The relationship between maximal strength and athletic performance is well documented (1). In addition, mechanical power production, which describes the rate at which force is applied (force × velocity), is particularly important for the successful execution of various sport-specific explosive movements (2). Greater mechanical power is associated with enhanced performance during sport-specific dynamic movements, such as sprinting, jumping, kicking, and rapid changes of direction (3,4). As such, an important focus of resistance training interventions designed to enhance sports performance is to maximize the development of muscular power (2).
Compared with traditional power-training methods, “contrast training” has received much attention as a potentially superior means of developing muscular power (5,6). The terms “complex” and “contrast” training have, at times, been used interchangeably in the literature; however, there is a distinction between these two training methods (7). Complex training typically involves alternating between sets of a high-intensity, strength-focused exercise (termed the “conditioning activity”) and a plyometric-type exercise [e.g., a set of heavy back squats (≥85% one-repetition maximum [1-RM]) followed immediately by unloaded countermovement jumps or bounds] (8–11). Contrast training, on the other hand, involves alternating between sets of a heavy-load exercise and a biomechanically similar lighter-load exercise, with both often performed with intended maximal movement velocity (e.g., a set of heavy back squats [≥85% 1-RM] followed immediately by a set of loaded squat jumps at 30% 1-RM) (12–14).
The purported ergogenic benefit of contrast training is based on the principle of postactivation potentiation (PAP), which describes the phenomenon by which intrinsic components of muscular performance, such as peak force, rate of force development (RFD) and contraction velocity, may be acutely enhanced as a result of contractile history, allowing for increased mechanical power output (6,15). It has been suggested that the cumulative effect of achieving higher-power outputs during contrast training may result in superior muscular power development (11,16).
Although many studies have demonstrated an acute ergogenic effect on power exercises during contrast training (e.g., [8–11,14,17–19]), others show either no effect (e.g., [20–25]) or even reductions in performance (26,27). These between-study differences suggest that the ergogenic effect of contrast training may be modulated by factors associated with the specific contrast training protocol used (6). These factors include the type of conditioning activity (e.g., isometric or dynamic), the intensity and volume of the conditioning activity, the recovery period between the conditioning activity and the subsequent power exercise, and the type of power exercise performed after the conditioning activity (15,24,28,29).
In addition to factors specific to the contrast training protocol, individual participant characteristics also strongly influence the ergogenic effects of contrast training (5,6). For example, it has been demonstrated that performance enhancement with contrast training is modulated by age (30), sex (29), muscle fiber–type distribution (31), and training status (3,9,29). In particular, stronger individuals have been reported to attain greater potentiation of performance in comparison to weaker individuals (3,11,32–35), which suggests performance enhancement with contrast training may be modulated by previous strength training experience (29). From a practical perspective, it is important to determine whether other individual characteristics, such as muscle power, also influence the ergogenic benefits of contrast training protocols. Such information is critical to aid decision making for the practitioner regarding which individuals may benefit from contrast training, and which may not.
Despite the potential implications of contrast training for improving power development, most studies have investigated the effects of contrast training on unloaded jump performance (e.g., [18,19,22,24,36]) or during other explosive activities, such as sprinting or throwing (10,37,38). Further research is required to investigate whether performance is enhanced during contrast training incorporating other common power-focused exercises, such as loaded squat jumps, which have received less attention in the contrast training literature (9,11,32). Furthermore, although the ergogenic benefits of contrast training have been demonstrated for single sets of contrast training (e.g., [3,8,11,32,36]), there is limited evidence that contrast training can enhance performance over multiple contrast training sets, as is typically implemented in applied settings. Such evidence is important if contrast training is to be advocated as a superior method for power development.
The purpose of this investigation was to assess whether mechanical power is enhanced during multiple sets of a contrast training protocol in subelite Australian Footballers. A secondary aim was to determine whether individual characteristics influence the degree of performance enhancement during contrast training. We hypothesized that peak squat jump power would be enhanced when performed after heavy half squats, but the degree of potentiation would be reduced when performance was averaged over a series of repetitions and sets. Based on previous investigations, we further hypothesized there would be a strong positive correlation between individual measures of maximal strength and the degree of performance enhancement during loaded jump squats.
Twenty-two male subelite Australian Football players (age, 19 ± 2 yr; body mass, 80.4 ± 9.4 kg; 1-RM half squat, 172 ± 18 kg; mean ± SD) were recruited from a team competing in the Western Australian Football League. All participants had at least 1 yr of prior experience with resistance training. Each participant was required to fill out a medical history questionnaire to eliminate individuals with contraindications from participating in the investigation. Before the study, all participants were informed of the study requirements, benefits, and risks before providing written informed consent. Ethical approval for the study was granted by the institutional research ethics committee.
