Vertical jump performance is an integral component for numerous athletic tasks. Thus, athletes and others interested in improved jump performance engage in exercise training programs specific to that task. A review of the literature shows that numerous exercise programs have been examined in an attempt to quantify improvements in parameters associated with the vertical jump (1,13,25). Resistance exercise (REX) is one modality that improves vertical jump performance (1,13,25). Exercises used in strength training programs have been classified as general, special, or specific as they relate to their resultant neuromuscular adaptations (1). Structured REX programs for the muscles used for the vertical jump, such as the knee and ankle extensors, increase strength and improve performance (1,13,25). However, training programs performed on different REX modes elicit varied jump-related adaptations and performance outcomes that in turn allow for the comparative assessment of each equipment modality. The ideal REX device for training aimed at improved jump performance has yet to be determined.
Standard REX equipment entails repetitions that are performed against a constant external load. Repetitions are typically performed until muscle forces are unable to overcome the resistance provided by the external load because of fatigue (10). A short rest period ensues, followed by another set of repetitions performed to momentary muscular failure. If REX training adaptations were localized to the knee and ankle extensors, vertical jump and related performance parameters should improve. One such device that operates in this manner is shown in Figure 1.
Unlike the manner in which standard REX devices operate, an ergometer (YoYo Technologies; Stockholm, Sweden) (Figure 2) that imposes inertial resistance with two flywheels also trains knee and ankle extensors. As concentric forces are exerted on a footplate, inertial resistance is overcome and the flywheels spin. Kinetic energy imparted by flywheel rotation reverses the footplates' direction as eccentric resistance is incurred. The eccentric resistance produced may exceed that for standard REX devices (2,3). Thus the ergometer also offers a potent REX stimulus that may improve vertical jump performance.
The disparate manner in which the devices shown in Figures 1 and 2 operate may yield unique and novel training adaptations when such equipment is part of structured REX programs. The purpose of the current study was to compare the effects of knee and ankle extensor REX performed on devices shown in Figures 1 and 2 on jump-related performance outcomes. Although a total-body REX program may be most beneficial to improve jump ability, the knee and ankle extensors are of primary interest because research show that those muscle groups contribute 49% and 23%, respectively, to work done during vertical jump tests (14). Thus total-body REX protocols may add little to gains incurred by a structured knee and ankle extensor program, as it increases the risk of overtraining from the increased volume of exercise. Results may reveal the best REX equipment modality for various jump performance measures. We hypothesized that each REX device would yield greater gains than those incurred by sedentary control (CTRL) subjects. Based on a similar design that compared strength and body composition changes (7), we also hypothesized that the standard REX device would evoke greater performance gains than those provided by the ergometer.
Experimental Approach to the Problem
With a matched-pairs design, healthy college-age subjects were assigned to one of three groups with no cross-over. The intent of the matched-pairs design was to ensure that groups were statistically similar in body mass, gender distribution, and anthropometric measurements as subjects began their participation. Two groups performed leg and calf press REX on Figure 1 or 2 devices, whereas a third acted as CTRLs. Group assignments lasted 6 weeks and were preceded and followed by tests that assessed jump performance-related parameters. Six weeks minimizes muscle hypertrophic influences on current results and thus may act as an early phase adaptation for REX periodization (16-18). Thus, the current study compared changes to REX-trained and CTRL groups over a period in which neural factors are the primary influence over strength gains (16-18,22).
The three groups were composed of standard (n = 10, seven men and three women), ergometer (n = 9, seven men and two women), and CTRL (n = 9, six men and three women) subjects. Informed written consent was obtained from subjects before their participation. All study procedures were approved by the institutional review board at The University of Tulsa.
