Maximal strength testing is central to the physical profiling of collegiate athletes, especially when establishing preinjury baselines,1 informing training interventions,2 determining key performance indicators,3 and identifying athletes at increased risk for musculoskeletal injury.4 In addition to repetition-maximum (RM) strength testing with free weights,1 vertical jump force–velocity testing can be used to quantify mechanical muscle function in athletes,5 providing a deeper physiological analysis of an athlete’s eccentric and concentric strength. This is due to the fact that mechanical muscle function is determined by the contractile properties of skeletal muscle including the force–velocity relationship.6 The force–velocity relationship describes that, when maximally activated, muscle produces less contractile force as the velocity of contraction increases.6,7 The force–velocity relationship also describes an increase in skeletal muscle contractile force during lengthening contractions (ie, eccentric muscle actions).8 The training and detraining responses of eccentric and concentric muscle strengths are divergent with respect to performance factors,9 injury risk,10 biological sex differences,11 and age-related adaptations,11 highlighting the importance of strength testing methods that can distinguish between eccentric and concentric mechanical muscle function.
Eccentric and concentric muscle strength capacities can be assessed using single-joint isokinetic dynamometry12 and multijoint testing.13 Single-joint muscle strength testing is often used in rehabilitation,14 but lower-limb multijoint testing is more specific to the requirements of competitive sport that involve running, jumping, and change of direction maneuvers. However, multijoint testing requires specialized dynamometers15 or the use of free weights through RM testing, which has limited applicability for assessing eccentric strength.13 Given the challenges associated with quantifying multijoint eccentric strength and the relevance of these muscle strength capacities in sport performance and noncontact sport injury mechanisms,16,17 alternative strength testing methodologies could assist practitioners and clinicians in establishing a more comprehensive assessment of lower-limb mechanical muscle function.
Countermovement jump (CMJ) testing is a multijoint movement that has similar kinetics to exercises traditionally performed as part of lower-body strength testing (eg, the back squat)18 and may address this gap. Similar to other strength testing exercises, the CMJ can be performed using an additional external load; however, the nature of jumping necessitates that external loading is submaximal, which may reduce safety concerns associated with RM testing. CMJ testing performed with multiple loads can be used to elucidate a force–velocity profile,19 and this approach has been shown to predict maximal lower-body isometric muscle strength.19,20 However, the relationship between loaded jump mechanics and multijoint leg extensor eccentric and concentric strength capacities is not fully understood. Early work by Hill7 described a hyperbolic force–velocity relationship in muscle by obtaining the shortening velocity against constant external loads. Similarly, a whole-body force–velocity relationship can be obtained by measuring vertical CMJ performance (ie, jump height or takeoff velocity [TOV]) as a function of increasing external loads.19 The CMJ TOV–external load relationship is often modeled as a linear relationship,21 and this supports the linear extrapolation of the model to obtain the maximal load intercept (L0; representing the theoretical maximal isometric force) and the maximal velocity intercept (V0; representing the theoretical maximal speed of unloaded muscle shortening).22 Further, a phase-specific analysis of the CMJ force–time curve allows the kinetics of the “braking” or eccentric deceleration phase to be assessed.23,24 Thus, CMJ testing permits a distinct analysis of the mechanical muscle function and strength capacities specific to the eccentric and concentric phases. But, considering the lack of studies exploring the relationship between multijoint eccentric, isometric, and concentric muscle strength, and CMJ velocity–load mechanics, more research is needed to explore the validity of the loaded CMJ method to predict contraction-type-specific leg strength. This information, in turn, may provide value in sport performance settings where 1 RM testing may not be appropriate or for preseason baseline testing to establish sport-specific neuromuscular benchmarks.25
The aims of this study were to evaluate the reliability of CMJ velocity–load testing and assess the relationship between CMJ velocity–load kinetics and lower-limb multijoint eccentric and concentric leg press strength on a robotic servomotor leg press in trained competitive athletes. We hypothesized that (1) CMJ velocity–load outcome measures show good reliability and (2) the CMJ eccentric deceleration phase and concentric phase kinetics predict maximal eccentric and concentric leg press strength, respectively.
