Sporer, BC. Effect of acute fatigue and training adaptation on countermovement jump performance in elite snowboard cross athletes. J Strength Cond Res 29(1): 37–46, 2015—Countermovement jump performance was examined in response to acute neuromuscular (NM) fatigue (study I) and chronic training (study II) in elite snowboard cross (SBX) athletes, through both typical (countermovement jump [CMJ]-TYP) and alternative (CMJ-ALT) CMJ variables. Seven (4 men and 3 women) elite (Olympic-level) SBX athletes participated in study I, and 5 of the same athletes (2 men and 3 women) participated in study II. Countermovement jump variables relating to force, velocity, power, and time were measured during both eccentric and concentric jump phases, with CMJ-TYP variables reflecting CMJ output and CMJ-ALT variables reflecting CMJ mechanics. In study I, CMJ performance was assessed before and after a fatiguing lower-body exercise protocol, and in study II, CMJ performance was examined before and after a 19-week structured training block. Meaningful differences in CMJ performance were examined using the magnitude of change (effect sizes [ES]) for group and individual changes. Acute fatigue decreased peak force and eccentric function, while the duration of the jump increased. The structured training block increased peak force and eccentric function, while jump duration markedly decreased. In both study I and study II, the largest ES were associated with CMJ-ALT variables. The CMJ test seems a suitable monitoring tool in elite SBX athletes for the detection of both acute fatigue and training-adaptation. Compared with CMJ output, CMJ mechanics exhibits more marked and divergent changes after both acute NM fatigue and a structured training block. CMJ-ALT variables should therefore be incorporated into CMJ analysis.

Introduction

The countermovement jump (CMJ) test is commonly used in high-performance sport to determine changes in neuromuscular (NM) function resulting from training and NM fatigue. Countermovement jump analysis is typically limited to analyses of CMJ output (i.e., peak/mean values specifically relating to the concentric phase) such as jump height and peak power (⁹). However, this approach provides limited information regarding the mechanical changes associated with longitudinal training-adaptation (⁹) and fatigue.

The CMJ test variables most sensitive to fatigue in elite athletes is unclear (²⁹). Previous investigations have reported no decreases in CMJ output and concentric variables (e.g., jump height, peak power) after fatiguing exercise (^5,6); these variables may therefore lack the requisite sensitivity to detect fatigue-induced changes in NM function. In contrast, NM fatigue may also manifest as a deviation in technique or strategy (^12,18), which could have important implications for all activities, with a strong technical component, planned by coaches and conditioning specialists. Examination of CMJ force-velocity and power-time traces can provide insight into CMJ mechanics (^9,12); thus, incorporation of such analyses into CMJ testing practice may reveal important information regarding athlete fatigue state.

Cormie et al. (^8–11) have used analysis of CMJ mechanics to examine chronic training-induced changes in NM function, gaining insight into the nature of training-adaptation, and the differences between athletes and untrained individuals (⁹). However, no examination has yet been performed on the effect of structured training in an elite athlete group. Given their highly trained status, it is possible that elite athletes may exhibit different training effects compared with lesser trained athletes.

The 2014 Olympics in Sochi marked the third appearance of snowboard cross (SBX) at Olympic games. However, at the time of writing, only 7 articles have been published on the sport, with 5 of these focused specifically on injury rates. Evidently, although associated injury risks are clearly of significance, published research on SBX also seems lacking.

Snowboard cross is a demanding sport requiring high levels of aerobic fitness and leg power (²⁶). Snowboard cross athletes therefore typically experience high levels of fatigue during training and competition. Snowboard cross also includes a large jumping component, with technical errors at jump take-off considered the cause of most SBX injuries (⁴). Accordingly, fatigue-induced changes in jump mechanics as Gathercole et al. (¹²), and others (²⁷), have previously observed may be of much importance to both SBX performance and the injury risks associated. Moreover, given the importance of jumping to SBX performance, the assessment of CMJ mechanics may provide an additional means of identifying potentially beneficial training-induced adaptations. Consequently, utilization of acute and longitudinal CMJ testing could provide a highly relevant feedback tool in elite SBX training periodization.

This study reports on 2 subinvestigations using a small cohort of elite SBX athletes (Olympic level), in which the suitability of the CMJ test to examine the effect of acute fatigue (study I) and the effect of a 19-week training program on CMJ test performance (study II) are examined. Here, CMJ test performance is analyzed in 2 ways: (a) through typical CMJ (CMJ-TYP) variables that typically relate to the CMJ output (i.e., concentric performance focused on jump outcomes; e.g., jump height, peak power) and (b) alternative CMJ (CMJ-ALT) variables, referring to those variables that are less common in CMJ analysis and relate to CMJ mechanics (i.e., the movement as a whole, describing both eccentric and concentric performance as well as jump duration).