REX Training Protocol
Standard and ergometer subjects performed REX twice per week for the 6-week period. Workouts were spaced at least 48 hours apart. To reduce the risk of injury, workouts began with a 5-minute warm-up on a stationary bicycle against 9.81 Newtons of resistance at a self-selected velocity. After the warm-up, subjects performed a three-set, 10-repetition leg press protocol for the knee extensors, followed by a similar calf press paradigm for the ankle extensors. Rest periods between sets lasted 2 minutes. All workouts for both groups were supervised by a project coauthor with an extensive REX background.
A week before their pre-test date, standard REX subjects had their initial 10-repetition maximum (10RM) loads determined for each exercise. Throughout their participation, as standard REX subjects became stronger, their resistance was increased by a coauthor (C.A.R.) such that they continued to perform only 10 repetitions per set. Over the 6-week period, increases in 10RM loads, from the initial to final workouts per exercise were recorded, so the magnitude of REX training progression could be quantified.
In contrast, because the ergometer does not use gravity-dependent resistance, subjects were constantly encouraged to increase the rates at which concentric muscle actions were performed. Encouragement was provided by a project coauthor (M.A.C.) who supervised all ergometer workouts. Increased rates of muscle shortening helped to evoke higher work outputs, as the knee extensors exerted greater force to overcome inertia. Higher rates also permitted the footplate to return to the starting position more quickly and thereby offer greater eccentric resistance. Data from ergometer subjects were recorded online at 100Hz and analyzed with methods previously described (10). Per exercise, data from the first and last workouts were used to calculate the total work performed to assess increases in work output over time. In addition, improvements for the two REX groups over the 6-week period were compared to determine whether the degree of intergroup training progression (progressive overload) was comparable.
Within a week of the start and completion of the 6-week intervention, a series of jump-related performance tests were administered to note the changes incurred per treatment group. All subjects engaged in the pre- and post-intervention tests. Each test session began with anthropometric data collection as subjects stood in a relaxed upright posture. Subject's body masses and heights were measured to the nearest tenth of a kilogram and centimeter, respectively. Upper and lower right leg lengths, as well as thigh and calf circumference, were assessed with a cloth tape measure. Upper leg lengths were measured from the anterior superior iliac spine to the patellar rim, whereas lower lengths quantified the distance from the fibular head to the lateral malleolus. Circumference was measured at equidistant lengths from each of the respective leg length sites. Also assessed at the circumference sites, subcutaneous skinfold thickness was determined at four points (anterior, medial, lateral, posterior) along the right thigh and calf with calipers (Slimguide Creative Health Products, Plymouth, MI). The average of three thickness measurements taken at each point was recorded to the nearest millimeter. Estimated thigh and calf cross-sectional areas (CSAs) were derived from circumference and skinfold values using previously established methods (17,18). All anthropometric and test measurements were administered by the principal investigator (J.F.C.).
After anthropometric data collection, all subjects performed three jump tests with a 156 × 83 × 30 mm instrumented switch mat (Probotics Inc., Huntsville, AL). Powered by a 9-volt battery, the mat had a reported accuracy of ±1.27 cm. Preceded by a 5-minute general warm-up of aerobic and stretching activity, subjects completed three trials of each jump test, with the best performance recorded for statistical analysis. A standing vertical jump test was recorded from the time spent in the air as subjects jumped as high as possible. Hang time, vertical height, and power output were variables collected from each trial. A four-jump protocol was next performed; subjects were told to repetitively jump as rapidly and as high as possible until the test was completed. Variables obtained included mean jump height, elapsed ground time, and explosive leg power factor (ELPF), which the mat manufacturers describe as an air/ground time ratio. The final test was a counter-movement depth jump from a 44-cm platform. Subjects landed on the mat placed next to the platform, then jumped as high as possible in one continuous motion. Subjects also landed on the mat at the conclusion of the test. Variables from the final test included hang time, vertical height, and power output.