Methods
Subjects
Canadian University sport (U SPORTS) athletes (n = 203; 52% females) from 6 field and court sports (ie, soccer, basketball, volleyball, field hockey, tackle football, and rugby) were recruited for annual baseline strength testing. U SPORT athletes are typically categorized as elite athletes who compete at a high level and have access to a strength and conditioning support staff.26 The average muscle strength and power results from baseline preseason testing are presented in Table 1. Participants were free from any musculoskeletal condition and recent concussions that limited their ability to perform maximal lower-body muscle strength testing. Prior to testing, all participants provided written and informed consent. The methodology for this study was approved by the University of Calgary Conjoint Health Research Ethics Board (REB15-1094) and conducted in accordance with the Declaration of Helsinki (without registration).
Participant Information and Normative Performance Values, Mean (SD)
| Sport | Sex | Age, y | Body mass, kg | CMJBW peak external mechanical power output, W/kg | CMJBW jump height, cm | Isometric LP strength, N | Isokinetic concentric LP strength, N | Isokinetic eccentric LP strength, N | Force–velocity L0, N | Force–velocity V0, m/s |
|---|---|---|---|---|---|---|---|---|---|---|
| Basketball | F (n = 13) | 20.6 (1.2) | 74.9 (8.3) | 46.1 (7.3) | 28.4 (5.7) | 1739 (369) | 1541 (268) | 1824 (354) | 2238 (237) | 3.5 (0.3) |
| M (n = 8) | 20.6 (2.3) | 87.9 (12.9) | 58.5 (4.3) | 39.5 (3.5) | 2180 (449) | 1898 (358) | 2140 (425) | 3130 (527) | 3.9 (0.3) | |
| Soccer | F (n = 23) | 19.9 (1.4) | 64.1 (8.2) | 42.0 (4.4) | 24.9 (3.3) | 1472 (309) | 1252 (261) | 1516 (325) | 2019 (288) | 3.2 (0.2) |
| M (n = 15) | 20.7 (1.6) | 74.9 (7.4) | 56.7 (7.0) | 39.0 (5.4) | 2220 (336) | 1889 (217) | 2035 (407) | 2774 (386) | 3.8 (0.3) | |
| Volleyball | F (n = 17) | 20.0 (1.1) | 71.4 (6.4) | 52.1 (4.9) | 34.0 (3.5) | 1614 (242) | 1533 (252) | 1743 (297) | 2343 (202) | 3.7 (0.2) |
| M (n = 14) | 21.1 (1.6) | 81.3 (8.4) | 60.5 (5.3) | 33.0 (5.1) | 2254 (395) | 1931 (304) | 2197 (383) | 2791 (40) | 4.1 (0.2) | |
| Tackle football | M (n = 61) | 20.4 (1.6) | 87.5 (10.8) | 61.3 (7.6) | 41.9 (5.7) | 2323 (455) | 2139 (401) | 2288 (498) | 3122 (367) | 4.0 (0.2) |
| Rugby | F (n = 31) | 19.6 (1.9) | 69.5 (11.5) | 43.3 (7.0) | 26.1 (5.6) | 1675 (292) | 1502 (244) | 1813 (333) | 2286 (501) | 3.3 (0.3) |
| Field hockey | F (n = 21) | 20.2 (2.3) | 63.1 (9.8 | 40.6 (4.2) | 24.8 (3.6) | 1352 (312) | 1270 (305) | 1482 (325) | 1872 (262) | 3.3 (0.3) |
Abbreviations: CMJBW, body-weight countermovement jump; F, female; LP, leg press; L0, the velocity–load relationship load intercept; M, male; V0, the velocity–load relationship velocity intercept.
