Referring to physical movements characterized by forceful contractions in response to myostatic stretching of active muscles (8), plyometrics is a means of encouraging the muscle to achieve maximal force rapidly and therefore serving to increase explosive-reactive power through a range of motion and is a popular training approach (8). By developing the efficiency of the neuromuscular system in achieving maximum explosive-reactive power, the application of plyometric training (PT) as a means to developing the performer has lead to wide use within sports that require fast, explosive movements (2).
Plyometrics having been established as a valuable strategy for enhancing force-generating potential of explosive-reactive movements, the effectiveness of such training depends on the incorporation of sport-specific movements applied through appropriate frequency and intensity over extended periods. With explosive sports requiring rapid maximal muscular force production, such appropriate manipulation of the stretch-shortening cycle to increase efficiency has had a consequential positive effect on sports performance (3). By developing the muscular reflex to withstand high-stretch loads, combined with a decreased neural disinhibition of muscle proprioceptors, PT produces neuromuscular adaptation and specific muscular activation for coordinated movement patterns, thereby enhancing contraction force (12).
Investigating the impact of PT on dynamic movement activity, Lathrop et al. (7) have reported substantial improvements in running economy (4.7%) and 3,200 m cross-country running times (3.9%) in adolescent performers, whereas Matavulj et al. (10) recorded enhancements in vertical jump economy and height of junior basketball players. With significant improvements also being found after PT in 40 m skating speed in junior ice hockey players (8), the appliance of sport-specific PT as a means of improving muscular functioning has a practical importance to sports performance. Despite the growing body of evidence elucidating the value of PT in a range of sporting activities, the value of such an approach to swimming has been given little investigation. Although, once a swimmer is in the water the performance benefits of PT may be negligible, when considering that the swimming block start (SBS), which requires explosive-reactive muscular response, accounts for up to 30% of the total 50 m race time (9), any improvements within this aspect of swim performance could have a significant impact on overall race success. Furthermore, with winning margins being as small as 0.01 of a second in sprint swimming events (11), an appropriate intervention strategy to enhance explosive power output and increase performance efficiency during the SBS may have a meaningful impact on overall swim time.
The age at which one is physically able to participate in a PT program has not been determined; however, guidelines have been prescribed to assist in developing appropriate programs for adolescents. To reduce the risk of injury and facilitate the performance of plyometric exercises, programs must initially focus upon technique using low-impact exercises before progressing onto higher-impact activities. It is recommended that athletes have a sound understanding of plyometric and landing techniques and possess a sufficient base of strength, speed, and balance. Other factors such as landing surface, training area, equipment, proper footwear, and supervision must also be assessed, as with any PT programs (1). Such considerations to the implementation of PT for developing performers must be paramount to those wishing to promote the effectiveness of such a training approach.
Considering therefore the important contribution SBS plays in overall swim performance and appreciating the explosive-reactive nature of such movements, the aim of this study was to evaluate the effectiveness of a swimming-specific PT program on start performance. By selecting adolescent competitive swimmers, the study researchers hope to provide the practitioner with appropriate guidelines when implementing plyometric drills to develop adolescent swimmers' performance capabilities.
Experimental Approach to the Problem
The research hypotheses were examined by way of an experimental design comprising 2 independent groups (independent variables). After the completion of a baseline trial, which measured a range of SBS performance-dependent variables, subjects were assigned through random selection to 1 of the groups. After which, the groups followed either their normal habitual training (HT) program or a plyometric program for an 8-week period. After the intervention period, both groups were reassessed by completing the SBS performance trial.
Subjects were recruited from local swimming clubs, and criteria for inclusion were applied to ensure all swimmers were aged 10 to 16 years and were engaged in a minimum of 8 hours of aquatic training per week. All were considered highly competent in executing the SBS by the lead investigator and coach, and all had achieved a minimum of regional standard within the past 18 months. Before the beginning of any engagement with the subjects, ethics consent was granted through the university ethics committee and were adhered to throughout the investigation. Subjects were informed of any potential risks to participation, and all provided written informed consent, including consent from a parent/guardian, in accordance with institutional regulations. The study experienced no withdrawals, and subject baseline measures and characteristics are displayed in Table 1.
