In most athletic activities (i.e., running, jumping, and throwing), the overall success of the athlete is dependent on the ability to quickly and maximally generate explosive leg motion. Therefore, strength and conditioning professionals continually seek new techniques and strategies to enhance performance of such activities. However, the focus of traditional training programs has been on increasing absolute strength, which may negate movement speed and explosive power. Over the past several decades, supplementary exercises, such as plyometrics, have been incorporated to facilitate improvement of power output and explosiveness (8). Unfortunately, there lacks a general consensus on the benefit of such combined training programs to enhance athletic performance (9,12) and it is critical for professionals to identify the potential transfer effects that different exercises have on overall athletic performance.
The guiding principles of strength training programs have been largely associated with the relation between muscular hypertrophy and strength gains with significant evidence supporting this viewpoint (19) but generally lack a clear relation to the level of success in athletic performance. An alternative approach to understand the relative influence of training is the theoretical perspective of transfer of learning found in the motor learning literature (11,16). Transfer of learning is broadly defined as the enhancement (or degradation) in motor performance as a result of practice on a different motor task. Therefore, in the strength and conditioning domain, the activities of a training program can either enhance or degrade other exercises and this framework allows practitioners to consider the relative influence of such activities on athletic enhancement.
Over the past century, transfer of learning has been extensively examined in a variety of cognitive and motor tasks (7) including movements such as rapid aiming (10), one-hand catching (4), and powerlifting (17). In the development of lower limb power, the application of transfer of learning has been indirectly investigated through studies examining the effect of different training programs (i.e., plyometric, Olympic-style lifts, strength) on similar power-related movements such as vertical jump (VJ) ability. In general, studies have failed to consider the overall demands of each movement and the potential transfer between different movements. The inconsistent findings on the relative effect of plyometric and strength training programs on power production (12) may be because of a variety of factors and the theoretical viewpoint of transfer of learning could be an alternative approach to better understand these varying findings (11,14).
The current study seeks to address the above limitations by using the transfer of learning perspective to address whether the addition of a novel plyometric exercise (i.e., knee-to-feet [K2F] jumps) to a training program results in positive transfer to similar power-related movements (VJ and hang clean [HC] exercises). For most athletes, a K2F jump is a novel movement pattern that has biomechanical and physiological similarities to the other movements. In a K2F jump (Figure 1), the individual starts in kneeling position with the knees and toes in contact with the floor. The execution of the jump phase requires an explosive movement that propels the center of mass (COM) in the vertical direction and a rapid hip flexion movement that allows for the feet to clear the floor. The aim of the current study is to examine the degree of effectiveness of K2F jump training and whether such training has a positive transfer effect on VJ and HC performance. Practically, the findings can inform strength professionals about the relative benefit of novel exercises and allow careful examination of specific activities within training programs.
In the transfer of learning perspective, the degree of similarity between practice and test conditions determines the level of transfer between the 2 activities. For example, Bebko et al. (3) showed how previous experience of a specific juggling task positively transferred to a novel juggling pattern compared with novice learners. The degree to which one skill transfers to the learning of a second skill is difficult to determine because of the presence of several confounding variables, including degree of initial learning, relative task difficulty, and similarity between the 2 tasks (7). Thus, the general viewpoint is that when transfer does occur, it is only partial. The application of motor learning approaches including attentional focus (15) and feedback information (13) has been examined in athletic movements but minimal work has explored the topic of transfer of learning to evaluate the effectiveness of training programs.
Combining plyometric exercises with a strength training program has been shown to enhance power production as measured by activities such as the VJ (1). However, contrary evidence exists with respect to the type of training program that can maximally enhance power production (9,12). Subsequently, there is a need to continue to examine how different training programs impact the development of strength and power in athletes so that professionals can address whether such activities yield enhanced athletic performance during competitions. The current study sought to investigate the effect of a training program on power development in athletes through the use of a novel plyometric exercise (K2F jumps).