Design
Participants performed a standardized warmup protocol27,28 including self-selected lower-body static and dynamic stretching, 10 minutes of moderate-intensity cycling on an ergometer, and 4 bouts of maximal intensity cycling intervals utilizing a 10:50 seconds work-to-rest ratio. Next, participants performed standardized warm up/familiarization trials for the loaded CMJ testing protocol (ie, 1–2 practice trials at each load using an identical cueing protocol as described in the CMJ velocity–load testing subsection) and on a robotic servomotor isokinetic leg press (ie, 5 concentric and eccentric leg press actions completed at progressively increasing rates of perceived effort; Fv Seger, Treadmetrix). Participants then performed 3 maximal voluntary contractions (MVCs) of isometric, eccentric (sled velocity = 0.2 m/s), and concentric (sled velocity = 0.2 m/s) leg press. For eccentric and concentric MVCs, a sled velocity of 0.2 m/s was selected based on previously reported average velocities obtained during 1 RM squat testing.29 Finally, according to a previously established methodology,30 participants performed CMJ velocity–load testing, which included unloaded CMJ testing and CMJ testing with an additional external load of 30% and 60% of the participants body weight.
CMJ Velocity–Load Testing
CMJ testing was performed on a dual force plate system (Accupower, Advanced Measurement Technology Instruments; sampling frequency = 1000 Hz). First, participants performed 5 maximal effort CMJs with instructions to keep their hands on their hips while they rapidly lowered to a self-selected CMJ depth prior to ascending with the intent to maximize their vertical jump height. Loaded CMJ trials were performed in the same manner except participants held a hexagonal trap bar with an additional external load equal to 30% (CMJ30; n = 3 trials) and 60% (CMJ60; n = 3 trials) of their body mass and an instruction to minimize any upper body vertical “pulling” motion during the jump. Testers monitored the CMJ testing and ensured participants performed a 5-second stationary quiet standing period before each CMJ trial prior to a strong verbal cue of “jump” at the onset of the each CMJ repetition.
Vertical ground reaction forces (Fz) were recorded synchronously using Noraxon Myoresearch software (version 3.20.14), and data were exported and analyzed using a custom-written computer program (MATLAB R2022a, MathWorks).23,24 The system mass (participants body mass plus the additional external load) was calculated as the summed left and right Fz signal during a 2-second period where the participant was stationary.23,24 As done previously,23 the CMJ force plate data can be used to calculate the CMJ acceleration-time signal, which was integrated to obtain the body center of mass (BCM) velocity–time curve and used to identify the eccentric deceleration and concentric phases23 (Figure 1). The eccentric deceleration phase was defined between the maximum downward velocity to the point of zero velocity, which corresponded to the lowest BCM position prior to the upward ascent.23,24 The concentric phase was defined from this point (BCM velocity equal zero, lowest BCM position) to the point of takeoff (toe-off).23,24
—(A) Vertical ground-reaction force–time curve overlaid with the body center-of-mass velocity–time curve showing the eccentric deceleration phase, concentric phase, TOV, and maximum downward velocity of the CMJ (Vmin). (B) Visual representation of CMJ velocity–load protocol and modeled parameter. CMJ indicates countermovement jump; L0, load intercept; TOV, takeoff velocity; V0, velocity intercept.
Citation: International Journal of Sports Physiology and Performance 20, 3; 10.1123/ijspp.2024-0439
The CMJ jump with the highest TOV from each loading condition was used for analysis and plotted against the total external load (body mass + external load). A linear regression model was fit using R: A Language and Environment for Statistical Computing (v. 4.3.1) to obtain the L0 and V0. Finally, the phase-specific CMJ kinetic impulse was obtained for the eccentric deceleration phase and the concentric phase. The CMJ outcome measures were selected based on their high reliability (see Supplementary Table S1 [available online]) and the mechanical relationship with CMJ performance outcomes, notably the impulse–momentum relationship. The heaviest loading condition, notably CMJ impulse with 60% of body mass, was also selected, given the goal of predicting maximal strength characteristics.