Before initiating the baseline SBS trial, subjects were required to conduct a standardized 15-minute aquatic warm-up as prescribed by club coaches, representing time allocated to swimmers before competition. Note of performed practices was recorded, and subjects were then asked to get themselves ready to perform under race conditions. In accordance with Amateur Swimming Association competitive start procedures, subjects mounted the blocks, adopted their preferred starting position, and were counted down. Throughout each SBS trial, a subject's performance was filmed using a Canon MVX460 camcorder at 50 Hz in the sagital plane of motion. The camera was positioned parallel to the motion 10 m from the subject along the poolside and 2 m from the starting blocks.
After completion of the baseline SBS trial, subjects were randomly assigned to either the PT group or HT group. Over a period of 8 weeks, all subjects followed the same normal habitual aquatic training patterns; however, the PT group was provided with an additional 2 hours per week of specific plyometric exercises. Exercises implemented into the PT program are displayed in Table 2. Included within the table are details pertaining to the number of sets, repetitions, recovery periods and box/hurdle heights. Subjects were required to conduct a 10-minute standardized dynamic warm-up before each PT session to increase the subject's mobility and ensure exercises could be performed safely in accordance with Radcliffe and Farentinos (14), who advocate that dynamic preparatory activities "excite the neuromuscular system without undue fatigue." The exercises implemented within the PT program were selected based upon their specificity to the SBS. Particular consideration was placed on ensuring boxes and hurdles were consistent with recommended heights for adolescents to ensure sufficient muscular overload could be achieved safely (see Table 2 for details of the training program). Normative values with reference to box and barrier heights ensured that training adhered to the requirements of safety and welfare of the subjects while ensuring that sufficient overload was achieved (refer to Table 3 for details).
Baechle and Earle (1) have reported that overload can be established with appropriate levels of training frequency, intensity, volume, and recovery in a 6- to 8-week period. Furthermore, in consideration of the subject group used in this study, the PT program was developed over an 8-week period to ensure gradual and safe progression for the adolescent performers. Chu (4) further noted that explosive-reactive power is established by implementing numerous sets of less than 6 repetitions at more than 90% percent of 1 repetition maximum with reference to the adult population. With Radcliffe and Farentinos (14) commenting that the intensity of a PT program should reflect that of competitive performance, but also considering the population-specific group in this study, variables affecting the intensity, such as box heights and repetitions, were adjusted to ensure they were appropriate to the group.
From the PT group, subjects were subjectively assessed by a qualified strength and conditioning specialist to determine a subject's levels of competency and adaptability to the PT program. If deemed necessary, intensity and volume were subsequently increased to ensure appropriate muscular overload. Intensity was increased by altering exercise variables including numbers of repetitions and sets or elevated heights of boxes and hurdles. Table 3 provides a guide to the predetermined training program, with any increases in overload appropriate to a subject's requirements, and the training recommendations as set by Baechle and Earle (1), Chu (4), and Radcliffe and Farentinos (14) to ensure safety.
For the HT group, monitoring occurred throughout the 8-week period; however, no manipulation of training intensity or volume was made. After the completion of the 8- week training program, all subjects returned to the pool to undertake a second SBS trial. Adhering to their baseline trial strategy, each subject undertook their 15-minute aquatic warm-up, after which they completed the trial under race conditions. The camera position was matched with that of the baseline trial, and performance was once again videoed, allowing for comparative analysis.
Through the use of Silicon Coach Pro (siliconCOACH, Ltd., Dunedin, New Zealand), each subject's footage was uploaded and subsequently analyzed to determine the time from the starting stimulus to contact of the head with the water surface, the distance from the starting stimulus to contact of the head with the water surface, the velocity of take-off to contact with the water surface, that angles relating to release off the blocks, and angle of entering the water. The dependant variables of time, distance, velocity, and angles were identified as the key determinants of the SBS technique (11). The time to complete a distance of 5.5 m from the starting stimulus was defined with visual reference points on the lane markers and poolside. The rationale behind inclusion of performance time to 5.5 m was that, with PT measuring the development of explosive-reactive power, any increase in distance would be reliant upon variables such as kick strength rather than SBS force production (5).