Methods

Experimental Approach to the Problem

This investigation adopted a longitudinal descriptive design. Subjects were Olympic-caliber SBX athletes examined during their daily training environment; consequently, participant numbers were small and no control group was included. In study I, participants performed CMJ testing before and 30 minutes after a lower-body fatiguing exercise protocol involving repeated stair climbs to fatigue. Presession data from study I were used in study II, with study II posttesting performed after a 19-week structured training period.

Subjects

Four male (26.5 ± 5.8 years; 86.2 ± 3.4 kg; 183.4 ± 3.8 cm) and 3 female (26 ± 6.1 years; 64.4 ± 4.5 kg; 165.7 ± 4.4 cm) national level snowboard-cross athletes participated in study I (n = 7), whereas 2 of the same male athletes (28.5 ± 9.2 years; 88.9 ± 2.1 kg; 181.6 ± 4.7 cm) and the same 3 female athletes participated in study II (n = 5). Four athletes had at least 1 Olympic, World Championships or World Cup podium, and all had been on the World Cup circuit a minimum of 3 years, recording multiple top 16 finishes. All athletes provided written informed consent. Informed consent and all aspects of data collection and management were conducted in accordance with the Declaration of Helsinki.

Procedures

In all testing sessions, participants performed a set of 6 CMJs. In study I, pretesting was performed at approximately 0800 hours, with posttesting taking place at approximately 1030 hours, 30 minutes after the lower-body fatiguing exercise protocol. On the day before study I, participants performed a full-body power workout and 60 minutes of low-intensity cycling. The same pretest data were used in both study I and study II. Posttesting in study II was performed at approximately 0800 hours. On the day before the posttraining test session, participants performed an upper-body maximum strength workout (∼1 hour 15 minutes) and 60 minutes of low-intensity cycling, so a similar level of acute fatigue was present in athletes before testing. The same standardized warm-up was performed before all testing sessions.

Countermovement Jump Testing Session

Participants performed a 15-minute dynamic warm-up consisting of light cycling (∼5 minutes), dynamic stretching (∼5 minutes), and 10- and 20-m sprints (5 each) of progressive speed completed within 5 minutes. Participants then performed 5 practice CMJ trials of increasing intensity, with session testing beginning approximately 2 minutes after. The CMJ was a familiar training exercise for participants, thus, additional familiarization was deemed unwarranted.

Subjects performed 6 CMJ trials with 1-minute rest between. Trials were sampled at 200 Hz using the Ballistic Measurement System and software (BMS; Fitness Technology, Adelaide, Australia; Version 2012.3.7), consisting of a force plate (400 series; Fitness Technology, Adelaide, Australia) and position transducer (Celesco, PT5A-0150-V62-UP-1K-M6, Chatsworth, CA, USA) situated to the side of the force plate and attached to a wooden dowel. The dowel was placed across participants back, as in a back squat. Participants were instructed to limit dowel movement, and the position transducer was zeroed to participant height before every jump. Data were collected immediately after zeroing until the jump was completed.

Experimental Protocols

The fatigue protocol performed in study I consisted of repeated stair climbs of different work-recovery durations. Participants were instructed to work at a perceived exertion of 9 of 10. This approach has been found to be an adequate gauge of exercise intensity in elite athletes (²⁸). Work-recovery durations comprised a 1-minute sprint followed by 5-minute rest, 3 × 45 seconds with 4-minute rest between, 3 × 30 seconds with 3-minute rest between, 3 × 10 seconds with 1-minute rest between, and a final 1-minute sprint. Total protocol time was approximately 35 minutes with 6 minutes 15 seconds being all out maximal effort (∼29 minutes of recovery time). The chronic training block performed in study II (Figure 1) consisted of 19 weeks of various loading and unloading weeks as well as maintenance phases, while athletes were training on snow. Total hours of conditioning sessions are included in Figure 1. Study II took place during the precompetitive phase and was directed by the head coach to maximize performance outcomes and so was not a dependent variable in study II.

Figure 1: The 19-week training program, the number, type, and total hours of conditioning sessions, and the period of pre-and posttesting for study I and study II.

