Validity of the Hawkin Dynamics Wireless Dual Force Platform System Against a Piezoelectric Laboratory Grade System for Vertical Countermovement Jump Variables.

ABSTRACT: Dos'Santos, T, Evans, DT, and Read, DB. Validity of the Hawkin dynamics wireless dual force platform system against a piezoelectric laboratory grade system for vertical countermovement jump variables. J Strength Cond Res 38(6): 1144-1148, 2024-The aim of this study was to determine the criterion validity of the Hawkin Dynamics (HD) wireless dual force platform system for assessing vertical countermovement jump (CMJ) variables, compared with those derived from a Kistler piezoelectric laboratory grade force platform system. During a single testing session, HD force platforms were placed directly on top of 2 adjacent Kistler force platforms to simultaneously collect vertical ground reaction forces produced by 2 male recreational soccer players (age: 29.0 ± 2.8 years, height: 1.79 ± 0.01 m, mass: 85.6 ± 4.7 kg) that performed 25 vertical CMJs each. Sixteen vertical CMJ variables pertaining to jump height (JH), flight time (FT), time-to-take off (TTT), countermovement depth, body weight (BW), propulsive and braking mean, and peak powers, forces, and impulses were compared between systems. Fixed bias was observed for 6 of 16 variables (peak and mean braking power, mean propulsion force, TTT, FT, and BW), while proportional bias was present for 10 of 16 variables (peak and mean propulsive and braking force, TTT, FT, peak and mean braking power, mean propulsive power, and BW). For all variables regardless of fixed or proportional bias, percentage differences were ≤3.4% between force platform systems, with near perfect to perfect correlations (r or ρ = 0.977-1.000) observed for 15 of 16 variables. The HD dual wireless force platform system can be considered a valid alternative to a piezoelectric laboratory grade force platform system for the collection of vertical CMJ variables, particularly outcome (i.e., JH, reactive strength index modified) and strategy variables (countermovement depth).

Validity and reliability of My Jump 2® app to measure the vertical jump on elite women beach volleyball players.

Purpose: The aim of this study was to assess the reliability and validity of the My Jump 2® app in measuring jump height, flight time, and peak power among elite women beach volleyball players on sand surfaces. Methods: Eleven elite female beach volleyball players (aged 23.6 ± 6.2 years; weight 66.3 ± 5.8 kg; height 174.4 ± 5.8 cm; with 8.4 ± 4.8 years of professional experience) participated in this study. Each player performed six countermovement jumps in a wooden box filled with sand on a force platform while simultaneously recording a video for subsequent analysis using the My Jump 2® app. Results: We found excellent agreement for flight time, jump height and peak power between observers (ICC = 0.92, 0.91 and 0.97, respectively). No significant differences between force platform and My Jump 2® app were detected in the values obtained for the three variables (P > 0.05). For the force platform and the My Jump 2® app, we found a good agreement measuring jump height and flight time (ICC = 0.85 and 0.85, respectively). However, we only found a moderate agreement for peak power (ICC = 0.64). The difference in jump height showed a limit of agreement between -4.10 and 4.74 cm in Bland-Altman, indicating a high level of agreement between the two measurement tools. Conclusion: Based on our findings, the My Jump 2® app reveals a valid tool for measuring jump height and flight time of CMJ on sand surfaces. However, more caution is needed when measuring peak power.

A Comparison of Different Strength Measurement in Taekwondo: Herman Trainer, Manual Tester, and Standing Long Jump.

Background and Objective: The accurate prediction of strength development relies on identifying the most appropriate measurement methods. This study compared diverse strength measurement techniques to assess their effectiveness in predicting strength development. Participants were taekwondo athletes competing at the red-black belt level or above. Methods: Technical striking forces (palding, dollyeo chagi, dwit chagi, and yeop chagi) were measured using a Herman Digital Trainer fixed to a striking stand. Quadriceps and hamstring strength were assessed with a Lafayette force measuring device. Explosive leg strength was evaluated through a standing long jump test, normalized for leg length. The Pearson correlation coefficient was used to examine relationships between measurement methods. Results: The standing long jump test showed no significant correlation with other strength assessments. A moderate positive correlation was found between Herman digital trainer measurements and Lafayette digital hand-held dynamometer results. A high positive correlation (r = 0.736, p < 0.001) emerged between hamstring strength and palding chagi technical strike force results. Technical strike kicks showed a significant positive correlation with each other and, also, a right foot-left foot correlation was observed. Conclusions: It was concluded that the standing long jump test, which was shown as one of the explosive leg strength measurement methods in field studies as an alternative to laboratory tests, did not correlate with other strength tests; therefore, this test is weak and insufficient to predict strength skills in taekwondo. In addition, this study showed that the hamstring muscle was more predictive in the measurement of technical strength. In future studies, it might be more useful to measure hamstring muscle strength or technical kick strength instead of a standing long jump field test.