The aim of the current investigation was to determine whether a 6-week training program of K2F jumps enhanced jump performance and whether the learning of this novel movement had a positive effect of transfer to similar power-related movements (VJ and HC). The selected exercises exhibit a degree of similarity in that rapid, explosive hip extension is required for successful performance of each movement pattern. According to the principles of transfer of learning (11,14), it was predicted that the 6-week training program would enhance K2F jump height; however, the movement patterns required to complete each of the selected exercises may have subtle differences that extend beyond the explosive hip extension action that limit the amount of transfer after the 6-week training program. Practically, the K2F jump has similar power demands to VJ and HC that some coaches and professionals may deem appropriate for power development and may consider incorporating into training programs; however, there lacks scientific evidence for such justification.
Experimental Approach to the Problem
Participants were counter-balanced into either an experimental group that completed a 6-week training program of K2F jumps or a control group that maintained normal weight training activities. Pre- and posttraining tests were separated by 6 weeks and consisted of assessment of K2F jump height, VJH, and 2-repetition maximum (RM) HC with appropriate warm-up and cool-down activities.
Twenty-six National Collegiate Athletic Association Division II athletes (15 men) were recruited to participate in this investigation. The athletes (age: 20.1 ± 2.3 years, range 19–23 years; height: 177.1 ± 11.6 cm; weight: 77.7 ± 14.9 kg) were from the following sports: football, wrestling, softball, basketball, and track. The inclusion criteria required men to have previously executed a 1RM HC at 115% of their body weight and women to have maxed 85% of their body weight. The study was approved by the Truman State University's Institutional Review Board. All subjects were informed of the testing protocol, the nature of the study, and experimental risks and benefits before signing an informed consent form.
To minimize any confound from in- and off-season training, each athlete was paired with another subject from his/her respective sport and position. After pairing, the subjects were randomly placed in either the control or treatment group. Athletes in the treatment group completed a 6-week training program of K2F jumps in addition to their respective sport-specific training program. The 6-week plyometric cycle was based on the principle of periodization (1,19) with volume decreasing as a function of intensity (Table 1).
All subjects participated in an initial learning session to understand the K2F movement. The initial session focused on movement technique and safety procedures to minimize risk of injury. Training has been shown to influence the circadian rhythms of athletes (6). Therefore, pre- and posttesting sessions were conducted during evening hours (i.e., 6–9 PM). Participants were asked to refrain from eating or drinking anything other than water at least 2 hours before testing; however, this information was not objectively captured or measured.
During initial and final testing (day 1 and day 18), each subject completed a warm-up session in accordance with the recommendations from the National Strength and Conditioning Association regarding specific weightlifting warm-ups. The activities included 2 sets of 10 small-arm circles (10 forwards and 10 backwards) and 2 sets of 10 large-arm circles (10 forwards, 10 backwards), 2 sets of 10 hurdle unders (10 each direction), and 2 sets of 10 light squats (warm-up and cool-down). Immediately after the warming up activities, subjects completed testing in the following order: VJH, maximum K2F height, and 2RM HC. Vertical jump height was assessed by 3 countermovement VJs (Jump pad; Probotics, Huntsville, AL) with appropriate period of rest (e.g., 60–120 seconds) between each attempt.
Knee-to-feet jump height was measured by having the subject start in a kneeling position with the measurement apparatus located directly in front of the athlete. To measure the height of each jump, a bungee cord was attached to 2 secured poles and the athlete placing his/her feet on top of the bungee cord defined a successful jump. Initial bungee height was set at 5 cm, and upon each successful jump, the cord was raised 2 cm until failure. Subjects were given one attempt per height, and the highest successful jump while landing with both feet on the bungee was recorded. If the subject missed after raising the bungee multiple heights, it would be lowered 1 cm at a time until completion or reaching the highest number previously cleared.
Hang clean performance was determined by a 2RM procedure. Each athlete determined the initial weight level. If the subject successfully completed the specific weight, an increment of 2.26 kg (5 lb) was added to the previous weight. Failure of the lift was determined through an unsuccessful HC movement or when the subject no longer felt comfortable increasing weight.
Upon completion of testing, each subject performed a cool-down adapted from the National Strength and Conditioning Association consisting of a 200-m jog followed by the standing gastrocnemius and soleus stretch for 30 seconds on each leg, standing right and left leg hamstring stretch for 30 seconds on each leg, and kneeling hip flexor stretch for 30 seconds on each leg. Identical testing procedures were conducted on day 1 and day 18.
