It has been demonstrated that training in explosive, powerful movements can be helpful in improving athletic ability and performance in both aerobic and anaerobic events (17). What is not entirely understood are the physiological mechanisms that allow for an increase in power output in humans. In addition, the relationships that dynamic measures of muscle power share with other performance variables such as peak isometric force (PF), isometric rate of force development (RFD), and other maximum-effort activities such as one-repetition maximum (1RM) are unclear (20). There are data that suggest that maximum strength is a key factor in determining the amount of power a person can produce, but the exact relationship between the amount of force that can be produced and the level of muscle power expressed by an individual remains undefined (22).
Rate of force development can be defined as a change in the level of force divided by the change in time (22). The importance of RFD has become more apparent in recent years, and it has been argued by some researchers that the ability to achieve maximum force production rapidly may be of greater importance than the actual levels of force achieved (1,22). This is because of the critical role that RFD is thought to play in both acceleration capabilities and explosive strength in athletic populations (22). Improvements in RFD are thought to result from increases in the rate of muscle activation by the nervous system, but they may also be related to patterns of motor unit recruitment, fiber type composition, and muscle hypertrophy (1,27). Although earlier studies have shown little or no relationship between RFD measured isometrically and more dynamic activities (6), other studies have reported a very strong correlation between more dynamic activities such as sprinting or jumping and RFD in isometric actions (1,7,22).
Several studies have shown very little increase in skeletal muscle hypertrophy or surface electromyography (EMG) changes after training in power activities, even though significant increases have been noted in strength and power performance (8). Two theories that address this lack of apparent response are that 1) changes in the neurological system are occurring to allow for an increase in power output that EMG is not sufficiently sensitive technique to detect and, 2) there are physiological, biochemical, or structural adaptations occurring within the muscle that are not reflected through changes in muscle hypertrophy. One example of such changes could include a shift in muscle fiber type between less powerful to more explosive fibers (26).
There is research showing that individuals who are involved in strength and power sports possess a greater percentage of type II skeletal muscle fibers than athletes involved in sports of a more aerobic nature (23). It has also been demonstrated that skeletal muscle has the ability to adapt very effectively to a training stimulus, but the dose-response relationship between ballistic resistance training and muscle plasticity in humans is unclear. Significant changes at the protein level have been discovered by Liu et al., with an observed decrease in myosin heavy chain (MyHC) and a resultant increase in MyHC2a after participation in a combination of ballistic and traditional resistance training exercises (11). Much of the research that has been performed in the past has used elite-level strength and power athletes compared with endurance athletes or, at the other end of the spectrum, completely sedentary individuals, in efforts to observe current fiber type expression among various populations, with differences being noted cross-sectionally (6,17). In recent years, the importance of power to nonathletic populations has been given increased attention in the literature. However, there has been only limited research performed to isolate the training response to muscle fiber types with recreationally trained individuals (6). In addition, although substantial evidence exists concerning strength training intensity and subsequent muscular adaptations, not as much research has been performed concerning the velocity of the training stimulus and subsequent adaptations of muscle fiber type (14).
The intentional manipulation of the various parameters of resistance exercise commonly referred to as periodization has garnered the attention of researchers in recent years (9). This focus has come as an attempt by scientists to understand the effectiveness of periodization as a tool to progress individuals in an exercise program and to improve performance. The majority of the research has indicated that the use of intentional variation in exercise parameters is an effective means for maximizing gains made during an exercise program over more monotonous programs as pointed out by Fleck in his critical review on the topic (5). Although the literature on the effects of periodization has become somewhat clearer in recent years with respect to traditional strength exercise, to date there are few available data indicating how the use of a periodized program might affect training in a ballistic manner (9). Given the growing popularity of ballistic forms of training such as the jump squat (JS) exercise with practitioners as well as researchers, the lack of data on the relationship between ballistic stimuli and adaptations represents a gap in the literature that should be addressed.
The purpose of this study was to determine the effect of an explosive resistance training protocol on performance variables and whether or not periodization would be an effective means of progression in an 8-week ballistic exercise protocol. Of additional concern was the relationship that the performance variables might have with muscle fiber types and any changes that might occur as a result of the training stimulus early on in the adaptive processes.
Experimental Approach to the Problem
The subjects involved in the experiment were randomly placed into one of two study groups: a training group (T) and a control group (C). All subjects had a muscle biopsy taken at the beginning of the study and performed a 1RM squat on a Smith machine using previously reported methods (29). Initial measures of peak power (PP) were obtained from a countermovement jump using a testing load of 30% of the subjects pretraining 1RM during both pre- and posttraining in a Smith machine while standing on a force plate (12,29). In addition, subjects performed a midthigh isometric pull while standing on a force plate to measure PF and RFD (7,20).