Participants visited the laboratory on four occasions. During the first visit, participants were familiarized with the procedure for determination of the 1-RM half squat. This was followed by familiarization of the loaded squat jumps using 20% of each participant’s predicted 1-RM half squat. On the second visit, the 1-RM half squat was determined, and participants were familiarized with the full protocol for determination of peak power during squat jumps performed with a load equivalent to 30% of the 1-RM. Testing was performed with a load equivalent to 30% of the participants 1-RM, as previous work has determined this to be the approximate load at which the mechanical power may be maximized during the squat jump (16). Participants were then matched for 1-RM and separated into two groups before completing the final two testing sessions in a random order. One visit served as the control trial (CTL condition) during which peak power values during repeated squat jumps (two sets of six repetitions) were determined. The other visit served as the experimental trial (contrast training [CST] condition), during which participants performed a simulated contrast training protocol to determine the effect of preloading using heavy squats on subsequent peak power during repeated squat jumps. A period of 48 to 72 h separated each visit, with 72 h of recovery allowed after the 1-RM and subsequent CTL or CST testing sessions to avoid the possible influence of muscle soreness and fatigue induced by testing. During the testing period, all participants were exempt from regular team-based training and were asked to refrain from any exercise or strenuous physical activity. All data collection and analysis was conducted by the primary investigator (K.T.S.).
1-RM half squat
Maximal strength during the half squat exercise was determined using the Plyometric Power System (PPS) (Norsearch, Lismore, Australia) (16). The PPS is a device whereby the displacement of the barbell is restricted to the vertical plane, similar to a Smith machine, as previously described (16). During both the warm-up and 1-RM testing, participants placed their feet at a set position on a floor grid (determined during familiarization). When ready, participants lowered the bar at a self-selected pace until a knee angle of 90° (i.e., half squat depth) was achieved. Knee angle was determined during familiarization using a manual goniometer, and the corresponding bar position was marked on the upright supports of the PPS using colored tape. Participants were able to see the tape in a mirror placed directly in front of them and were instructed to lower the bar until it was in line with the tape before commencing the concentric phase of the lift. The placement of the tape was recorded according to a ruler fixed to the upright supports of the PPS and was kept constant for all squat and squat-jump testing for each participant. After a warm-up consisting of 10 repetitions at 50% of predicted 1-RM, five repetitions at 70% of predicted 1-RM, three repetitions at 80% of predicted 1-RM, and one repetition at 90% of predicted 1-RM, participants performed three attempts to determine their actual 1-RM to the nearest 2.5 kg. Three to 5 min of passive recovery was allowed between 1-RM attempts.
Power output during squat jumps (CTL condition)
Baseline values for peak power during squat jumps were determined using the PPS. The PPS software was used to calculate peak power (as well as other kinetic measures including peak force, peak velocity, and maximal RFD) during the concentric phase of the squat jump, based on the displacement of the barbell, time of displacement, and the mass of the system (barbell mass plus body mass). Before testing, participants performed a standardized warm up involving 5 min of cycling on a cycle ergometer at 80 W, 2 min of passive rest, six submaximal jumps at 20% of the 1-RM squat load, 2 min of passive rest, and three maximal jumps at 30% 1-RM. Five minutes of passive rest was allowed after the warm-up before the commencement of testing. During testing, participants placed their feet at a predetermined position on the floor grid and were instructed to lower the bar at a self-selected pace until a knee angle of 90° was achieved (as per the 1-RM squat). The 90° knee angle has been previously used in the testing of the strength-power qualities of elite athletes (32,39). Upon reaching the bottom of the movement, participants immediately propelled themselves upward as explosively as possible, releasing the bar at the top of the movement. At this point, the unidirectional electromagnetic braking system of the PPS was engaged, thereby preventing downward movement of the barbell. Upon landing, the bar was lowered down to the participant, and they were given time to place their feet back in position and prepare themselves before the next jump. This resulted in a period of approximately 4 to 6 s between jumps. A total of six jumps were performed during the first set, and the peak concentric power of each jump was recorded for analysis. Participants were then given a 4-min passive rest period before performing a second set of six jumps using the same procedures.