Subjects receiving treatment participated in a 6-week K2F jump program. In addition to the normal lifting program for their respective sport, the athletes performed K2F jumps 3 times a week (Table 1). Subjects logged the number of successful jumps completed during each training session and returned it to the investigators at the completion of the program (97% of the workouts were completed by the experimental group). Subjects in the control group continued their normal lifting program without adding any K2F jumps.
To examine the effect of K2F jump training, the dependent variables were independently evaluated in separate 2-way (2 × 2) repeated measures analysis of variance with group (i.e., treatment vs. control) and time (pre- vs. posttest) as the main factors. Additionally, pretest data of the dependent variables were correlated with each other to examine the degree of relation (i.e., similarity) between these variables. All statistics were evaluated using SPSS (IBM, Chicago, IL, USA), and significance was defined when there was less than a 5% chance (p ≤ 0.05) of making a type I error.
The aggregate values of VJH, K2F jump height, and 2RM HC performance as a function of test are shown in Figure 2. Vertical jump height significantly increased from 24.3 ± 0.94 cm, 95% confidence interval (CI) (22.38–26.25) to 24.9 ± 0.91 cm, 95% CI (23.01–26.8) between pre- and posttest (p ≤ 0.05). No statistical differences were found between groups or for the group × time interaction. The mean K2F jump height during pretesting was 19.27 ± 1.88 cm, 95% CI (15.16–23.38) and during posttest was 22.08 ± 1.90 cm, 95% CI (18.15–26.00) resulting in a statistical difference for the main effect of time (p ≤ 0.05). There was a significant group × time interaction (p ≤ 0.05) on K2F jump height. No statistical difference was found on 2RM HC.
Pretest data were further examined through pairwise correlations between the dependent variables. Figure 3 illustrates the correlations between K2F jump height and VJH (R2 = 0.40, p < 0.01), 2RM HC and K2F jump height (R2 = 0.23, p ≤ 0.05), and VJH and 2RM HC (R2 = 0.38, p < 0.001).
The aim of the current investigation was to examine the effectiveness of K2F jump training on jump performance and the degree of transfer to similar explosive power-related movements such as VJ and HC. After a 6-week training program of K2F jumps, individuals increased K2F jump height compared with controls; however, nonsignificant changes were observed in VJH and 2RM HC. Significant correlations were found between the exercises with VJH and HC showing the highest correlation followed by VJH-K2F and HC-K2F, respectively. The findings were inconsistent with our hypothesis that training would exhibit positive transfer to other power-related movements based on the similarity of hip extension movement associated with each exercise.
Plyometric exercises are a key component of most training programs for athletes, but there lacks a definitive design that optimally enhances both strength and power (9,12). The current study incorporated the theoretical approach of transfer of learning to examine whether the effect of training of this novel movement (K2F jump) exhibited positive transfer to similar power-related activities including VJH and HC performance. This approach is well established with cognitive and motor tasks and offers a window into how professionals need to consider the design of training programs related to the potential influence on enhancing athletic performance.
When evaluating transfer of learning, it is important to note that similarity can be assessed on the movement pattern characteristics (14), the context of the practice environment (4), or the dominant sensory source guiding the action (17). Here, we focused on the similarity of the movement pattern characteristics, specifically, the ability of the hip extension muscles to generate maximal power that would propel the COM in the vertical direction. This action is intimately related to functional movements of running, jumping, and throwing and a key component in most training programs designed by strength and conditioning professionals. Therefore, identifying exercises that promote positive transfer between different power-related movements has important practical implications.
Although each exercise shares the similarity of requiring explosive hip extension movement to execute the action, the overall task goals are distinctly different. For example, the goal of VJ is to maximally displace the COM with body mass as the primary load. In contrast, HC performance requires a smaller displacement of the COM and includes additional loads beyond body mass. For K2F jumps, a greater amount of rapid hip flexion movement is needed to allow for foot clearance compared with VJ and HC execution. The subtle difference in task outcome may have limited the potential transfer between these movements.