Multijoint Leg-Press Strength Testing
Participants were positioned in the robotic servomotor leg press device by laying on a recumbent chair, placed along a 2-dimensional track, with their feet set on the leg press platform in a standardized position.27 Each participant’s range of motion was standardized and corresponded to knee joint flexion angles between 10° and 100° and programmed into the leg press software (Accupower, version 2024; sampling frequency = 200 Hz). After the warmup contractions, participants performed three 3 seconds unilateral MVCs of isometric leg press extension at 60° of knee flexion angle with the cue “push as fast and as hard as possible.”31,32 Finally, participants performed 3 MVCs of isokinetic leg press extension at a sled velocity of 0.2 m/s, with a 0.5-second pause between concentric and eccentric contractions. During isokinetic tests, the cue “push as hard as possible throughout the duration of the repetition” combined with “push, push!” during concentric actions and “resist, resist!” during eccentric actions were used to ensure a maximal effort was given. Isometric, concentric, and eccentric MVCs were analyzed in the same manner: First, for each repetition on the left and right limb, the peak force (in newtons) was obtained. Next, the maximum force from the limb that achieved the highest peak MVC force output was used for the analysis.
Statistical Analysis
Statistical analyses were conducted using R: A Language and Environment for Statistical Computing (v. 4.3.1).33 Descriptive statistics are presented at the group means and SDs. To assess the intraday reliability of CMJ velocity-load testing and the associated CMJ kinetic metrics, a subset of 14 athletes (8 males, 6 females; age: 22 [2] y; body mass: 71.2 [10.9] kg; CMJBW jump height: 37.9 [8.1] cm; CMJBW peak mechanical power: 53.8 [9.0] W/kg) were recruited by convenience sampling and volunteered to perform CMJ velocity–load testing on 2 occasions separated by 20 minutes of active rest. Intraday reliability was assessed using the coefficient of variation (CV) with 95% CI and the intraclass correlation coefficient (ICC) using a 2-way mixed-effects model (ICC 3,1) with 95% CIs, which were computed using the “agRee” package (v. 0.5-3).34 ICC ≥ .70, CV ≤ 10% and ICC ≥ .80, CV ≤ 5% were set as the thresholds for acceptable and good reliability, respectively.35,36
To quantify the relationship between maximal muscle strength capacity (ie, MVC force) and velocity–load CMJ metrics, the statistical analysis was completed in 2 parts. For part I (model construction and identification of model parameters), linear mixed-effects models were constructed to assess the effects of (1) CMJ60 concentric impulse, CMJ60 net concentric impulse, CMJ60 TOV, L0, and athlete sex on maximal concentric isokinetic leg press strength; (2) L0 and athlete sex on maximal isometric leg press strength; and (3) CMJ60 eccentric impulse, CMJ60 net eccentric impulse, CMJ60 maximum downward velocity of the BCM, and athlete sex on maximal eccentric isokinetic leg press strength. Models were constructed, and the collinearity of the fixed effects was evaluated using the “car” package (version 3.1-2),37 where a variable inflation factor of greater than 4 was considered collinear and the associated fixed effect was removed from the model.38,39 Next, the final models were constructed with a forward and backward step wise model selection by Akaike Information Criterion40 using the “MAAS” package (version 7.3-60.2).41 For these models, an alpha level of .05 was used to determine statistical significance. For part II (evaluation of the model’s predictive performance), the data set was randomly split into training (70%; n = 142) and testing (30%; n = 61) sets. Using the model parameters predetermined from the stepwise analysis, the training set was used to train a linear mixed-effects model, and the test data were used to evaluate the trained model and to calculate the absolute percent prediction error between the predicted force and actual force.
Results
Loaded and unloaded CMJ kinetic metrics displayed good reliability (see Supplementary Table S1 [available online]). Further, the velocity–load L0 and V0 demonstrated good reliability (CV = 5.9%; 95% CI, 4.0–8.6 and CV = 4.1%; 95% CI, 2.9–6.0; respectively); whereas, the slope of the velocity–load relationship (CV = 9.9%; 95% CI, 6.8–14.5) exhibited poor reliability. A full description of the calculated intraday reliability metrics is presented in the Supplementary Table S1 (available online).