To determine whether the PT program had a significant impact on measures monitored during the SBS trial when compared with HT, data were subjected to a 2-tailed independent t-test on the change between baseline-post implementation of the training intervention. To evaluate within group differences for performance measures, a two-tailed dependent t-test was administered. Before the completion of the inferential analysis, a Kolmogorov-Smirnov test for normality and Mauchley test for homogeneity of variance were conducted. Further correlational analysis in the form of Pearson product moment coefficients were performed to establish relationships between SBS parameters and 5.5 m performance time. For all statistical analyses, alpha was set at the 95% probability level (p < 0.05). Standard error of the estimates (SEE) were calculated from linear regression analyses to express the precision of estimate in absolute terms.
Comparison of SBS performance measures revealed significant differences between the PT and HT groups (Table 4). Between group baseline-post trial scores for swim performance time to 5.5 m revealed a significantly greater change for the PT group when compared with the HT group (p < 0.01). Similarly, significantly greater change occurred across baseline-post trials for the PT group compared with the HT group for velocity of the take-off to contact (p < 0.01), distance to head contact (p < 0.01), and time to head contact (p = 0.023). Interestingly, no significance was found for the angle out of blocks (p = 0.12) and angle of entry into water (p = 0.27) between the PT and HT groups.
Analysis of the within group differences between baseline-post trial measures revealed (Table 4) that the swim time of 5.5 m was statistically lower (p < 0.001) for the PT group; however, no significance was found for the HT group (p = 0.11). Further examination also identified time to head contact from the PT group to be significantly lower after PT intervention (p < 0.01), whereas no statistical difference was found for the HT group (p = 0.42). Within training groups, significant differences for angle out of blocks, distance to head contact, and SBS velocity were also found after the respective 8-week training program (Table 4).
The association between SBS velocity and swim performance time to 5.5 m for both groups across baseline and post-trials was determined to identify the extent of correlational change after 8 weeks of specified training (Figures 1 and 2). For the PT baseline trial, analysis revealed a significant relationship between the measures (r = −0.66, p < 0.05), which was strengthened after the intervention period (r = −0.91, p < 0.01), as detailed in Figure 1. Accounting for 83% of the total variance explained, SEE for the 2 associations were reduced by 50%, from 0.14 to 0.07 seconds. For the HT group, a significant association was found between SBS velocity and performance time to 5.5 m for the baseline measures (r = −0.69, p < 0.05); this, however, remained relatively unchanged for the post-trial (r = −0.58, p < 0.05), as detailed in Figure 2. Examining the SEE for both associations revealed a marginal increase in error for the post-trial (0.32 s) when compared with the baseline assessment (0.35 s).
Distance to head contact and swim time to 5.5 m was also significantly correlated for the baseline assessment for both the PT (r = 0.82, p < 0.01) and HT group (r = 0.66, p < 0.05). The post-trial association between distance to head contact and time to 5.5 m in the PT group was strengthened slightly (r = 0.88, p < 0.01) after training; however, for the HT group, a reduction in the degree of association was found (r = 0.31, p > 0.05). Interestingly, an insignificant association was revealed for time to head contact and time to 5.5 m for the baseline assessment in the PT group (r = 0.019, p >0.05) and the HT group (r = 0.10, p > 0.05). Evaluating the post-trial associations, however, revealed a marginal change for PT group (r = 0.13, p > 0.05) and HT group (r = 0.40, p > 0.05).
The purpose of this study was to evaluate the effectiveness of a swimming-specific PT program on SBS performance in adolescent competitors. The major finding was that, by engaging in explosive-power training sessions in addition to habitual aquatic regimes, swim time to 5.5 m was significantly improved, on average by 0.59 seconds, equating to a 15% improvement in performance time. With the HT group not exhibiting any significant change in time to 5.5 m, it can be assumed that exposure to 2 hours of supplementary training per week had a meaningful impact on the swimmers' ability to cover the distance more quickly.
Enhancements of flight dynamics after swim block release are thought to significantly impact the quality of SBS performance (16). The present study confirms that, as a consequence of PT, significant reductions in time to head contact and greater distance to head contact likely contributed to the improvements in performance time. This indicates that performers were able to generate greater release speed and power off the block, translating into faster times to 5.5 m, signifying to the practitioner that supplementary PT offers a valuable means of developing this important aspect of overall swim performance (9).