Also, the coordination pattern between the hip extension/flexion cycle in HC and K2F jumps reflects an aspect of similarity not evident in VJ performance. Both K2F jumps and HC performance require the athlete to quickly and forcefully perform hip flexion to allow for proper catching of the load. The transfer of learning perspective (11,14) would predict that K2F jump training would exhibit a stronger influence on HC than VJ performance, but the findings did not support this prediction. Proper technique is essential for successful performance, but particularly in HC, the ability to properly catch the load is a critical aspect during initial acquisition. The benefit of K2F jump training may be better realized for novice learners of the HC by accentuating foot clearance/catching phase of the movement through the rapid hip flexion component. The participants involved in the current study were athletes with previous HC experience and demonstrated solid technical form that appeared to not have benefitted from the K2F jump training.
Another aspect in which the exercises differed was with respect to the stretch/shortening cycle of the movements. In most plyometric exercise, the force of gravity is used to facilitate the storage of elastic energy and generation of kinetic energy that is subsequently transmitted to the positive phase of the jumping activity. Studies have shown the importance of storing elastic energy during the stretch cycle (2,12). Here, the different initial starting positions of these exercises likely influenced the storage of elastic energy. A countermovement component was included in the assessment of VJH that facilitated the storage of elastic energy. Comparatively, the starting position of the K2F jumps reduced the range of motion during the loading phase and therefore minimized the potential availability of stored elastic energy compared with VJ performance. Consequently, the observed effects of training may be more evident in static rather than the countermovement jump, which could be explored in future studies.
After the 6-week training program, individuals showed significantly higher K2F jump heights compared with the control group. Four to 6 weeks of plyometric training appears to be the optimal training length that allows for adaptations within the central nervous system and minimizes excessive strain or fatigue (19). Additionally, this duration of training is typically linked to neural adaptations, and alterations in the coordination pattern as a result of training likely occur across a different time scale of training. Overall, the findings indicated that short-term, acute changes after training failed to transfer to the other power-related movements and a longer training period may be needed to realize transfer between these exercises.
Previous evidence suggests that power development through high-speed training also requires longer training phases than the current program (18) and represents a potential factor to the lack of transfer to other power-related movements (VJ and 2RM HC) found in the present investigation. Therefore, strength and conditioning professionals can use this finding when considering the micro- and macro-structures (time-line) of training programs.
After resistance training, there are a number of potential sites that alter how the central nervous system recruits trained muscles both for the specific exercise and during related functional movements (5). Additionally, these changes impact movement execution and control. According to Carroll et al. (5), there needs to be a better understanding of the neuromuscular adaptations associated with the response on training to more effectively predict whether transfer of learning will occur between exercises such as plyometric and functional athletic movements. The goal would be to determine the relation of such adaptations between typical resistance training exercises and functional tasks related to athletic performance. Thus, the principles of transfer of learning (11,14) may provide a theoretical framework to better understand the overall impact of resistance training on enhanced athletic performance.
The findings of the current study illustrated that K2F jump training had an isolated effect on performance as evident by the minimal transfer to similar power-related movements. The results also highlight the need for further investigation at the biomechanical and neurophysiological levels of analysis to understand the principles that govern the influence of resistance training on movement control (5). When designing resistance training programs, professionals need to consider the numerous demands placed on the athlete and the desired effect of transfer to functional performance. The application of K2F jump training may be potentially useful in other activities such as power cleans and short-distance sprinting; however, additional research is needed to fully evaluate the potential transfer between these power-related activities.
Coaches and strength professionals continually seek novel approaches to enhance the development of athletic performance. This study showed that K2F jump training enhanced only K2F jump height and did not show a positive transfer to other power-related movements. Plyometric exercises foster the development of lower body explosive power and are often suggested to be the link between strength and speed. In terms of selecting plyometric exercises, practitioners may find the current study's findings useful for their training programs. Although the K2F jumps reflect a degree of similarity to other plyometric exercises such as box or depth jumps, there are subtle differences between the movement patterns that limit the implied benefits of transfer. Knee-to-feet jumps may be a supplemental plyometric exercise, but practitioners need to carefully consider introducing novel movements into training programs.
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