At the completion of the jump tests, test sessions concluded with a protocol that estimates the percentage of fast-twitch knee extensor muscle fibers. Seated on an isokinetic dynamometer (System 3; Biodex Corp., Shirley, NY), all subjects performed concentric exercise with their right quadriceps from 90o (lower leg perpendicular to femur) of knee flexion through 0o (full extension) of motion. Per subject, dynamometer settings were maintained among test sessions such that the shaft of the powerhead was aligned through the right knee's frontal axis of rotation. To prevent extraneous body movement, Velcro straps were placed across the pelvis, right distal femur, and tibia to secure the leg to the dynamometer's attachment arm. Subjects kept their arms folded over their chest as they performed 50 maximal-effort knee extensor repetitions at a rate of 3.14 rad·s−1. Subjects were instructed not to pace themselves throughout the protocol. Subjects exerted no knee flexor torque as the dynamometer's attachment arm returned to the original position to begin new repetitions. Average peak torque values from the initial and final three repetitions were used to calculate a fatigue product (FP). Estimated fast-twitch percentage (FT%) was calculated with the following equation: 0.9 × (FP) + 5.2 = FT% (24).
To assess whether the magnitude of training progression was similar between the two REX groups, their initial and final workout data values were compared as a percentage change with t-tests. Workout changes to 10RM loads (standard REX) and total work performed (ergometer REX) incurred over the 6-week period were compared to assess the degree of progressive overload for the two REX groups.
Anthropometric data were examined with 3 × 2 (group × time) mixed-factorial ANOVAs with repeated measures applied to the time variable. Dependent measures from the jump and estimated knee extensor FT% tests were assessed with two different 3 × 2 ANCOVA procedures with repeated measures applied to time in each case. The first procedure employed a summed thigh and calf CSA value, and the second used pre-intervention values per dependent variable as statistical covariates, respectively. The CSA values were used as covariates based on previous research that showed that the knee and ankle extensors contribute 49% and 23%, respectively, to work done during a vertical jump test (14). Pre-intervention values were also examined as covariates to reduce intragroup variability, as the current study had a consistently higher level of within-group heterogeneity from the standard REX group. Interactions were further treated with a simple comparisons test to establish the source of the difference. ANOVA assumptions (normality, homogeneity of variance, sample independence) were met before administration of the statistical treatment (15). An α value of 0.05 was used for all analyses.
No subjects were injured through their project participation. With an ω2 calculation to estimate sample size and a 0.80 power level for a medium treatment effect approximation, the current study had sufficient data for the ANCOVA statistical procedure (15). Based on percent changes over the 6-week period, both REX groups incurred a training progression. However t-tests revealed a significantly (p < 0.05) greater leg press progressive overload from standard REX training, whereas the same group showed a trend (p = 0.10) toward greater improvements vs. ergometer subjects with the calf press exercise. Per test session and subject, three trials were performed for each of the three jump tests. Intraclass correlation coefficients for the vertical, depth, and four-jump tests were 0.92, 0.90, and 0.88, respectively.
After 6 weeks, anthropometric variables (Table 1), which included estimated thigh and calf CSA values, showed insignificant changes. Thus, current results likely were not affected by muscle hypertrophy (16-18), which suggests neural adaptations were the primary factor behind strength and jump performance gains. With summed estimated thigh and calf CSA values as covariates, 3 × 2 repeated measures ANCOVAs revealed mean height from the four-jump test (Table 2), and mean torque output from the final three isokinetic repetitions (Table 3) were statistically (p < 0.05) significant. Post hoc analysis revealed the following relationship: standard REX, ergometer REX > CTRL. A 3 × 2 repeated-measures ANCOVA with the same covariate showed a group × time interaction (p < 0.05) for ELPF from the four-jump test. Post hoc analysis revealed that post-intervention standard REX values were the source of the interaction.
With pretest values per dependent variable as statistical covariates, a 3 × 2 ANCOVA showed a time (post > pre; p < 0.05) effect for vertical jump power output and a group (standard REX, ergometer REX > CTRL; p < 0.05) effect for vertical jump hang time (Table 2). A trend toward a group × time interaction (p = 0.10) for hang time from the depth jump test occurred, as ergometer REX values increased over 6 weeks, whereas the other groups did not improve. Analysis of Table 3 data shows time (post > pre) effects (p < 0.05) for mean torque from the first three repetitions, FP, and estimated FT%. A group (standard REX, ergometer REX > CTRL) effect (p < 0.05) also occurred for mean torque from the last three repetitions.