During isokinetic leg press strength testing, the eccentric-to-concentric force ratio was 1.14 (0.17), and eccentric force was 3% greater than isometric force (eccentric-to-isometric strength ratio = 1.03 [0.18]; Figure 2A). The CMJ TOV–load relationships including the load (L0) and velocity (V0) for each participant along with the mean value for male and female participants are shown in Figure 2B.
—(A) Average body-mass-normalized eccentric:isometric:concentric ratio force-production capacity across males and females. (B) The takeoff velocity–load relationships for each participant along with the mean value for male and female participants. CMJ indicates countermovement jump; Ecc, eccentric; Iso, isometric; Con, concentric.
Citation: International Journal of Sports Physiology and Performance 20, 3; 10.1123/ijspp.2024-0439
The stepwise model fits and associated predictors are shown in Table 2. The final isometric model (Isometric strength (N) ∼ L0 (N) (0.39) + Athlete sex (Male = 1) (370.90) + 732.03; F2,200 = 129.8, P < .001, R2 = .565) had a prediction error of 14% for isometric leg press force. A significant fixed effect of L0 on isometric strength (β = 0.393, SE = 0.063, P < .001) was found, and males had a higher isometric force than females (β = 370.90, SE = 74.03, P < .001; Figure 3).
Model Development and Included Variables Through Removal of Collinearity and Stepwise Model Selection
| Response variable | Model | Included predictor variables | AIC | R2 |
|---|---|---|---|---|
| Maximum concentric force | 1 | CMJ60 takeoff velocity, m/s CMJ60 net concentric impulse, Ns CMJ60 concentric impulse, Ns Force velocity L0, kg Athlete sex (male = 1) | 2842.2 | .69 |
| 2a | CMJ60 takeoff velocity, m/s CMJ60 concentric impulse, Ns Force velocity L0, kg Athlete sex (male = 1) | 2861.1 | .66 | |
| 3b | CMJ60 concentric impulse, Ns Force velocity L0, kg Athlete sex (male = 1) | 2859.3 | .66 | |
| Maximum isometric force | 1 | Force velocity L0, kg Athlete sex (male = 1) | 2955.8 | .57 |
| Maximum eccentric force | 1 | CMJ60 maximum downward velocity, m/s CMJ60 net eccentric impulse, Ns CMJ60 eccentric impulse, Ns Athlete sex (male = 1) | 2984.0 | .45 |
| 2a,b | CMJ60 maximum downward velocity, m/s CMJ60 net eccentric impulse, Ns Athlete sex (male = 1) | 3014.9 | .36 |
Abbreviations: AIC, Akaike information criterion; CMJ60, countermovement jump with additional load (60% bodyweight); L0, the velocity–load relationship load intercept.
aRemoval of colinear variables (>4 variable inflation factor). bFinal model determined through forward and backward step-wise model selection.
—Stepwise regression model (the model for male participants is shown in a dashed line and the model for the female participants is shown in a solid line) demonstrated that athlete sex and the force velocity intercept (L0; in newtons) were significant predictors of maximal isometric leg-press force (in newtons; P < .001). Open (blue) circles are data points for female participants, and closed (red) circles represent male participants.
Citation: International Journal of Sports Physiology and Performance 20, 3; 10.1123/ijspp.2024-0439
The final concentric model (Concentric strength (N) ∼ L0 (N) (0.3217) + CMJ60 Concentric Impulse (Ns) (0.8739) + Athlete sex (Male = 1) (213.56) + 175.52; F3,199 = 127.3, P < .001, R2 = .657) had a prediction error of 10% for concentric leg press force. A significant fixed effect was found for L0 (β = 0.322, SE = 0.056, P < .001) and CMJ60 concentric impulse (β = 0.874, SE = 0.194, P < .001) on maximal concentric force, and males had higher concentric force than females (β = 213.56, SE = 58.59, P < .001; Figure 4).