Countermovement Jump Variables

The techniques used to derive these variables have been described in detail by our laboratory previously (¹²). Briefly, the BMS software was used to calculate CMJ-TYP variables, whereas CMJ-ALT variables were calculated using custom-designed software written in Python (Python Software Foundation, www.python.org, USA). Relative force at zero velocity ([email protected]) and area under the eccentric phase of the relative force velocity trace (F-V AUC) were calculated using the force-velocity trace (Figure 1A). Mean relative eccentric and concentric power (EccConMP) and eccentric (EccDur), concentric (ConDur), and total duration (TotalDur) were calculated using the power-time trace (Figure 1B).

Statistical Analyses

The 4 most consistent CMJs from the 6 collected were used for analyses. Selection was based on our previously used methods, which determined the 4 most similar jumps in terms of jump duration and mean power output using the EccConMP variable (¹²). The coefficient of variation (CV) was calculated using raw data. Other analyses were performed using log-transformed data, with back transformation after statistical analyses. Pre-to-post differences were examined through the magnitude of change. Effect sizes (ES) were based on within-individual variability (i.e., typical error [TE]). Group ES were determined using the mean TE in the pretest session, whereas individual ES were determined using the TE for each individual within the pretest session. The following inference thresholds used (trivial: <0.3; small: <0.9; moderate: <1.6; large: <2.5; very large: <4.0; extremely large: ≥4.0) (^15,32). Effect sizes were based on previously used methods (⁶) and classified as substantial, probable, trivial, or unclear. Substantial changes were identified when ±90% confidence intervals (CIs) of the ES did not exceed either trivial ES boundaries (i.e., ±0.3). Probable changes were determined when there was a >75% likelihood of the ±90% CI of the ES being equal to or greater than trivial ES boundaries. If the likelihood of the ±90% CI of the ES was <75%, then ES were deemed trivial, while if the ±90% CI of the ES spanned both trivial ES boundaries, then changes were deemed unclear.

Results

Study I

The intrasession CV ranged from 2.0 to 16.2% for all variables (Table 1), with smaller CVs generally observed for CMJ-TYP variables (intrasession CVs of 5% or less [excluding JH]). Figure 2 shows the mean force-velocity and power-time traces for pre- and postexercise for all athletes. The power-time trace clearly shows an increase in jump duration (EccDur and TotalDur; Table 1), whereas the force-velocity trace highlights the decreases in eccentric function ([email protected] and FV-AUC; Table 1) postexercise.

Table 1: Acute fatigue effect (study I): group mean and SD, ES, and interpretation between pre- and postexercise (n = 7; 8 CMJ per participant) for (A) CMJ-TYP and (B) CMJ-ALT variables.

Figure 2: Acute fatigue (study I): force-velocity (A) and power-time trace at pre- and postexercise (n = 7; 8 CMJ trials from each participant) (B). Mean jump traces were calculated by normalizing each jump to the same number of data points. For the power-time trace, time was then reintegrated, with each jump standardized to the same zero-second start.

Body mass decreased pre-to-post exercise (pre: 76.0 ± 12.8 kg; post: 75.1 ± 12.7 kg). Eight CMJ-TYP and all CMJ-ALT variables displayed changes with ES greater than trivial (Table 1), whereas moderate and large changes were observed in peak and mean force, [email protected], F-V AUC, EccConMP, EccDur, and TotalDur. Small increases were also observed in peak power, peak velocity, and jump height.

Figure 3 illustrates the mean and 90% confidence limit for the individual ES between pre- and postexercise in comparison with the mean intrasession CV for select variables. Five variables (peak power, [email protected], EccConMP, EccDur, and TotalDur) displayed probable or greater changes.

Figure 3: Acute fatigue (study I): effect sizes (ES) (mean ± 90% confidence limits [CLs]) for the change between pre- and postexercise. ↑ = increase; ↓ = decrease; substantial = ±90% CL exceeds either ±0.3 (i.e., trivial ES); probable = if >75% of ±90% CL exceeds beyond either ±0.3 ES; trivial = if <75% of ±90% CL exceeds beyond either ±0.3 ES; unclear = If ±90% CL exceeds both ±0.3 ES thresholds; [email protected] = force at zero velocity; F-V AUC = force-velocity area under the curve; EccConMP = mean eccentric and concentric power; EccDur = eccentric duration; ConDur = concentric duration; TotalDur = total duration; T = trivial; S = small; M = moderate; L = large; VL = very large; EL = extremely large; trivial ES boundaries are shaded in gray.