Current results show that either REX mode evokes greater strength and jump performance gains than a CTRL condition. Closer data inspection suggests, perhaps because of more progressive overload incurred by standard REX subjects, Tables 2 and 3 time effects were primarily the result of the effort provided by that group. Nonetheless, current study REX contributed to the time effects (post > pre) seen with isokinetic dynamometry and vertical jump power output. The current study hypothesis was in part supported, as standard REX subjects incurred gains that led to an ELPF interaction obtained from the four-jump test. Yet depth jump hang time displayed a trend for an interaction, with post-test ergometer REX data as the source of this difference. Current results suggest REX mode-specific training adaptations occurred. Such an effect was previously noted (1).
Although muscle control and coordination influence vertical jump performance (4), muscle strength increases from REX typically improve jump ability (1,6,13,25). A 12-week lower body REX program, in which previously untrained men worked out 3 days per week, caused significant improvements in vertical jump height, power output, ground time, and flight time (13). In addition, the 12-week protocol led to significant strength gains in the squat and leg press exercises (13). With a REX program equal in duration to the current study, female track athletes performed three sets of bilateral seated knee extensions with an 8-10RM load 3 days per week (25). Results included significant increases in relative knee extensor strength and vertical jump height (25). Finally, a study of national-caliber Olympic lifters (n = 64) also showed counter-movement and static vertical jump peak power were each highly correlated with 1RM squat loads (6). Thus, concurrent strength and jump performance gains in current study REX-trained subjects concur with previous investigations (1,6,13,25).
Significant time effects (post > pre) occurred for estimated FT%, FP, and mean torques from the first three isokinetic repetitions. Results show that 6 weeks of REX increased knee extensor force output. Yet higher torques from the current 50-repetition isokinetic test likely increased intramuscular metabolite build up, which in turn yielded higher FP and estimated FT% values. REX adaptations include greater FT representation by muscle area and IIx → IIa fiber conversions (5,22,23). Different REX protocols evoke various levels of hypertrophy among the three main fiber (I, IIa, IIx) types (5,23). Subjects assigned to “low” or “intermediate” repetition REX for 8 weeks had greater IIa and IIx fiber percentage area increases than did a high-repetition group (5). Whereas 20 weeks of lower body REX enlarged vastus lateralis CSA for all three fiber types, IIa area incurred a greater percentage increase than type I fibers (23). Altered myosin heavy chain activity initiates myofibril phenotype changes, which can occur well within the current study time frame (22). Like the intermediate repetition protocol (9-11 repetitions/set) of the Campos study (5), current study REX may have preferentially targeted FT fibers. Because 6 weeks of REX does not elicit muscle hypertrophy (16-18), current adaptations were confined to higher force outputs, which in turn may have evoked intramuscular metabolite build-up during the post-intervention isokinetic to yield higher FP and estimated FT% values.
Perhaps the most interesting current finding is training-specific adaptations offered by each REX mode. Analysis of depth jump hang time data showed a trend for post-intervention ergometer REX values as an interaction source. ELPF data were statically significant; post hoc analysis revealed post-intervention standard REX values as the source of the interaction. A recent leg press study compared strength and body composition changes in subjects assigned to a standard REX or ergometer REX treatment with no cross-over (7). After 10 weeks, both groups incurred similar muscle mass, strength, and fat loss improvements; however, standard equipment REX evoked significant total- and lower-body bone mineral density gains, whereas the same variables were unchanged by ergometer workouts. Between-group differences were attributed to the greater peak forces that led to higher strain magnitudes applied to bones that resulted from standard REX (7). Greater peak forces and strain magnitudes resulted from the faster rates that repetitions were performed with standard REX (7). In like fashion, repetition rate differences in the current study may have led standard equipment REX to elicit faster muscle efforts that in turn produced higher ELPF values.
