Speed and strength are 2 of the most sought-after physical qualities in athletics. In many cases these qualities are trained independent of each other, when in fact these qualities are related (3). The amount of force that an athlete is able to apply to the ground will determine how fast he or she can run, how effectively he or she can change direction, and how high he or she can jump-all actions predominant in many athletic activities. However, improving athletic performance is not as simple as getting as strong as possible.
Athletes do not have the luxury of an abundance of time to generate what strength levels they do possess. It has been shown that 0.3 to 0.4 seconds or more are required to reach maximum force levels (23). Further, during maximum lifts in the traditional squat and deadlift movements, 0.6 seconds elapses before the movement is complete (10).
These may seem like relatively short time periods. However, the time available to produce force in athletics is much lower. For example, in explosive movements, such as running and jumping, force has to be produced in less than 0.3 seconds. In fact, it is usually closer to 0.1 to 0.2 seconds (13,18,31). Strength is not the sole determinant of optimal force output. The rate of force development (RFD) is much more important than strength alone.
An athlete can possess high strength levels and still be deficient in the ability to generate force quickly (11,20). Traditionally, many resistance training programs simply instruct athletes to move the external resistance as fast as possible to achieve adaptations in RFD. A major shortcoming to this methodology, however, is that a large portion of the movement range is spent decelerating the resistance (6,10,17,18,20,33).
A training method that addresses the deceleration period and the need for RFD is called variable resistance training (4,5,28,30). This training method allows the athlete to use his or her mechanical advantage to generate both high force and high power levels in selected resistance training movements (2,8,30). Theoretically, increased muscle activation occurs throughout the concentric phase of a given movement as a result of the progressively increasing resistance the athlete has to overcome with variable resistance training. In addition, when elastic bands are used for variable resistance training, theoretically the athlete also is able to store elastic energy in his or her muscles and tendons, resulting in greater RFD.
Very little research has been done on the performance-enhancing effects of variable resistance training. In fact, to our knowledge only Wallace et al. (31) and Newton et al. (20) have examined the use of variable resistance training with regard to RFD variables. Wallace et al. (31) determined that variable resistance training with elastic bands led to increases in peak power and peak force when compared to resistance training without elastic bands, whereas Newton et al. (20) found that velocity and power values were higher at the early stages of the lift with variable resistance training.
The purpose of this study was to assess the effect of heavy/slow movements and variable resistance training on peak power and strength development. It was hypothesized that variable resistance training would lead to significant improvements in peak power when compared to traditional resistance training.
Experimental Approach to the Problem
College athletes from a variety of sports (baseball, track, football, and basketball) were randomly divided into 3 groups and participated in a training intervention differing only by the speed of movement in the squat exercise and the use of accommodated resistance. Lower-body power was tested pre- and post-intervention and compared statistically for differences between groups. The methods used in this study were reviewed and approved by an Institutional Review Board for research with human subjects. All participants provided written informed consent prior to participation.
Forty-eight NCAA Division I athletes (age: 21.4 ± 2.1 years, all men) were recruited for this 12-week training intervention study. All subjects were apparently healthy, reporting no contraindications to high-intense physical conditioning, and were free from any injury experienced 1 year prior to the study. All participants reported active participation in consistent resistance training and plyometric exercise (at least 3 days per week) in the year prior to participating in this study. Each athlete committed to adhering to the training protocol and refraining from performing alternative or supplemental workouts for the lower body. Subjects also agreed to refrain from cardiovascular endurance training more than 15 minutes per day to avoid any effect on power development (26).
Strength and Power Measurements
Muscular strength was measured via the 1-repetition maximum (1RM) on the barbell back squat exercise. Each athlete had previously performed this test numerous times in conjunction with the normal sport conditioning program, for purposes of monitoring strength development, and therefore was well familiarized with the procedures of the test. Athletes were required to perform a nonspecific warm-up of running and to participate in dynamic stretching before performing approximately 10 squat repetitions with a light resistance. The resistance was then progressively increased to amounts estimated to be less than the participant's 1RM for several subsequent warm-up sets. Finally, for the 1RM test, the resistance was increased in incremental loads following each successful 1RM attempt until failure. All 1RM values were determined within 3 to 5 attempts to ensure reliability.