Group T participated in 8 weeks of ballistic resistance exercise in addition to their normal resistance training regimen and was not to do any loaded ballistic activity outside of the parameters of the study. Group C did not participate in the 8-week training block and was advised to make no changes to their prior resistance training program. Additionally, group C was given instructions prohibiting the performance of any loaded ballistic activity during the course of this study. Both groups participated in unsupervised traditional resistance exercise outside of this study. Specific details were not recorded because they were different on an individual basis. After the 8-week exercise session, postintervention measurement of PP was performed at 30% of original 1RM for groups T and C in the JS. All other strength and power measurements (1RM, PF, RFD) were repeated for analysis of between-group effects. In addition, muscle tissue samples were used for comparative analysis of pre-post muscle fiber type expression.
Fourteen male subjects between the ages of 18 and 30 years completed the study (Table 1). During the course of the investigation, two subjects from group C withdrew. The subjects were defined as recreational athletes who were currently participating in resistance training activities for at least 3 months but who had not performed ballistic and/or explosive type resistance exercises in the last 12 months. After initial testing, subjects were paired according to strength in the Smith machine squat 1RM, and then they were randomly assigned into either the T (n = 8) or C (n = 6) group. This experiment was approved by the University of Wisconsin-La Crosse institutional review board for the protection of human subjects, and all subjects were informed of the procedures and potential risks involved before they gave their written consent to participate.
Subjects in group T trained at 26-48% of their individual 1RM on the JS exercise for three sets, three times per week on nonconsecutive days for 8 weeks in supervised resistance training sessions. Training loads were consistent with those recommended in the available literature, which has suggested that the optimal loading for PP in the JS should be between 20 and 60% of 1RM (22). The training program was periodized to decrease volume and increase intensity linearly during the course of the 8-week program, but it did include within-microcycle variations in an undulating manner on a session-by-session basis (Table 2) (5,19). Subjects in both study groups were instructed to maintain their previous training protocols outside of this study for the duration of the project. Subjects were advised against any ballistic activities with loading in addition to their body weight outside of the training associated with this study for a duration of 8 weeks. All sessions were supervised by a certified strength and conditioning specialist, and there was 100% compliance with the training protocol.
Muscle biopsies were obtained at week 0 and week 9 and were extracted using the percutaneous needle biopsy technique with suction (2,4). Tissue samples were taken from a site one third of the length from the proximal lateral edge of the patella to the anterior superior iliac spine of the vastus lateralis muscle. To ensure adequate sample sizes, the double-chop method was used during the biopsy procedure (21). Approximately 100-150 mg of skeletal muscle was removed, oriented with respect to muscle fiber direction, and mounted on cork. After the mounting procedure, muscle tissue was frozen in isopentane precooled in liquid nitrogen to −159°C and stored at −80°C for later analysis.
Fiber Type Analysis
Standard histochemical analysis was performed to determine muscle fiber typing (3). Serial sections (12 μm) were cut in a cryostat at −20°C for histochemical analysis. Fiber type analysis was performed using the myosin ATPase (mATPase) method (pH 9.4) after optimized acid (pH 4.54) preincubations. Fibers were classified as I, IIA, and IIB according to staining intensities. To confirm the accuracy of the fiber type staining, samples were stained at a preincubation pH of 10.3.
The percentages of at least 50 fibers per major type (I, IIA, and IIX) per biopsy were determined using the staining methods mentioned above. Previously published research has indicated that 50 fibers per biopsy are of sufficient amount to accurately measure fiber type percentage and cross-sectional area in skeletal muscle (13). In the case of our samples, hybrid fibers comprised a very small percentage of the total number of muscle fibers, making it impossible to identify 50 such fibers in a single biopsy sample. As a result, fiber type percentages were only determined for the major fiber classifications. Reliability data for this technique are high and have been previously established by our authors (ICC = 0.92-0.98 and CV = 2.12-3.67%).