Validating the Fitbit Charge 4© wearable activity monitor for use in physical activity interventions.

OBJECTIVES: Commercially available wearable activity monitors can promote physical activity behaviour. Clinical trials typically quantify physical activity with research grade activity monitors prior to testing interventions utilising commercially available wearable activity monitors aimed at increasing step count. Therefore, it is important to test the agreement of these two types of activity monitors. OBJECTIVES: Observational. METHODS: Thirty adults (20-65 years, n = 19 females) were provided a Fitbit Charge 4©. To determine reliability using an intraclass correlation coefficient, two, one-minute bouts of treadmill walking were performed at a self-selected pace. Subsequently, participants wore both an ActiGraph wGT3X-BT and the Fitbit for seven days. To determine agreement, statistical equivalence and the mean absolute percentage error were calculated and represented graphically with a Bland-Altman plot. Ordinary least products regression was performed to identify fixed or proportional bias. RESULTS: The Fitbit showed 'good' step count reliability on the treadmill (intraclass correlation coefficient = 0.75, 95 % CI = 0.53-0.87, p < 0.001). In free-living however, it overestimated step count when compared to the ActiGraph wGT3X-BT (mean absolute percentage error = 26.02 % ±â€¯14.63). Measurements did not fall within the ± 10 % equivalence region and proportional bias was apparent (slope 95 % CI = 1.09-1.35). CONCLUSIONS: The Fitbit Charge 4© is reliable when measuring step count on a treadmill. However, there is an overestimation of daily steps in free-living environments which may falsely indicate compliance with physical activity recommendations.

Reliability and validity of field-based tests for assessing physical fitness in gymnasts

Objective: This systematic review aimed at analysing the reliability and validity of field-based tests for assessing physical fitness in gymnasts.MethodThree electronic databases (PubMed, SPORTDiscus, and Scopus) were searched up to March 2022, in order to identify studies that assessed the psychometric properties of field-based physical fitness test among gymnastics.ResultsA total of 16 studies on several gymnastics modalities (artistics n = 11; rhythmic n = 3, artistics and rhythmic n = 1; aerobic n =  1), were analyzed. All studies reported on reliability measured through test-retest design. Validity was reported in only four studies. Regarding specific tests, the split test (ICC = 0.998), and the handstand (ICC= 1) showed the highest test-retest reliability. The greater validity values were achieved by the split test (r2 = 0.52), hanging pikes test (r2 = 0.86), and handstand test (r2 = 0.65).ConclusionA great variety of both specific and non-specific physical fitness tests have been analyzed in the field of gymnastics. The side split test, the handstand test, the vertical jump test, the 20-m run test, the agility test, and the aerobic gymnast anaerobic test could be useful tools to assess flexibility, strength, balance, muscular power, speed, agility, and cardiorespiratory fitness in gymnasts. Further investigations analyzing absolute reliability and criterion validity are needed. (AU)

A Clinician's Guide to Smartwatch "Interrogation".

PURPOSE OF REVIEW: Wearable technology is rapidly evolving and the data that it can provide regarding an individual's health is becoming increasingly important for clinicians to consider. The purpose of this review is to help inform health care providers of the benefits of smartwatch interrogation, with a focus on reviewing the various parameters and how to apply the data in a meaningful way. RECENT FINDINGS: This review details interpretation of various parameters found commonly in newer smartwatches such as heart rate, step count, ECG, heart rate recovery (HRR), and heart rate variability (HRV), while also discussing potential pitfalls that a clinician should be aware of. Smartwatch interrogation is becoming increasingly relevant as the continuous data it provides helps health care providers make more informed decisions regarding diagnosis and treatment. For this reason, we recommend health care providers familiarize themselves with the technology and integrate it into clinical practice.