Study II

The intrasession CV ranged from 1.7 to 12.0% for all variables (Table 2). The mean force-velocity and power-time traces for pre- and posttraining for all athletes are shown in Figure 4. The power-time trace highlights the increase in eccentric and concentric relative power production as well as decreased jump duration, whereas the force-velocity trace highlights the increase in eccentric loading and take-off velocity after training.

Table 2: Chronic training effect (study II): group mean and SD, ES, and interpretation (p ≤ 0.05) between pre- and posttraining (n = 5; 8 CMJ per participant) for (A) CMJ-TYP and (B) CMJ-ALT variables.*

Figure 4: Chronic training (study II): force-velocity (A) and power-time trace at pre- and posttraining (n = 5; 8 CMJ trials from each participant) (B). Mean jump traces were calculated by normalizing each jump to the same number of data points. For the power-time trace, time was then reintegrated, with each jump standardized to the same zero-second start.

Body mass was decreased after training (73.2 ± 13.5 kg; post: 71.1 ± 14.3 kg). Nine CMJ-TYP variables (all but absolute peak power) displayed posttraining increases (Table 2), with large increases in peak force (absolute and relative), mean force, and peak and mean power (relative only). All CMJ-ALT variables exhibited large posttraining changes, with increases in [email protected], F-V AUC, and EccConMP, and decreases in EccDur, ConDur, and TotalDur.

Nine variables (peak power, mean power, peak force, mean force, [email protected], F-V AUC, EccConMP, EccDur, ConDur, TotalDur) showed probable or greater changes (Figure 5).

Figure 5: Chronic training (study II): effect sizes (ES) (mean ± 90% confidence limits [CLs]) for the change between pre- and posttraining. ↑ = increase; ↓ = decrease; substantial = ±90% CL exceeds either ±0.3 (i.e., trivial ES); probable = if >75% of ±90% CL exceeds beyond either ±0.3 ES; trivial = if <75% of ±90% CL exceeds beyond either ±0.3 ES; unclear = if ±90% CL exceeds both ±0.3 ES thresholds; S = small, M = moderate; L = large; VL = very large; EL = extremely large; trivial ES not shown. Note: ES are calculated using pretest within-session SD instead of intersession SD. This may have contributed to the high ES observed.

Discussion

This 2-part investigation examined the suitability of the CMJ test to monitor acute fatigue- (study I) and chronic training-induced changes (study II) in NM function in a group of world-class SBX athletes. Our results suggest that the acute fatiguing protocol decreased CMJ force production and prolonged jump duration. Conversely, chronic training decreased jump duration and increased force and power production. The larger magnitude of change typically evident with CMJ-ALT variables indicates that the changes associated with CMJ mechanics were greater than the CMJ output. CMJ testing, with examination of both CMJ-TYP and CMJ-ALT variables, therefore seems a useful athlete-monitoring tool for both acute fatigue and training-induced adaptive responses in highly trained populations, particularly for athletes involved in large jumping-component sports such as SBX.

To the best of our knowledge, this is the first investigation to examine fatigue-induced changes in CMJ mechanics (i.e., CMJ-ALT variables), specifically in a group of elite athletes. The workload, described in the methods, was targeted to an athlete's RPE score of 9 of 10, which has been shown to induce significant fatigue previously (²⁸). We therefore speculate that athletes were acutely fatigued by the protocol.

Typical countermovement jump variables displayed trivial to large group changes pre-to-post exercise in response to acute fatigue, with small increases observed in peak-power (PP), peak velocity, and jump height. Previous studies have reported various PP changes after fatiguing exercise, with increases (^5,6), no changes (^{6,13,14,17,31}), and decreases (^12,22) all observed. These investigations used either specifically designed fatiguing protocols or competitive matches, and so the degree of fatigue likely differed. Interestingly, Cormack et al. (⁶) reported increased relative PP but no change in absolute PP after a competitive Australian Rules Football match. The alternating use of absolute and relative values may therefore have also contributed to differences in the literature. Nevertheless, our results show similar changes for both relative and absolute PP, and so it seems that PP production was perhaps enhanced, or at least maintained, despite acute fatigue.