Power output during CST condition
The CST protocol involved a standardized warm-up (5 min of cycling on a cycle ergometer at 80 W, 2 min of passive rest, ten repetitions of squats at 30% 1-RM, 2 min of passive rest and six repetitions of squats at 50% 1-RM. Five minutes of passive rest was allowed after the warm-up before the commencement of testing. Testing involved four alternating sets of half squats (performed to a 90° knee angle) and squat jumps performed on the PPS. For each set of half squats, a total of six repetitions were performed using a load corresponding to 85% of the 1-RM. We used an 85% 1-RM load as it has been demonstrated as an appropriate load to achieve potentiation (18), and higher relative intensities have been shown to be more beneficial for potentiation than lower intensities (i.e., up to 90% 1-RM showing more benefit than <75% 1-RM) (19,20). Six repetitions was used primarily as participants were able to perform six repetitions at this intensity, and we aimed to mimic a chronic training program aimed at simultaneously developing strength and power in this level of athlete. For each set of subsequent squat jumps, six repetitions were performed using a load corresponding to 30% of their squat 1-RM, and the peak power was recorded during the concentric phase of each jump. A passive rest period of 4 min (11) was allotted between each set of the testing protocol (Fig. 1). As per 1-RM and CTL testing, participants were required to achieve a knee angle of 90° during the eccentric phase of each repetition of the half squats and squat jumps. During both the CTL and CST conditions, the highest value recorded for each variable (peak power, peak force, peak velocity, and maximal RFD) during the first three jumps of set 1 was deemed the “peak” respective value. Average peak power was calculated as the average of the peak power values for all six squat jumps for both sets 1 and 2, respectively.
On a separate occasion to the CTL and CST trials, the test–retest reliability for peak power, peak force, peak velocity, and maximal RFD during the squat jump, and average peak power over 6 squat jumps, was established using 17 athletes (see Table, Supplemental Digital Content 1, Measurement reliability for all squat jump measures, http://links.lww.com/MSS/B212).
Outcome variables were log-transformed before analysis to reduce nonuniformity of error (40). The magnitude of differences in outcomes between the CTL and CST trials was quantified using the standardized difference (effect size [ES]) as previously described (41), with the default threshold of 0.2 defined as the smallest worthwhile effect. Magnitude-based inferences about effects were made by qualifying the effects with probabilities that reflected the uncertainty in the magnitude of the true effect (41); 25% to 75%, possibly; 75% to 95%, likely; 95% to 99.5%, very likely; >99.5%, most likely. We considered substantial effects as those that were at least 75% “likely” to be greater than the smallest worthwhile effect (according to the overlap between the effect magnitude, the uncertainty in the magnitude of the true effect, and the smallest worthwhile effect ()). A summary of all magnitude-based inference data for all statistical comparisons in this study are presented in supplementary information (see Table, Supplemental Digital Content 2, Summary of magnitude-based inference (MBI) data for all within-condition comparisons, http://links.lww.com/MSS/B213). Exact P values were also determined for each comparison, derived from paired t tests, with a Bonferroni correction applied to correct for multiple comparisons (SPSS, Version 21; IBM Corporation, New York, NY). Relationships between individual participant characteristics, including peak squat jump power and 1-RM half squat strength, and the potentiation of squat jump power by CST were determined using Pearson correlation coefficient (GraphPad Prism 7; GraphPad Software, La Jolla, CA). Data are reported as the mean percentage difference between CTL and CST conditions ±90% confidence limits, unless otherwise specified.
After initial analyses on combined participant data, and subsequent findings regarding the strong negative relationship between the individual power-to-strength ratio and performance enhancements with CST, we used the median-split technique to separate participants into two groups (32); this created one group with the lowest power-to-strength ratios (PSR) (LPSR group; 15.4–19.1 W·kg−1; n = 11) and another with the highest PSR (HPSR group; 19.4–24.7 W·kg−1; n = 11). There were no differences between the LPSR and HPSR groups for 1-RM half squat strength (170.9 ± 15.8 kg vs 172.7 ± 21.5 kg, respectively; P = 0.824) or age (19.2 ± 2.8 yr vs 19.2 ± 2.3 yr, respectively), although the HPSR group was taller (178.2 ± 5.7 cm vs 188.5 ± 4.1 cm, respectively; P < 0.001) and heavier (75.8 ± 8.5 kg vs 85.1 ± 8.1 kg, respectively; P = 0.016) compared with the LPSR group (see Table 1). All data were analyzed for differences in key outcomes between the CTL and CST conditions for the entire cohort, and then separately for the LPSR and HPSR groups.