Functional movement screen dataset collected with two Azure Kinect depth sensors.

This paper presents a dataset for vision-based autonomous Functional Movement Screen (FMS) collected from 45 human subjects of different ages (18-59 years old) executing the following movements: deep squat, hurdle step, in-line lunge, shoulder mobility, active straight raise, trunk stability push-up and rotary stability. Specifically, shoulder mobility was performed only once by different subjects, while the other movements were repeated for three episodes each. Each episode was saved as one record and was annotated from 0 to 3 by three FMS experts. The main strength of our database is twofold. One is the multimodal data provided, including color images, depth images, quaternions, 3D human skeleton joints and 2D pixel trajectories of 32 joints. The other is the multiview data collected from the two synchronized Azure Kinect sensors in front of and on the side of the subjects. Finally, our dataset contains a total of 1812 recordings, with 3624 episodes. The size of the dataset is 190 GB. This dataset provides the opportunity for automatic action quality evaluation of FMS.

Exercise heat acclimation causes human responses and safety performance improvements.

Heat acclimation (HA) is a widely recognized physiological phenomenon of human body in hot environments. HA has many benefits, such aspreventing hyperthermia responses, and is an efficient way to improve human responses to hyperthermal environments. However, it is not known whether HA is dependent on the environmental conditions. Moreover, its mechanism and effect on the safety performance remain unexplored. In this study, we created a climate chamber to simulate a hyperthermal environment. Thirty healthy males were recruited for this study, who were then trained under the same ambient conditions (temperature of 38 °C and relative humidity (RH) of 40%). The training involved running on treadmills (at 5 km/h) to simulate heavy manual labor, and performing heat stress tests (HST) under six different conditions (32 °C/40% RH, 35 °C/40%, 38 °C/40%, 32 °C/70%, 35 °C/70%, and 38 °C/70%). Their physiological indices (rectal temperature, heart rate, sweat loss and skin temperature) and one psychological index (thermal sensation) were measured. Furthermore, a hazard avoidance test device (HATD) was designed to evaluate the individual safety performance by detecting human errors. The results show that training and environmental conditions have different effects on HA. After HA, the physiological and psychological strain were significantly improved. More importantly, HA also helped improve the participants' awareness of the dangers and required emergency responses to face potential hazards. Overall, a reasonable HA training under proper conditions is helpful to ensure the safety of human beings. More research is needed to study the role of HA on safety performance.

Treadmill-based task for assessing spatial memory in head-fixed mice.

Several mouse in vivo neuronal recording techniques require head fixation. Head-fixed treadmill walking can be used to design tasks that enable the study of neural activity in the context of behavior. Here, we provide a detailed protocol for constructing a treadmill with tactile spatial cues, training mice on a rewarded behavioral task, and analyzing behavioral data. We discuss common problems and solutions we have developed to optimize training. Finally, we demonstrate how to test spatial memory performance using this task.

Velocity of the Body Center of Mass During Walking on Split-Belt Treadmill.

ABSTRACT: Walking on split-belt treadmills (each belt rotating at a different velocity) has inspired a growing number of researchers to study gait adaptation and rehabilitation. An overlooked peculiarity of this artificial form of gait is that the mean velocity adopted by the participant, considered as a whole system represented by the body Center of Mass, can be different from the mean velocity of the two belts. Twelve healthy adults (21-34 yrs) were requested to walk for 15 mins on a treadmill with belts rotating at 0.4 and 1.2 m sec-1, respectively (mean = 0.8 m sec-1). Each belt was supported by four 3-dimensional force sensors. For each participant, six strides were analyzed during the 1st and the 15th minute of the trial. The mean Center of Mass velocity was computed as the sum of the velocities of each belt weighted by the percentage of time during which the resulting forces, underlying the accelerations of the Center of Mass, originated from each belt. Across early and late observations, the median Center of Mass velocities were 0.72 and 0.67 m sec-1, respectively (P < 0.05). Therefore, the real velocity of the Center of Mass and its time course should be individually assessed when studying walking on split-belt treadmills.

Current limits for flowmeter resistance in metabolic carts can negatively affect exercise performance.