In contrast, both absolute and relative peak force (PF) displayed large and moderate decreases after exercise indicating decreased force-generating capacity. Previous research has reported decreased PF (^5,6,22) or no changes (^13,14,17,31) after fatiguing exercise. In this study, peak velocity and jump height both exhibited small increases after exercise. Although increased jump height and decreased PF seem contradictory, jump height was determined directly from linear position transducer-derived velocity data and so, as peak velocity increased, so did jump height. Decreased peak velocity has been observed after a simulated handball match (³¹) and a fatiguing running protocol (¹²), whereas jump height has been found to increase (⁵), remain the same (¹⁹), or decrease (^2,6,31,34) after various fatiguing activities. Accordingly, fatiguing exercise seems to elicit inconsistent responses in CMJ-TYP variables, with factors such as athlete training status and genetic make-up, the multifactorial mechanisms of fatigue, the activity performed, and the time after the fatiguing exercise bout, likely to contribute to the varied responses.

Alternative countermovement jump variables tended to be associated with larger postexercise changes (Table 1 and Figure 3) and so may provide greater insight and sensitivity to NM fatigue compared with CMJ-TYP. In a previous study, we observed decreased [email protected], F-V AUC, and EccConMP, and increased EccDur and TotalDur after a fatiguing running protocol (¹²). In contrast, Thorlund et al. (³¹) reported no changes in eccentric, concentric, and total CMJ duration after a simulated handball match. However, Thorlund et al. (³¹) did not use the same standardized jump start threshold as this study and examined only 1 jump per participant, thus, they may have lacked the sensitivity to detect such changes.

In this study, postexercise testing took place 30 minutes after exercise cessation. Postactivation potentiation dissipates after 5–6 minutes (²⁰), and so the small increases in PP and peak velocity seem unlikely the result of a potentiating effect. Wadden et al. (³³) concluded that at around 40 minutes after fatiguing stretch-shortening cycle exercise, decreases in PF were the result of mechanical changes within the muscle rather than central or metabolic changes. Meanwhile, excitation-contraction coupling failure has also been suggested to affect force production (¹), and so these mechanisms may explain the decreased PF observed here. Postexercise changes in CMJ-ALT variables suggest that participants took longer to perform the jump and that eccentric function had diminished. The mechanical efficiency (i.e., the ratio of work performed to energy expenditure) of a CMJ is determined by the force produced and duration of the eccentric phase (²¹). Our findings therefore indicate mechanical efficiency was decreased postexercise, possibly through decreased elastic energy storage and utilization (²¹).

Changes in CMJ-ALT variables point to the adoption of an alternative CMJ NM strategy. Interestingly, our results suggest that concentric CMJ variables are not necessarily decreased as a result of a fatigue-induced shift in CMJ technique. Similar observations have recently been made by Schmitz et al. (²⁷), who found no change in jump height in response to fatiguing exercise, although CMJ biomechanics were markedly altered. Given that NM fatigue may elicit deviations in technique without decreases in capacity (¹⁸), such observations likely reveal important information about an athlete's NM fatigue state. Accordingly, the detection of acute NM fatigue and its interpretation may be enhanced through incorporation of CMJ-ALT variables into CMJ analyses.

To the best of our knowledge, this is the first investigation to examine the effect of training on both CMJ mechanics and output in elite SBX athletes. Chronic training markedly improved CMJ performance, with moderate and greater improvements in both CMJ-TYP and CMJ-ALT variables (Table 2 and Figure 5).

Notably, the posttraining decrease in body mass contributed to many of the CMJ-TYP changes, as the largest changes tended to be demonstrated by the CMJ-TYP variables in their relative forms. These changes would still however confer a competitive edge in SBX. Previous investigations have reported increases in CMJ PP, PF, velocity, and jump height after 10 (^10,11) and 12 weeks (^9,16) of varied strength and power training regimens. However, these investigations examined nonelite cohorts, and so the extent of change, or potential room for increase, was likely greater than in trained elite athletes. For example, 4 weeks of strength- and speed-power training produced no change in absolute PP in elite rugby athletes (³). These results mirror our findings, as we also observed no change in absolute PP after training. It has been suggested that the extent of power development slows after a year of high-performance training (²⁴), and so these SBX athletes may have had limited capacity to improve PP. Nevertheless, our results highlight that although PP development may have been trivial, other components of NM function can still show marked improvements.

One such example is PF, which demonstrated very large improvements in both absolute and relative values, whereas individual mean ES showed extremely large changes (Figure 5). Training-induced improvements in PF are considered to result from morphological changes increasing contractile capacity (^10,24); therefore, the chronic training performed here seemed to elicit such adaptations in the SBX athletes. Although the improvements in both PF, and to a lesser extent, peak velocity, may seem incompatible with the unchanged absolute PP; this is because PF does not occur at the same time as PP. This is illustrated in Figure 4A, which also displays the similarity in force produced at PP at pre- and posttraining, despite the distinctly different PF. These results highlight that changes can occur within the CMJ movement that may be overlooked in traditional analyses.