Lower-body peak power was identified through the use of the TENDO FiTROdyne Powerlizer (Fitro-Dyne; Fitronic, Bratislava, Slovakia) according to protocols suggested by Jennings et al. (15). Athletes weighed in and performed a basic warm-up consisting of light aerobic exercise, dynamic stretching, and low intense jumping prior to performing a maximal countermovement jump test. The same warm-up session was used before pre- and post-testing to control for any warm-up effect. To effectively test power output during the countermovement vertical jump, the TENDO unit cord was attached to the back waistband of each subject's athletic shorts. This arrangement allowed for the base of the TENDO FiTROdyne unit to be positioned on the floor behind the athlete during the test in such a way that valid readings could be obtained without impeding jump technique or performance. To calculate power output in watts (W), each athlete's body mass was imported into the Fitrodyne microcomputer. The TENDO unit proficiently computed peak power according to the speed of movement in the concentric phase of the jump test and each respective athlete's body mass. Five jumps were awarded to each athlete with the highest power measurement recorded. Certified strength and conditioning specialists and investigators oversaw all testing processes to ensure proper technique and safety.
Athletes were randomly assigned to 1 of 3 training groups: heavy resistance/slow movement (Slow), lighter resistance and fast movements (Fast), or fast movements with accommodated resistance (FACC). Three weeks of accustomization (12 training sessions) were included prior to testing, during which time exercise technique was taught and basic fitness trained. Compliance to training was monitored by the researchers and strength and conditioning staff responsible for training sessions. Exclusion criterion for analyses was set at anything in excess of 2 missed workouts over the entire 12-week training intervention.
All training groups performed similar training programs composed of free weight resistance training with lower-body compound exercises (e.g., back squat, powercleans, standard deadlifts, dumbbell walking lunges, and Romanian deadlifts) (Table 1). In conjunction with traditional resistance training, the training control groups performed running and plyometric drills, including variable-distance sprints, split-squat jumps, variable-height depth jumps, and hurdle jumps. Athletes from all treatments followed a periodized training program with resistance exercises performed 2 to 3 days per week, and sprint/plyometric training 1 to 2 days per week, for 12 total weeks. The volume of resistance training exercises averaged 4 sets per muscle group, at a mean intensity of 75 to 85% of 1RM. These training values were selected because they have been shown to be the most operational combination of training stimuli for eliciting maximal strength among athletic populations (21,24). Sets and repetitions for each exercise were periodized in daily-undulating (23) fashion.
The only differences between the training interventions was the speed at which subjects performed the squat exercise and the use of elastic bands to provide progressive resistance during the exercise. Speed was prescribed for each workout and monitored through the use of the TENDO FiTROdyne Powerlizer. With a cord attached to the bar, this apparatus monitors the speed of each repetition and provides immediate feedback to the lifter on the microcomputer screen. The Slow group performed squat repetitions at speeds ranging from 0.2 to 0.4 meters/second. Resistance was set at the highest amount possible that could be moved at the prescribed speed; therefore, subjects did not intentionally move at a slow speed.
The Fast group performed repetitions at 0.6 to 0.8 meters/second with the resistance set at the highest level allowing the speed in the prescribed zone. The FACC group trained at the same speed (0.6-0.8 meters/second) with the addition of accommodated resistance in the form of large elastic bands. During the accustomization period, participants were instructed and trained regarding their prescribed speed, which was monitored throughout the training intervention for control and accuracy. For training, 50% of the athlete's 1RM for back squat was set with the addition of bands as needed to keep training speed within the prescribed training speed. Bar weight was added in small increments (5-10 pounds) as performance increased to progress the resistance accordingly.
Descriptive data (means ± standard deviation) for the various tests were computed and analyzed with the statistical software SPSS version 13 (SPSS Inc, Chicago, Illinois, USA). Level of statistical significance was set at p ≤ 0.05 for all analyses. To examine for normal distribution, Kolmogorov-Smirnov tests were conducted on each independent variable. Data were found to be normally distributed suggesting the appropriateness of parametric statistics. Change in power from pre- to post-testing was calculated and analyzed by multivariate analysis of variance. Meaningfulness of differences was determined by the use of effect sizes calculated by determining the difference between pre- and post-test means, divided by the pre-test standard deviations and interpreted according to a scale previously proposed by Rhea (25).