One-Repetition Maximum Testing
One-repetition maximum testing was performed in a Smith machine as described previously by Wilson et al. (29). Foot position was determined using a weighted pendulum to ensure that the subjects' feet were directly under the bar. Foot position was marked and measured to allow for precise placement in both pre- and posttesting. Multiple warm-up trials were given before actual 1RM testing (percentages are given of subjects' estimated 1RM), 10 repetitions at 30% followed by 2 minutes of rest, seven repetitions at 50% followed by 2 minutes of rest, four repetitions at 70% followed by 3 minutes of rest, and one repetition at 90% followed by 3 minutes of rest. From the last warm-up set, loading was increased through subject feedback on the level of repetition intensity so that 1RM was achieved within three trials. Four minutes of rest were given between each 1RM effort. Before testing, foot and grip placement were measured for each subject to maintain consistency between pre- and posttesting conditions. For each repetition, subjects were asked to lower the bar until their hips were parallel to their knees, and they were advised that, on reaching the bottom of the squat, they should immediately move the bar upward until the start position was reached. Subjects were allowed to self-select the tempo of their lifts during this protocol. The reliability of this method of 1RM testing in our laboratory with similar subject demographics is high (ICC = 0.98).
Peak Power Testing
Peak power was measured during the 30% 1RM countermovement JS test, which was performed on a Quattro Jump Force Plate (Kistler Instrument Corporation, Amherst, NY). Thirty percent of the 1RM pretraining value was used for both pre- and posttesting. The knee angle at the bottom of the movement was self-selected by each subject and maintained pre- and posttesting by visual observation. Foot placement was identical to that used in determination of the back squat 1RM. Peak power was calculated by using the mass of the jumper and the weight on the bar (F = ma) to generate an acceleration-time graph. This graph was integrated to create a velocity-time graph, which was then multiplied by the original force-time graph to generate a power-time graph. Subjects performed three sets of one repetition each and were allowed 2 minutes of rest between sets. The highest value of the three trials was used for analysis. Coefficients of variation of 2.5 and 3.4% were obtained for PP and jump height using similar methods and the Quattro Jump Force Plate with test-retest reliability testing in our lab. A familiarization session for all testing was conducted on a separate day.
Isometric Midthigh Pull Test
Peak force and RFD were tested simultaneously by having the subject perform an isometric midthigh pull on the Smith machine using the methods described by Stone et al. (22) and McGuigan et al. (15). The bar height was adjusted at 2-cm increments so that the knee angle was 130° (straight leg = 180°). Foot placement and hand grip width were measured to ensure consistency between pre- and posttesting sessions. Subjects were instructed to pull on the immovable bar as quickly as possible and were required to maintain effort for 5 seconds. Rate of force development was determined using the slope of the force tracing in the first peak. Previous research using isometric performance testing has suggested that subjects achieve similar PF and higher RFD values when instructed to achieve maximal force as quickly as possible (20). Subjects performed three sets and were allowed 2 minutes of rest between sets. The highest value of the three trials was used for analysis. We have previously reported the test-retest reliability of this test as being high for PF and RFD (R > 0.96 and CV < 10%) (15).
Statistical analysis on the performance data and muscle fiber typing was performed using a repeated-measures analysis of variance. Correlations between variables were determined by a Pearson correlation coefficient. All tests were calculated using SPSS 11.0 for Windows (SPSS Inc., Chicago, Ill). For all comparisons, the statistical significance was set at p ≤ 0.05.
All data are presented as means ± standard deviations. Group T showed a 28% improvement in PP from 4088.9 ± 520.6 to 5737.6 ± 651.8 W pre to post in the 30% JS test (effect size [ES] = 3.17) (Figure 1). In addition, there was a statistically significant difference (33%) observed between groups for PP after the training intervention, with group T being higher. Rate of force development in the isometric midthigh pull test improved 49% pre to post for group T from 12687.5 ± 4644.0 to 25343.8 ± 12614.4 N·s−1 (ES = 2.73) (Figure 2). Peak velocity (PV) improved significantly in group T from 1.59 ± 0.41 to 2.11 ± 0.75 m·s−1 (ES = 1.27). No significant differences were noted in squat 1RM or PF for group T (Figures 3 and 4). No significant changes in any of the performance variables were observed in group C. The significant changes in PP and RFD were significantly correlated to each other in group T (R = 0.735, p = 0.038).
Muscle Fiber Type Expression
Before the training intervention, there were no significant differences between groups for muscle fiber type percentage (Table 3). In addition, there was no statistically significant change in muscle fiber type percentage in either group pre to post.
A significant increase in muscle power and RFD was observed as a result of ballistic resistance exercise training. These changes occurred independently of significant strength increases because there was no difference noted in either squat 1RM or PF as measured in a midthigh isometric pull. Previous studies that have investigated the effects of ballistic resistance training on performance did find changes in strength resulting from the training stimulus they applied (12,24). However, one of these studies used untrained subjects and used ballistic resistance training in combination with heavier forms of resistance training (24).