Investigating the association between SBS velocity and performance time to 5.5 m before and after the intervention in both training groups revealed further important practical implications to the coach. Examination of the findings indicated that, as a consequence of PT, the velocity from release to water contact resulted in a stronger relationship (r = −0.91) with performance time than before the intervention (r = −0.66). Such improvement in the association between an important SBS variable and the key performance indicator, coupled with a meaningful reduction in the SEE, signifies the important contribution SBS velocity must play in reducing swim time over the first 5.5 m. For the coach working with the swimmer, exercises that focus on developing the velocity of release will have a significant impact on overall swim start performance. By ensuring the performer is able to maximize the power off the block by accelerating quickly, increases in distance and time to head contact will allow for faster transit through the water once entered. Establishing whether significant improvements in SBS exist after intervention requires practitioners to provide quantitative interpretations of the "flight phase" before contact with the water (15). In consideration of the complexity of such an approach, research studies investigating SBS variables and their impact on key performance indicators are needed. This being the case, the present study is the first to provide conclusive evidence that swim performance time enhancement is attributable to both increased flight distance and time.
With significant changes reported for angle out of the block, distance to head contact, time to head contact, and consequently SBS velocity, it would appear that the implementation of the PT intervention provided an adaptive response that allowed for greater power production off the blocks. Although documented evidence does highlight the importance of sport-specific PT to the enhancement of muscular force-generating capacity and neuromuscular development (3,7,8,10,12), limited evidence linking PT and swimming-specific block starts has been reported. One such study to do so, however, did conclude that, after a training intervention program, no significant improvements in SBS were found (6). Examination of that approach does reveal, however, that implementation of the PT intervention did not take into consideration the adaptation and progression through the 6-week schedule. For the current study, increases in intensity and volume reflected levels of competency and adaptability to the PT program over the 8-week period. Continually assessing the swimmers and adjusting their training stimulus accordingly ensured that all swimmers engaged in activities suited to the adaptive responsiveness to the program and provided a more realistic approach to the management and implementation of tailored training regimes.
With swimmers continuing to engage in their normal HT patterns, the study provided further insight into SBS performance from nonplyometric-specific training. Findings indicated that, despite the habitual group significantly increasing their take-off angle out of the blocks across trials, swimmers showed a decrease in both distance to head contact and SBS velocity, with a concomitant increase in time to head contact. With the HT group not performing any land-based PT exercises, it can be concluded that although some aspects of SBS performance were altered, an inability to produce sufficient power to transfer the increased take-off angle into meaningful improvements resulted in insignificant changes in swim performance time.
Sport-specific PT effectively stimulates muscle spindles, which involves agonist muscle preloading and elastic energy storage. Through the manipulation of intensity and volume, the force-generating capacity of the stretch-shortening cycle to initiate forceful muscular contractions can be altered (1). By allowing for more purposeful and deliberate exercise tasks, Brandon (2) showed, therefore, the important responsibility practitioners and coaches have in implementing appropriate training frequency and intensity to enhance such force-velocity relationships of skeletal muscle. The optimization of eccentric force production significantly develops elastic-muscular components and explosive-power production through enhanced motor unit firing rates and development of contraction intensity involved in neurophysical potentiation (13). By influencing muscular power output and force production, the safe implementation of PT in addition to habitual aquatic-based drills in the current study successfully demonstrated the ability of swimmers to explosively maneuver from the block start position to cover greater distances in significantly faster times.
In consideration of the importance of the SBS to overall race performance, the development of effectively managed PT programs for adolescent swimmers can have a meaningful impact on overall race success. When devising such PT programs, it is imperative that the athletes are provided with adequate supervision and assessment to avoid injury caused by poor technique and excessive overload to the muscles. The initial development of the program should therefore be low to moderate in intensity and focus upon the technical proficiency of the exercises used to avoid injury. The exercises should be specific to the movements in which increased power production is sought and should provide a progressive overload through increases in both intensity and volume. Progression to higher overload intensities can be manipulated through altering exercises and increasing the number of repetitions and sets or elevated heights of boxes and hurdles.
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