—Stepwise regression model (the model for male participants is shown in a dashed line and the model for the female participants is shown in a solid line) demonstrated that force velocity (A) load intercept (L0; in newtons), (B) the CMJ60 concentric impulse (in newton seconds), and athlete sex (shown in panels A and B) were significant predictors of maximal concentric leg-press force (in newtons; P < .001). Open (blue) circles are data points for female participants, and closed (red) circles represent male participants. CMJ60 indicates countermovement jump with an additional external load equal to 60% of the participant’s body mass.
Citation: International Journal of Sports Physiology and Performance 20, 3; 10.1123/ijspp.2024-0439
The final eccentric model (Eccentric strength (N) ∼ CMJ60 maximum downward velocity (m/s) (−305.55) + CMJ60 eccentric Impulse (Ns) (1.17) + Athlete sex (Male = 1) (368.42) + 955.43; F3,199 = 37.1, P < .001, R2 = .359) displayed a 15% prediction error for maximal eccentric leg press force. A significant fixed effect was found for CMJ60 eccentric impulse on eccentric strength (β = 1.167, SE = 0.341, P < .001). Further, males had higher eccentric force than females (β = 368.42, SE = 76.7, P < .001). No significant fixed effects were noted for CMJ60 maximum downward BCM velocity on eccentric strength (β = −305.55, SE = 156.25, P = .052; Figure 5).
—Stepwise regression model (the model for male participants is shown in a dashed line and the model for the female participants is shown in a solid line) demonstrated that the (A) CMJ60 maximum downward velocity (in meters per second), (B) CMyJ60 eccentric impulse (in newton seconds), and athlete sex (shown in panels A and B) were significant predictors of maximal eccentric leg-press force (in newtons; P < .001). Open (blue) circles are data points for female participants, and closed (red) circles represent male participants. CMJ60 indicates countermovement jump with an additional external load equal to 60% of the participant’s body mass.
Citation: International Journal of Sports Physiology and Performance 20, 3; 10.1123/ijspp.2024-0439
Discussion
This study established the reliability of CMJ-derived velocity–load testing and examined the relationship between CMJ velocity–load mechanics and multijoint, lower-body eccentric, isometric, and concentric maximal strength. The primary findings from this study were that (1) all loaded and unloaded CMJ kinetic metrics as well as the intercepts of the velocity–load relationship displayed good reliability and (2) a phase-specific kinetic analysis of loaded CMJ trials during velocity–load profiling demonstrated good predictive validity of lower-limb, multijoint maximal isometric, eccentric, and concentric strength in university athletes. Building from previous studies that employed traditional maximal muscle testing approaches, we utilized a robotic servomotor leg press device to quantify lower-limb eccentric, isometric, and concentric maximal strength. This approach reduced the technical demand of the strength testing exercise and minimized interathlete variability in lifting proficiency, which may improve the validity of quantifying muscle strength capacities.13 The robotic leg press device also permitted maximal eccentric force generation, and our results of ∼14% higher eccentric force compared with concentric force align with experimental research into the eccentric force capacity of isolated muscle.4
Reliability of the outcome measures were in line with previous work42 for the L0 (CV = 5.9%) and V0 (CV = 4.1%). However, the velocity–load slope had poor reliability (CV = 9.9%). These results suggest that CMJ velocity-load testing and quantifying the L0 and V0 are relevant for routine monitoring of strength characteristics in an athlete population. Maximal strength is fundamentally related to performance, training interventions, injury risk reduction, and injury rehabilitation in athletes;4,43 yet, there are limitations to conventional maximal strength testing including interathlete variation in lifting proficiency and the requirement for high external loading.13 The fact we evaluated maximal strength on a robotic isokinetic leg press minimized these factors while permitting an assessment of the neuromuscular ceiling for eccentric force production, which is rarely acquired using multijoint lower-limb testing in elite athletes. This may explain the discrepancy between our finding of an eccentric:concentric force ratio of 1.14 and the findings of others using a motorized isovelocity belt squat that yielded a ratio of ∼1.00.44 Nuzzo et al11 report that different movement velocities impact the measured eccentric:concentric force ratio where the eccentric:concentric force ratio is largest at faster velocities, perhaps due to changes in concentric force production as a product of the force–velocity relationship, or as a result of viscoelastic cross bridge properties (ie, titin) that may enhance eccentric force production at faster velocities.11 The movement velocity in our study was 0.2 m/s, which was marginally slower than the movement velocity adopted by Armstrong et al44 (0.25 m/s) and found a 1.00 eccentric:concentric force ratio. The fact we observed an eccentric:concentric force ratio of 1.14 compared with the value of 1.00 observed by Armstrong et al44 highlights the importance of task specificity in the eccentric:concentric force ratio.11 However, it should be noted as well that the eccentric:concentric force ratio of 1.14 in our study is more in line with expectations for the eccentric force capacity of whole muscle.7 Additionally, the robotic leg press allowed us to capture a more complete profile of the eccentric–isometric–concentric force capacity of the participants, which may not be possible using alternative testing procedures such as conventional RM testing with the squat or deadlift or isometric midthigh pull testing.