PURPOSE: To investigate whether a metabolic cart using a flowmeter in the upper range of accepted resistance to airflow (<1.5 cmH2 O∙L-1 ∙s-1 for flows up to 14 L∙s-1 , American Thoracic Society) negatively impacts exercise performance in healthy individuals. METHODS: 16 recreationally active males (age 25 ± 1 years, height 180 ± 6 cm, weight 73.5 ± 5.8 kg, all mean ± SD) performed two incremental tests on a bicycle ergometer on each of two visits, using a metabolic cart with a flowmeter of either low (Oxycon Pro) or high (Innocor) airflow resistance. Mouth pressures, gas exchange, blood lactate concentration [La- ], perception of breathlessness, respiratory, and leg exertion were assessed throughout the tests. RESULTS: Tests performed with the Innocor were significantly shorter (15.3 ± 3.2 vs. 15.8 ± 3.3 min, p < 0.0001) and showed higher maximal flow resistance (1.3 ± 0.2 vs. 0.3 ± 0.0 cmH2 O∙L-1 ∙s-1 , p < 0.0001). At end-exercise, peak oxygen consumption (-200 ± 220 ml.min-1 , p < 0.0001), minute ventilation (-19.9 ± 10.5 L.min-1 , p < 0.0001), breathing frequency (-5.4 ± 5.2 breaths.min-1 , p < 0.0001), heart rate (-2.1 ± 3.6 bpm, p = 0.002) and [La- ] (-0.7 ± 1.0 mmol.L-1 , p < 0.0001), but not tidal volume (-0.1 ± 0.2 L, p = 0.172) were lower with the Innocor, while the perception of breathlessness was higher (+3.8 ± 5.1 points, p < 0.0001). CONCLUSIONS: Airflow resistance in the upper range of current guidelines can significantly affect exercise performance and respiratory pattern in young, healthy males during incremental exercise. The present results indicate the need to revisit guidelines for devices used in ergospirometry.

Maximal Fat Oxidation: Comparison between Treadmill, Elliptical and Rowing Exercises.

Fat oxidation during exercise is associated with cardio-metabolic benefits, but the extent of which whole-body exercise modality elicits the greatest fat oxidation remains unclear. We investigated the effects of treadmill, elliptical and rowing exercise on fat oxidation in healthy individuals. Nine healthy males participated in three, peak oxygen consumption tests, on a treadmill, elliptical and rowing ergometer. Indirect calorimetry was used to assess maximal oxygen consumption (V̇O2peak), maximal fat oxidation (MFO) rates, and the exercise intensity MFO occurred (Fatmax). Mixed venous blood was collected to assess lactate and blood gases concentrations. While V̇O2peak was similar between exercise modalities, MFO rates were higher on the treadmill (mean ± SD; 0.61 ± 0.06 g·min-1) compared to both the elliptical (0.41 ± 0.08 g·min-1, p = 0.022) and the rower (0.40 ± 0.08 g·min-1, p = 0.017). Fatmax values were also significantly higher on the treadmill (56.0 ± 6.2 %V̇O2peak) compared to both the elliptical (36.8 ± 5.4 %V̇O2peak, p = 0.049) and rower (31.6 ± 5.0 %V̇O2peak, p = 0.021). Post-exercise blood lactate concentrations were also significantly lower following treadmill exercise (p = 0.021). Exercising on a treadmill maximizes fat oxidation to a greater extent than elliptical and rowing exercises, and remains an important exercise modality to improve fat oxidation, and consequently, cardio-metabolic health.

Movement direction impacts knee joint kinematics during elliptical exercise at varying incline angles.

BACKGROUND: Elliptical trainers are a popular cardiovascular exercise for individuals with injuries or those post-operation. There is currently limited data on the impacts of direction while on elliptical trainers for knee joint kinematic risk factors. This study compared lower extremity kinematics between the forward and reverse direction at varying inclines on an elliptical trainer modified with converging footpath and reduced inter-pedal distance. METHODS: Twenty-four college age participants exercised on the modified elliptical in both directions at four ramp inclines: 6°, 12°, 25°, and 35°. Three-dimensional kinematics were collected for each direction and ramp incline. A 2 × 4 (direction × incline) repeated measures analysis of variance was run with an alpha of 0.05. Simple effects analysis was run with Bonferroni correction for significant interaction or main effect of ramp incline. RESULTS: The reverse direction had significantly greater peak knee valgus at 6° incline (mean difference [MD] = 1.35°, p < 0.014, d = 0.31) and 12° (MD = 2.41°, p < 0.001, d = 0.55), peak hip abduction at 6° (MD = 2.86°, p = 0.002, d = 0.49) and 12° (MD = 2.91°, p < 0.001, d = 0.51), but decreased peak knee flexion angles (p = 0.032) at all inclines. CONCLUSIONS: Individuals with knee pathologies such as knee osteoarthritis or anterior knee pain should exercise in the reverse direction at lower inclines. However, switching to the forward direction and/or increasing incline may increase quadriceps strength during a safe activity such as elliptical trainers.