While plyometric workouts increase rates of force development to yield greater hang times and vertical displacements, REX may improve jump- and power-related variables to an equal extent (13). Research suggests tasks in which no external load is lifted, like the current four-jump test from which ELPF was derived, exertion of higher power outputs per unit of body mass or force per unit time are thought more important than the absolute level of power output (6). However, such studies may also note the higher peak torques and forces incurred by REX improves jump performance parameters. In accordance with current results, other studies also noted peak torque and force gains from standard REX devices. Significant knee extensor peak torque gains occurred from REX given over the same time duration as the current study (11). Longer REX interventions also noted significant gains in torque and force (17,19-21). Furthermore, REX set repetition protocols used in some previous investigations (19-21) are comparable to those of the current study. Thus, previous results support the merits of standard REX devices to yield higher ELPF values (6,7,11,13,17,19-21).
In contrast to ELPF results, current study depth jump data show that ergometer REX evoked a trend for greater post-intervention hang times than the other treatment groups. Had the current intervention been longer, statistical significance may have occurred. Because of an exaggerated counter-movement and musculotendinous lengthening, the depth jump induces more knee and ankle extensor pre-stretch than the other current tests (1). In similar fashion, the ergometer induces a greater pre-stretch than standard REX devices. The exaggerated pre-stretch influences adaptations that result from work done on the ergometer and may account for differences in performance outcomes obtained from the REX devices used in the current study. For instance, an examination of the effects of contractile mode on the net caloric cost for leg presses done on the ergometer yielded novel results (9). Whereas the inclusion of eccentric leg press actions to concentric REX on standard equipment increases net caloric cost by roughly 17% (12), a similar protocol done on the ergometer showed that eccentric actions did not raise net caloric cost above concentric-only REX values (9). Results were attributed to greater elastic energy utilization from increased knee extensor pre-stretch seen with ergometer REX (9).
Integrated electromyography (IEMG) and anthropometric data served as predictor variables to explain the variance with calf press REX performance done on the ergometer (8). Analysis of 10-repetition sets revealed ankle extensor IEMG explained little final-repetition performance variance (8). It was concluded that greater elastic energy utilization from the triceps surae-Achilles tendon complex lowered motor unit activity (8). In similar fashion, current ergometer workouts likely conditioned the knee and ankle extensors to better utilize stored elastic energy for tasks such as the depth jump. In conclusion, REX mode-specific adaptations likely accounted for current improvements in strength and jump-related performance parameters. Given the duration of the current study, such gains resulted from neural adaptations (16-18). With the dependent variables measured in the current study, it is difficult to discern whether the improvements resulted from physiological changes to the neuromuscular system or whether a learning adaptation occurred over the 6-week period. Perhaps a combination of both factors contributed to REX mode-specific strength and jump-related performance adaptations. Continued research may help to confirm these effects and elucidate how such adaptations occur.
The current study workout paradigm may induce neurally mediated early-phase periodization adaptations consistent with preseason preparation. Results suggest a 6-week period with either REX mode increases muscle strength. Perhaps the outcome of most interest to coaches and practitioners is the REX mode-specific performance gains. For jump-related variables that are performed in a more rapid fashion, gains may be optimized on standard device workouts that enable repetitions to be executed quickly to yield higher peak torques and power outputs. Such a training stimulus transfers well to high-speed jump-related tasks. In contrast, ergometer workouts seem better for jump-related tasks that require increased musculotendinous pre-stretch or elastic energy utilization. Thus, the different REX devices elicit task-specific training adaptations.
Project funding was provided by the University of Tulsa College of Business Administration's 2006 Summer Research Grant. J.F. Caruso and M.A. Coday are participants in the Tulsa Undergraduate Research Challenge at the University of Tulsa.
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