Stretch-shortening-cycle function, which is assessed readily in CMJ testing, is fundamental to sport performance movements such as sprinting and running.45 The CMJ testing method is a cornerstone for evaluating athletes in the context of training, injury, and fatigability.45 Additionally, force velocity mechanics and maximal strength underpin mechanical muscle function and the capacity to generate contraction-type-specific force. Alongside traditional multijoint strength assessment methodologies (eg, RM back squat testing and isometric midthigh pull testing),13 velocity–load CMJ testing is a valid and reliable approach to evaluate lower-limb muscle strength in athletes19 with a comparably minimal safety risk.13 Additionally, CMJ testing shares biomechanical similarities with the back squat.18 Performing the CMJ on a force plate system allows for the quantification of performance and biomechanics, which has supported numerous explorations linking CMJ outcomes to athletic performance46 and sport injury,47 as well as in the context of velocity–load testing where CMJ kinetics are strongly associated with isometric midthigh pull strength,18 peak knee extensor isometric strength,12 isometric squat strength,48 and maximal dynamic tasks such as the 1 RM back squat.20 The results of our study are consistent with previous literature20 and demonstrate a relationship between the force or, in our case, L0 and maximal isometric muscle strength, as well as between the L0, CMJ60 impulse, and maximal concentric leg press strength (cf Figure 4). However, it is interesting to note that where most previous studies have explored CMJ velocity–load kinetics in relation to isometric strength, we report stronger predictive capacity for the concentric model (10% prediction error, R2 = .67) compared with the isometric model (14% prediction error, R2 = .57). This finding is in line with studies elucidating the distinct neuromuscular mechanisms underpinning concentric and isometric muscle actions and provides additional support for the use of ballistic motor assessments (ie, CMJ) to quantify maximal concentric muscle strength especially in relation to sport performance.
With respect to maximal eccentric strength, these results illustrate that quantification of the CMJ60 eccentric phase impulse and CMJ60 maximum downward velocity of the BCM offers a new and practical approach for multijoint lower-limb eccentric strength testing. Most commonly, eccentric leg muscle strength has been measured using single-joint methodologies, such as isokinetic devices, instrumented fly-wheel devices, and bodyweight resisted protocols (eg, the Nordic hamstring curl).49 However, it is generally thought that multijoint eccentric strength testing, including an analysis of the braking, or eccentric deceleration phase kinetics in the stretch-shortening-cycle, are more transferable to sport performance than single-joint testing.50 Although eccentric muscle strength has been implicated in sport performance4 and sport injury mechanisms,43 including anterior cruciate ligament injuries,51 further research is needed to understand the underlying mechanisms associated with eccentric muscle actions in humans. In our study, we note that the coefficient of determination for the eccentric strength model (R2 = .36) was lower than that of the isometric (R2 = .57) and concentric (R2 = .66) models, pointing toward distinct neuromuscular mechanisms underpinning these strength capacities. For instance, passive components of the muscle cross bridge (ie, titin) and muscle tendon unit (eg, tendon) provide a relatively greater contribution to force production in eccentric actions compared with concentric actions.8 Further, the neural mechanisms associated with muscle activation during eccentric actions are not fully understood but are thought to be related to motor control and potentially protection against acute damage due to high force output.52,53 Nevertheless, we demonstrate that, despite the complexity of eccentric contractions, the CMJ eccentric phase deceleration mechanics during velocity–load testing may provide practical insights into the dynamic multijoint eccentric strength capacity of elite athletes. This has important implications for monitoring strategies aimed toward understanding the role of eccentric muscle strength on sport injury and sport performance.