Performance Enhancement during Squat Jumps with Contrast Training
Peak power during squat jumps was not different between the CST versus CTL conditions for either set 1 (mean change: ±90% confidence interval, 2.8% ± 2.0%; ES: ±90% confidence interval, 0.13 ± 0.09; P = 0.079) or set 2 (0.3% ± 1.7%; ES, 0.01 ± 0.08; P = 0.781; Table 2). Average peak power during squat jumps was not different between the CST versus CTL conditions during either set 1 (1.9% ± 1.9%, ES, 0.09 ± 0.09, P = 0.158) or set 2 (0.6% ± 1.5%, ES, 0.03 ± 0.07, P = 0.530; Table 2). Peak force during squat jumps was not different for CST versus CTL during either set 1 (0.3% ± 1.9%, ES, 0.02 ± 0.14, P = 0.792) or set 2 (−1.2% ± 1.7%, ES, −0.09 ± 0.13, P = 0.225; Table 2). Peak velocity during squat jumps was higher for CST versus CTL during both set 1 (2.3% ± 1.4%, ES, 0.20 ± 0.12, P = 0.023) and set 2 (2.1% ± 1.1%, ES, 0.18 ± 0.09, P = 0.017; Table 2). Maximal RFD during squat jumps was not different for CST versus CTL during either set 1 (−8.1% ± 6.8%, ES, −0.28 ± 0.24, P = 0.101) or set 2 (−4.6% ± 6.1%, ES, −0.13 ± 0.18, P = 0.280; Table 2).
Relationships between Individual Strength and Power Characteristics, and the Change in Peak Squat Jump Power with Contrast Training
Given the absence of overall improvements in peak and average peak squat jump power with contrast training, we then investigated if there were any individual strength and power characteristics associated with performance changes with contrast training. Correlation analysis revealed no substantial relationship between either absolute (r2 = 0.001, P = 0.884, Fig. 2A) or relative (r2 = 0.14, P = 0.088, data not shown) 1-RM half squat strength and the percent potentiation of peak squat jump power with CST. However, strong negative correlations were found between individual peak squat jump power at baseline and the percent potentiation of peak squat jump power with CST (r2 = 0.44, P < 0.001, Fig. 2B), and, in particular, between the individual PSR and the percent potentiation of peak squat jump power with CST (r2 = 0.65, P < 0.001, Fig. 2C). Statistically significant negative correlations were also found between the individual PSR and the percent potentiation of the average peak power (of all six jumps) with CST for both set 1 (r2 = 0.58, P < 0.001) and set 2 (r2 = 0.32, P = 0.006; data not shown).
Based on the strong relationships between the PSR and performance enhancement with CST, we conducted further comparisons between participants based on their individual PSR. Further correlation analyses revealed a very strong negative relationship between the PSR and percent potentiation of peak squat jump power with CST for the LPSR group (r2 = 0.91, P < 0.001), but not for the HPSR group (r2 = 0.008, P = 0.792; Fig. 3A). This same pattern was evident for the CST-induced potentiation of average peak power for both set 1 (LPSR: r2 = 0.69, P = 0.002 vs HPSR: r2 = 0.27, P = 0.102; Fig. 3B) and set 2 (LPSR: r2 = 0.40, P = 0.036 vs HPSR: r2 = 0.04, P = 0.568; Fig. 3C).
Performance Enhancement during Squat Jumps with Contrast Training for the LPSR versus HPSR Groups
Given the very strong associations between the individual PSR and performance enhancement during squat jumps with contrast training, we conducted further comparisons between the LPSR and HPSR groups for each performance variable assessed during squat jumps.
Peak power was substantially enhanced for the CST versus CTL conditions during set 1 for the LPSR group (8.1% ± 3.9%; ES, 0.44 ± 0.21; P = 0.004; Fig. 4A), yet was reduced for the HPSR group (−2.1% ± 1.3%; ES, −0.14 ± 0.09; P = 0.010; Fig. 4B). During set 2, peak power was not substantially different during the CST versus CTL conditions for either the LPSR group (2.3% ± 3.1%; ES, 0.13 ± 0.17; P = 0.240; Fig. 4A) or the HPSR group (−1.6% ± 2.0%; ES, −0.11 ± 0.14; P = 0.166; Fig. 4B).