It would seem from the results observed in this study that, in recreationally trained athletes, ballistic resistance training alone is not a sufficient stimulus to cause strength increases. In addition, as the testing load for both pre and post PP testing was 30% of the pretest 1RM, improvements in PV and/or RFD are the most likely causes of the noted increase in muscle power in this study. These results agree with the findings of Kyrolainen et al., who have reported a significant change in power production and mechanical efficiency (ME) independent of improvements in force production or strength (10). Because the formula for mechanical power is power = force × velocity, improvement in velocity of movement may be a significant contributor to the noted increase in PP. This occurred coincidentally with an increase in RFD during the isometric midthigh pull, which has been related to dynamic muscle power capabilities (22).
No changes were noted for within-group or between-group effects pre-post for muscle fiber type expression. The lack of physiological response to ballistic resistance training observed in this particular study has been noted by several other researchers (10,12,14). However, it is acknowledged that subtle changes in fiber type composition or changes in fiber type composition from other muscles, such as the triceps surae, might have contributed to the observed change in performance in jumping. Potteiger et al. (18) did show shifts in muscle fiber type associated with improvements in power after only 8 weeks of training. However, in the current investigation, the use of the ATPase staining method and the small number of hybrid fibers did not allow for accurate subtyping of muscle fibers, so shifts in the levels of IIAB fibers, for example, would not have been detected (13). Furthermore, it may be simply that in the early stages of adaptations to ballistic forms of training, changes are more closely related to issues of recruitment, rate coding, and other neural modifications rather than more structural changes such as a fiber type shift. This is supported by our noted increase in RFD, which many researchers speculate to be largely a result of neural adaptations (1).
There are several other factors that could have contributed to the noted increase in PP and RFD that were not investigated as part of this particular study. Further research in this area of study should consider these factors as possible explanations for our lack of noted physiological response. Possible examples of these factors are 1) changes in muscle recruitment strategies by the subjects, 2) changes in muscle architecture, such as fiber orientation or angle of pennation as a result of the training stimulus, could cause a subsequent increase in muscle cross-sectional area and may contribute to ME and power production, 3) alterations in the level of tendomuscular stiffness could improve the return of stored elastic energy in a loaded countermovement jump, 4) possible changes in the passive elastic structures of skeletal muscle such as collagen that could affect ME, and 5) biochemical adaptations and alterations in sarcoplasmic reticulum function, such as the rate of calcium ion uptake and removal or the level of calcium ATPase activity in the muscle cell itself (10,25).
Another possible contribution to the noted increase in PP could have come from motor learning, because the subjects in group T did perform 8 weeks of JS exercise, which was the task that they were being tested on. However, when one considers the noted pre-post increase in RFD in what was, for all subjects, a novel task, it is likely that other adaptations were occurring.
The results of this study are consistent with previously published research on the effects of using a periodized program in resistance training (5,919). According to a review by Fleck, correctly manipulating the variables of volume, intensity, and rest can optimize results from resistance exercise (5). A study performed by Newton et al. found similar results with the use of a periodized program to increase both power and strength in young and older men (16). This is one of the first studies to use periodization in conjunction with ballistic training alone. It would seem that providing variation in intensities, volume, and rest time is an effective technique when prescribing training designed to increase power production in a moderately trained population (16).
In conclusion, the results of this study indicate that the use of a periodized ballistic resistance exercise program is an effective method for improving PP and RFD. Further research needs to examine possible mechanisms for the adaptations in the performance variables tested. Increasing the duration or the volume of the training protocol may be an effective method to highlight any adaptations in muscle fiber type expression that may occur as a result of training. In addition, using a more sensitive technique for analysis of muscle fiber characteristics may allow for identification of any resultant shifts in muscle fiber subtype.
The data from this current investigation support the use of ballistic resistance exercise for the improvement in PP in a moderately trained population. In addition, our results demonstrate that periodizing training variables in an undulating fashion can be an effective planning method when implementing ballistic resistance exercise into a training regimen. Coaches and other practitioners wishing to improve power performance should consider the use of these methods.
The authors would like to acknowledge the grant committee from the University of Wisconsin-La Crosse Graduate Council for their support of graduate student research and the staff of the University of Wisconsin-La Crosse Student Health Center for their technical assistance in performing the muscle biopsies from this study.
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Keywords:© 2008 National Strength and Conditioning Association
training; resistance; isometric; jumping; force