Ankle inversion proprioception measured during landing in individuals with and without chronic ankle instability.

BACKGROUND: Research evidence has suggested that a more sensitive ankle proprioceptive testing method with higher ecological validity is needed for assessing proprioceptive deficits in individuals with chronic ankle instability (CAI). OBJECTIVES: (1) To determine the test-retest reliability of a novel ankle proprioception assessment tool, the Ankle Inversion Discrimination Apparatus for Landing (AIDAL); (2) To assess whether AIDAL scores were sufficiently sensitive to detect proprioceptive deficits in chronic ankle instability (CAI); and (3) To examine whether AIDAL scores correlated with Cumberland Ankle Instability Tool (CAIT) scores. DESIGN: Cross-sectional study. METHODS: The AIDAL was purpose-built to assess ankle discrimination in four positions of ankle inversion (10°, 12°, 14° and 16°) upon landing from a 10cm drop. Area Under the Receiver Operating Curve (AUC) was employed as the ankle proprioceptive discrimination score. Seven-day test-retest reliability was evaluated with 23 university students (12 CAI and 11 non-CAI), and another 36 university students (18 CAI and 18 non-CAI) were in the comparison study. RESULTS: The test-retest reliability ICC score for the whole group was 0.763 (95% CI=0.519-0.892), which showed an excellent reliability level. ICC (3,1) was 0.701 for the non-CAI group (95%CI=0.210-0.910) and 0.804 for the CAI group (95%CI=0.451-0.939). The CAI group performed at a significantly lower level on the AIDAL assessment than the non-CAI group (0.777±0.05 vs. 0.815±0.05, F=5.107, p=0.03). The discriminative AUC value for the AIDAL test was 0.756 with a cut point of 0.819 (sensitivity=0.733, specificity=0.800). The MDC90 scores for CAI and non-CAI groups were both 0.04. Spearman's correlation showed that the CAIT scores were significantly correlated with the ankle proprioceptive discrimination scores (rho=0.401, p=0.015). CONCLUSION: The AIDAL showed good test-retest reliability for both non-CAI and CAI groups. Measuring ankle inversion proprioception during landing may be important for assessing the outcomes of CAI rehabilitation, as proprioceptive performance obtained from the AIDAL was significantly correlated with severity of functional ankle instability CAIT scores.

The effect of posture on maximal oxygen uptake in active healthy individuals.

PURPOSE: Semi-supine and supine cardiopulmonary exercise testing (CPET) with concurrent cardiac imaging has emerged as a valuable tool for evaluating patients with cardiovascular disease. Yet, it is unclear how posture effects CPET measures. We aimed to discern the effect of posture on maximal oxygen uptake (VO2max) and its determinants using three clinically relevant cycle ergometers. METHODS: In random order, 10 healthy, active males (Age 27 ± 7 years; BMI 23 ± 2 kg m2) underwent a ramp CPET and subsequent constant workload verification test performed at 105% peak ramp power to quantify VO2max on upright, semi-supine and supine cycle ergometers. Doppler echocardiography was conducted at peak exercise to measure stroke volume (SV) which was multiplied by heart rate (HR) to calculate cardiac output (CO). RESULTS: Compared to upright (46.8 ± 11.2 ml/kg/min), VO2max was progressively reduced in semi-supine (43.8 ± 10.6 ml/kg/min) and supine (38.2 ± 9.3 ml/kg/min; upright vs. semi-supine vs. supine; all p ≤ 0.005). Similarly, peak power was highest in upright (325 ± 80 W), followed by semi-supine (298 ± 72 W) and supine (200 ± 51 W; upright vs. semi-supine vs. supine; all p < 0.01). Peak HR decreased progressively from upright to semi-supine to supine (186 ± 11 vs. 176 ± 13 vs. 169 ± 12 bpm; all p < 0.05). Peak SV and CO were lower in supine relative to semi-supine and upright (82 ± 22 vs. 92 ± 26 vs. 91 ± 24 ml and 14 ± 3 vs. 16 ± 4 vs. 17 ± 4 l/min; all p < 0.01), but not different between semi-supine and upright. CONCLUSION: VO2max is progressively reduced in reclined postures. Thus, posture should be considered when comparing VO2max results between different testing modalities.