Average peak power
Average peak power was substantially improved for CST versus CTL during set 1 for the LPSR group (5.6% ± 3.5%; ES, 0.36 ± 0.22, P = 0.016; Fig. 4C), but not for the HPSR group (−1.7% ± 1.8%; ES, −0.11 ± 0.13, P = 0.117; Fig. 4D). During set 2, average peak power was not substantially improved during the CST versus CTL conditions for either the LPSR (2.7% ± 3.0%; ES, 0.17 ± 0.19; P = 0.124; Fig. 4C) or HPSR groups (−1.4% ± 1.7%; ES, −0.09 ± 0.12; P = 0.158; Fig. 4D).
Peak force was “possibly” increased for the LPSR group for the CST versus CTL condition during set 1 (3.3% ± 3.6%; ES, 0.25 ± 0.27, P = 0.101; Table 2), but was reduced for the HPSR group (−2.6% ± 1.5%; −ES, 0.20 ± 0.12; P = 0.009). During set 2, peak force was not substantially different during the CST versus CTL condition for either the LPSR group (−0.3% ± 2.8%; ES, −0.02 ± 0.22; P = 0.869) nor the HPSR group (−2.1% ± 2.1%; ES, −0.16 ± 0.16; P = 0.087; Table 2).
Peak velocity was improved during the CST versus CTL condition during set 1 for the LPSR group (4.2% ± 2.4%; ES, 0.55 ± 0.31, P = 0.027), but not the HPSR group (0.5% ± 1.4%; ES, 0.07 ± 0.21, P = 0.525; Table 2). For set 2, peak velocity was also improved for CST versus CTL for the LPSR group (3.3% ± 1.2%; ES, 0.43 ± 0.16, P = 0.039), but not the HPSR group (0.8% ± 1.2%; ES, 0.13 ± 0.18, P = 0.243; Table 2).
Maximal RFD was “likely” reduced for the CST versus CTL condition during set 1 for the LPSR group (−10.6% ± 8.5%; ES, −0.32 ± 0.27; P = 0.146), but not for the HPSR group (−5.6% ± 12.0%; ES, −0.15 ± 0.32; P = 0.435). There were no substantial differences in maximal RFD during set 2 for either the LPSR (−9.7% ± 10.1%; ES, −0.29 ± 0.31; P = 0.162) or HPSR (0.8% ± 7.4%; ES, −0.02 ± 0.18; P = 0.869; Table 2) groups.
The primary purpose of this study was to investigate whether a contrast training protocol incorporating heavy half squats (85% 1-RM) was able to enhance power output over multiple sets of subsequent squat jumps (30% 1-RM) in subelite Australian Rules Football players. In contrast to our hypothesis, the first major finding was that neither peak power, nor average peak power, during loaded squat jumps was enhanced by prior heavy half squats when the entire cohort was analyzed as a whole. There was, however, statistically significant negative relationships between individual participant characteristics, including baseline peak power (r2 = 0.44) and, in particular, the ratio between baseline peak power and 1-RM half squat strength (i.e., the PSR; r2 = 0.65), and the potentiation of peak squat jump power with contrast training. Subsequent analyses revealed that improvements in peak power, average peak power, and peak velocity during squat jumps with contrast training were noted for individuals with a LPSR (≤19.1 W·kg−1), whereas individuals with a HPSR (≥19.4 W·kg−1) experienced reductions in both peak power and peak force during squat jumps with contrast training. Together, these data suggest that individual characteristics, particularly the PSR, influence the effect of contrast training for enhancing subsequent explosive strength performance.
Although we noted no effect of prior heavy half squats on changes in subsequent peak squat jump power for the cohort as a whole, there was a large degree of intersubject variability in this response. For example, the percent potentiation of peak squat jump power with contrast training was 3.1% ± 7.6% (mean ± SD), with a range of −5.5% to 20.7%. This is consistent with other investigations that have also observed a large intersubject variability in performance enhancement with complex training (18,19). Indeed, Weber et al. (19) noted an increase in peak jump height of 4.6% ± 4.8% (mean ± SD: range, −1.7% to 13.2%), and an increase in ground reaction force of 4.6% ± 7.4% (mean ± SD: range, −5.4% to 19.6%) with a complex training protocol (five half squat repetitions at 85% 1-RM, followed 3 min later by five unweighted squat jumps). Such high intersubject variability in these responses is likely a consequence of individual participant characteristics that influence the magnitude of the PAP response (5).