Large changes in CMJ-ALT variables reveal that CMJ mechanics differed markedly between pre- and posttraining, with posttraining jumps taking substantially less time to perform with greater eccentric power production (Figures 4A, B). Previous studies have reported improved eccentric power production (^9,11) alongside decreased (^10,16), or unchanged (^9,10) jump duration, after strength and power training. Determining the specific NM adaptations behind these observed changes in CMJ mechanics is complicated by many factors (e.g., muscle fiber composition or neural activation) (¹⁰); however, the enhanced eccentric function and decreased jump duration point to a more time-efficient NM strategy. Improved eccentric capacity is considered the result of enhanced stretch-shortening cycle function, possibly through increased musculotendinous stiffness (¹¹), and/or superior mechanical efficiency. Optimization of the stretch-shortening cycle is also thought to increase PF (¹⁰), possibly contributing to the large improvements in PF observed here. Therefore, chronic training seemed to elicit marked changes in CMJ mechanics, possibly through improved stretch-shortening cycle function; thus, the inclusion of CMJ-ALT variables into CMJ analysis seems to permit clearer interpretation of chronic training-induced changes in NM function.

A limitation of this study design is the lack of control group. Because participants comprised more than half of the nation's elite SBX athletes and were in the midst of preparing for World and Olympic championships, an elite control group was not feasible and the use of a general population control group is unsuitable, as comparisons between these groups would be meaningless. To account for the small sample size, we have however based our analyses and interpretations on the magnitude of change. In addition, we collected 4 CMJ trials from each athlete at each time point, reducing the typical error of measurement (³⁰), and so we believe the precision of our measures to be greater than most previous CMJ investigations. Although we lack a control group, the repeatability of CMJ-TYP variables is well established (^{7,12,23,25,30}) and has also been reported for CMJ-ALT variables (¹²). Moreover, Cormie et al. (¹¹) observed that a control group maintaining normal activity levels displayed no changes in CMJ force-velocity data over a 10-week period. As such, the changes that we have observed here seem due to training effects alone.

These investigations clearly demonstrate that acute NM fatigue and a training block elicit marked changes in CMJ mechanics (i.e., CMJ-ALT), with the magnitude of these changes typically greater than associated with CMJ output (CMJ-TYP) variables. It is therefore recommended that practitioners incorporate CMJ-ALT variables into CMJ analyses practice to better determine the acute fatigue state of their athletes and/or to longitudinally assess NM training adaptations over weeks to months.

Practical Applications

Our investigations suggest that the CMJ test is a useful tool for monitoring both acute fatigue- and chronic training-induced changes in NM function. Nevertheless, current CMJ methodology (i.e., CMJ variables focusing on single points [e.g., peak power] and/or jump output [e.g., jump height]) may overlook a wealth of information. For example, NM fatigue seemed to influence the CMJ strategy (i.e., mechanical and time-efficiency of movement) more so than the CMJ output (Figure 2), whereas chronic training elicited opposing effects (Figure 4). Incorporation of CMJ mechanics (CMJ-ALT variables) into CMJ test analysis is therefore likely to enhance the usefulness of the CMJ test.

Given the importance of movement speed, timing, and mechanical efficiency in many sports, these results have many important performance and training implications. For example, in SBX, a longer duration spent achieving maximal push-off in response to a passing move or avoiding another rider will likely affect race outcome. Similarly, an indirect effect of decreased mechanical efficiency when fatigued may be an acceleration of further fatigue-induced performance declines through increased energy expenditure performing the same movement. Interestingly, the greatest injury risk in SBX is associated with technical error at jump take-off (⁴). Fatigue-induced shifts in NM strategy, as revealed through CMJ testing approaches highlighted in this article, may therefore directly relate to enhanced monitoring and the potential for reduced injury susceptibility. Consequently, in highly technical sports such as SBX, where altered or mistimed movement can result in injury, it is perhaps ill-advised for athletes to perform technical practices in a fatigued state.

Acknowledgments

The authors acknowledge the contribution of Ryan Brodie in developing the software to perform the CMJ analysis, the athletes, and coaches for their participation and support of this study, and Anthony Findlay for his assistance with the testing and training of the athletes. No funding was received for this research.