Finally, biological sex was an explanatory variable in each model, and we noted specific sex differences. Maximal eccentric, isometric, and concentric strength was lower in university athlete females than in males (Figures 3–5). This is a common finding in the literature; however, future research should investigate how normalizing external loads to lean body mass (rather than whole-body mass) during velocity–load testing impacts the strength of these predictive models. Second, in a post hoc analysis, we found that the eccentric:concentric force ratio was lower in the male athletes compared with female athletes (β = 0.108, SE = 0.022, P < .001), which is consistent with the literature.11 More research is needed to better understand the biological sex dependencies of mechanical muscle properties such as the eccentric:concentric force ratio and force–velocity mechanics and how they contribute to CMJ velocity–load testing.
Practical Applications
Strength testing is a cornerstone in the physical assessment of elite athletes, but there are instances when traditional RM strength testing is disadvantageous (eg, when used with athletes who have poor strength training technique or when maximal eccentric strength must be assessed). Additionally, practitioners and clinicians may be interested in the physiological underpinnings of multijoint lower-limb mechanical muscle function, notably the force–velocity relationship of the leg extensors, to better evaluate athletes before and after injury. Although muscle strength testing technologies, such as an isokinetic leg press dynameter are available, these devices are often prohibitively expensive and not readily available in a range of high-performance and clinical settings. However, alternative methods such as the loaded CMJ velocity–load protocol require only a validated force plate system, which have low financial and expertise barriers to entry. One of the advantages of conducting loaded CMJ testing with force plates is the ability to dissect the kinetics and biomechanics of the vertical jump alongside performance. This is fundamental to an analysis of the eccentric deceleration phase mechanics. Although, alternative testing strategies such as field testing protocols primarily permit an analysis of performance outcomes (eg, jump height or flight time), field tests may still have value to practitioners seeking to quantify concentric and isometric maximal strength in competitive athlete populations.27 In our study, we experienced no adverse effects associated with loaded CMJ testing; this may indicate that this protocol can be expanded into other nonathlete populations with relative ease. Practitioners can use the methods and regression equations presented in this article to enhance assessments of lower-limb mechanical muscle function and gain insight into muscular strength that is contraction-type specific. Evaluation of muscle strength has many applications including in the context of athlete monitoring25 where evaluation of muscle strength capacities from baseline testing protocols can be used to individualize training programs to enhance performance,30 decrease musculoskeletal injury risk,27 and evaluate function postinjury.54
Conclusions
Given the challenges associated with differentiating between eccentric, isometric, and concentric strength capacity, along with the relevance of each strength capacity for sport performance and injury prevention, a kinetic analysis of the loaded countermovement-jump (CMJ) test can help practitioners and clinicians tailor strength-testing methodologies to better understand muscle mechanics across isometric, eccentric, and concentric contractions. Further, the formula presented in this study provides a practical solution to difficulties in evaluating maximal muscle-strength capacities across eccentric, isometric, and concentric actions. The loaded CMJ test is a practical strength-testing method that can help uncover whole-body muscle mechanics and can be easily implemented in various clinical and high-performance sport settings within a framework of athlete monitoring and to support individualization of athlete strength-training interventions.
Acknowledgments
This project was funded by the University of Calgary Dinos Athletics Department in the form of a PhD stipend.
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