Statistical power for the analyses averaged 0.67. Descriptive data (means ± standard deviation) are presented in Tables 2 and 3. At baseline, no difference was found between groups in strength or power measures (p > 0.05). Post-test data revealed a significant difference between power improvements between the Slow and FACC groups (p = 0.02). FACC training elicited statistically greater power improvements than the slow training speed (p = 0.02). Statistical differences between Fast and FACC groups were identified at p = 0.09, a level approaching but not reaching the p < 0.05 level, with no statistically significant difference between the Fast and Slow groups (p = 0.45). Percent increases and ESs demonstrated a much greater treatment effect in the FACC group (17.8%, ES = 1.06) with the Fast group (11.0%, ES = 0.80) adapting more than the Slow group (4.8%, ES = 0.28). These ES calculations (Figure 1) demonstrate a large treatment effect among the FACC group, a moderate effect for the Fast training speed, and a small effect for the Slow training speed. Strength data demonstrated no significant differences between groups at either pre- or post-test (p > 0.05); however, statistical power may have limited the analysis because percent increases and effect size calculations demonstrated a sizeable difference between groups. The FACC and Slow groups improved strength comparatively (FACC: 9.44%, ES = 1.10; Slow: 9.59%, ES = 1.08). The Fast group improved strength considerably less-3.20% with an effect size of only 0.38.
The results of this study demonstrate a definitive advantage to training with faster movement speeds with the inclusion of variable resistance for the development of lower-body power among collegiate athletes. Slow training speed was shown to have little impact on jumping power, with both FACC and Slow groups increasing maximal strength to a similar degree. Although percent increases and effect size calculations showed an advantage of training with faster speeds, that benefit was not fully realized until variable resistance was included.
These results support the notion that RFD can be improved through the use of variable resistance training with elastic bands. Several other studies have shown benefits from the use of variable resistance (1,7,8,12,13,19,30). However, only 2 of these studies specifically examined variable resistance training with regard to RFD variables (19,30). Wallace et al. suggested that RFD could be increased as a result of the longer peak velocity phase seen with variable resistance training. Because the resistance increases as the mechanical advantage of the athlete increases, he or she theoretically is able to generate the highest force levels during the concentric phase of the lift when the muscles are at or near their optimal length-tension relationship (9).
Another variable likely responsible for increases in RFD from elastic band training is the exploitation of the stretch-shortening cycle of muscle tissue. The muscle is able to store elastic potential energy during the eccentric phase of the lift and then release this energy as kinetic energy during the concentric phase of the lift (14). This added benefit of training with elastic bands is unseen with traditional resistance training or variable resistance training with chains (28).
It is interesting to note that the FACC group increased maximal strength to a similar amount as the Slow group, whereas the Fast group experienced a small treatment effect for this measure (see effect size calculations). Power, representing the speed at which high levels of force can be generated, is a combination activation of muscle tissue, synchronization of motor unit firing, and rate coding (the speed at which multiple firing signals are sent by motor nerves). Strength, where time to reach maximal force is not as limited, relates more to activation of muscle mass with some relationship to synchronization. It appears, based on the fact that those athletes training with accommodated resistance increased both strength and power, that training with bands may stimulate adaptations in activation, synchronization, and rate coding simultaneously. Although plausible, this theory would require additional research to validate.
Further research should examine the differences in variable resistance training with bands vs. chains in regard to RFD variables. Additionally, long-term effects of variable resistance training, especially with respect to the training maturity of the subjects used, should be studied. It has been established that as the training maturity of the athlete increases, different strength characteristics become the limiting factor(s) in improving performance in a given sport (18,27,29,32).
Variable resistance training with elastic bands appears to provide performance benefits with regard to peak force and peak power. If athletes and coaches can get beyond the limitations of variable resistance training (i.e., cost of equipment and additional set-up time), it seems that this is a viable method of training for athletes who participate in sports where RFD is a key to high levels of performance.
When implementing variable resistance training, care should be taken to adjust the additional resistance in accordance with the 1RM of the athlete. Band tension and/or chain weight must increase as the strength of the athlete increases so as to optimally exploit the mechanical advantage of the athlete. Further, a load must be used that will allow the athlete to accelerate the weight through the entire range of motion of the lift. Loads that are too high will not allow the athlete to gain maximally from the peak power benefits to be had from variable resistance training. Finally, although variable resistance training has been shown to provide significant benefits, it may not be the best training method for every athlete. Athletes with a low training maturity may, in fact, benefit more from training targeted at increased maximal strength. Athletes with a higher training maturity who have already attained high levels of maximal strength may benefit more from a training method such as variable resistance training that allows them to transfer their maximal strength into explosive movements during a given sporting practice/competition.
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