Exerted running results in altered impact mechanics and footstrike patterns following gait retraining.

Exertion may alter running mechanics and increase injury risk. Effects of exertion following gait-retraining are unknown. OBJECTIVES: To determine how exertion effects load rates, footstrike, and cadence in runners following a transition to forefoot strike (FFS) or increased cadence (CAD) gait-retraining. METHODS: 33 (9 M, 24 F) healthy rearfoot strike runners were randomized into CAD or FFS groups. All runners received strengthening exercises and gait-retraining. 3D kinetic and kinematic motion analysis with instrumented treadmill at self-selected speed was performed at baseline & 1-week post-intervention, including an exerted run. Exertion was ≥17 on Borg's Rating of Perceived Exertion scale or voluntary termination of running. RESULTS: Within group comparisons between fresh and exerted running: Cadence not affected in either group. Foot angle at contact became less plantarflexed in FFS (-2.2°, ±0.4) and was unchanged in CAD. Both groups increased vertical average load rate (FFS +16.9%, CAD +13.6%). CAD increased vertical stiffness (+8.6 kN/m). FFS reduced ankle excursion (1.8°). (p ≤ 0.05 for all values listed). CONCLUSION: Both FFS and CAD exhibited increased load rates with exertion. Variables that may have increased load rates were different for each group. CAD runners had increased vertical stiffness while FFS runners had reduced plantarflexion at contact and reduced ankle dorsiflexion excursion.

Reliability of cable downswing load-velocity performance in golf swings.

The cable-pulley downswing is a movement similar to the golf downswing, and therefore may offer a valuable golf specific rotational diagnostic and training tool. However, to be of value, measurements need to be stable across testing occasions. Therefore, the aim of this study was to quantify the test-retest reliability of the cable downswing across a spectrum of load-velocities. Ten male participants (21.7 ± 3.0 years, 84.6 ± 9.8 kg, 1.80 ± 0.05 m) volunteered to participate over 3 testing sessions' separated by a minimum of 3 days. Participants performed maximal velocity cable downswings across eight loads (1.25-18.75 kg), which were incrementally increased by 2.5 kg. Vertical cable stack velocity was collected at 50 Hz via a GymAware linear position transducer. Downswing velocity across all eight loads was observed to be extremely reliable (change in mean = -5.1% to 2.9%, coefficient of variation = 1.5-6.4% and intra-class correlation = 0.70-0.98), with reliability increasing with increasing trials. In conclusion, the cable downswing is a reliable method of tracking rotational ability similar to the golf downswing. Practitioners should establish an upper load relative to the apparatus and participant. Future research should determine the utility and sensitivity of this measure.

Validation of an opto-electronic instrument for the measurement of weighted countermovement jump execution velocity.

The purpose of this study was to analyse the reliability and validity of an opto-electronic sensor system (Velowin) compared to a linear velocity transducer (T-Force System) considered as the gold standard. Mean velocity (MV) and peak velocity (PV) generated in the Smith machine bar placed on the shoulders in counter-movement jump exercise (CMJ) were analysed. The study was conducted with a sample of 21 men with experience in resistance training. Five measurements were analysed for CMJ exercise in concentric phase using a progressive loading increase. Three jumps were made per load with a 3-4 min recovery between loads. The analysis of the variance confirmed that there were no significant differences (p > 0.05) in the execution velocity between Velowin and T-Force with each of the loads. The reliability analysis showed, with each of the loads, high values of the intraclass correlation coefficient (ICC = 0.95-0.99) and a 'substantial' Lin´s concordance coefficient in MV (CCC ≥0.96) and between 'substantial' (CCC = 0.98) and 'almost perfect' (CCC = 0.99) in PV. These results confirm the reliability and validity of the Velowin device is reliable for measuring the execution velocity in loaded CMJ exercise.