Previous investigations have suggested that an individual’s level of strength can influence the potentiation achieved with contrast training (11,32–35), although others have not observed such a relationship (22). For example, both Young et al. (11) and Duthie et al. (32) found no overall improvement in weighted squat jump performance with contrast training; however, both studies reported a statistically significant positive correlation between the absolute strength (either 5-RM half squat, or predicted 1-RM squat) and the degree of performance enhancement with contrast training. Additionally, when the participants in the study by Duthie et al. (32) were divided into two groups based on predicted 1-RM strength, a statistically significant performance enhancement was noted with contrast training in the high-strength group, but not in the low-strength group. Unlike some previous investigations (11,32–35), we found no statistically significant correlations between either maximal (r2 = 0.001; Fig. 2A) or relative strength (r2 = 0.14; data not shown) and the potentiation response. Aside from methodological differences, it is worth noting that these studies typically included smaller sample sizes, and generally reported a greater heterogeneity in participant strength levels. For example, the strongest participant in Young et al. (11) had a 5-RM strength level greater than three standard deviations above the mean value. Nevertheless, consistent with the present data, Jensen and Ebben (22) also failed to observe a relationship between participant strength levels and the ergogenic effect of 5-RM squats on subsequent unweighted jump performance. Furthermore, the results of a meta-analysis (6) suggest there is little difference in performance enhancement with contrast training in stronger (ES, 0.41; 95% confidence interval, 0.31–0.51) versus weaker (ES, 0.32; 95% confidence interval, 0.16–0.50) individuals.
A novel finding of this investigation was the strong negative association between individual peak squat jump power and the contrast training-induced potentiation of peak power (r2 = 0.44; Fig. 2B) and, more predominantly, between the individual PSR and the potentiation of peak power (r2 = 0.65; Fig. 2C). Based on this finding, we separated participants into two groups (n = 11 per group) based on their individual PSR, via the median split technique (32). Subsequent analyses revealed those participants with a relatively low PSR (i.e., the LPSR group) had a substantial increase in peak power, as well as average peak power, during the first set of contrast training (but not the second set). In contrast, the HPSR group experienced a reduction in peak squat jump power during the contrast training protocol, with no effect on average peak power. These data are the first to suggest that performing heavy half squats (85% 1-RM) can enhance subsequent power output of the lower-body musculature in individuals with low power relative to their maximal strength (i.e., a low PSR). Additionally, these data also suggest those individuals already capable of producing high levels of power relative to their maximal strength (i.e., those with a high PSR) do not benefit from contrast training, and may in fact experience compromised peak power output during contrast training.
The improvements in peak (and average peak) squat jump power induced by contrast training for the LPSR group occurred alongside an improvement in peak squat jump velocity, with minimal changes in peak squat jump force. On the other hand, the HPSR group experienced a decrease in peak squat jump power, a decrease in peak squat jump force, with trivial changes in peak squat jump velocity with contrast training. It therefore appears that the ergogenic effect of contrast training in the LPSR group was associated with an improvement in movement velocity, whereas the negative effect of contrast training noted for the HPSR group was associated with a decrease in subsequent force-generating capacity. Performance enhancement after contrast training is considered dependent on the net balance between the degree of potentiation and fatigue induced by the conditioning activity (5,6). It would therefore appear that contrast training had a potentiating effect on movement velocity in the LPSR group, whereas fatigue induced by the conditioning activity may have negatively influenced force-generating capacity in the HPSR group. It should be noted, however, that individuals with different strength levels may experience an optimal net balance between potentiation and fatigue (and therefore performance enhancement) at different time points after a given conditioning activity. Indeed, there is evidence that longer recovery intervals (e.g., 5 to 7 min) are more effective at inducing PAP in stronger individuals compared with shorter recovery intervals (e.g., 0.3 to 4 min), whereas recovery length has less influence on PAP effects in weaker individuals (6). Given the 1-RM half squat strength (both in absolute and relative terms) of the athletes in the present study are similar (or even higher) to that reported in weightlifters (42,43) and elite soccer players (44), it is possible that a longer recovery interval may have been required to induce greater PAP effects in this cohort.
In addition, although there were no differences in absolute maximal strength between the LPSR and HPSR groups, the HPSR group had lower relative strength compared with the LPSR group. Based on these differences in relative strength, one might expect that longer recovery periods may have been required to realise greater PAP effects in the LPSR group. Despite this, the LPSR group still showed performance enhancement effects following the contrast training protocol, which may suggest that the postconditioning activity time period had less influence on our findings, and potentially highlights that the PSR is a more powerful modulator of performance effects with contrast training. Nevertheless, we cannot discount the possibility that our results might have differed if longer postconditioning activity periods were used.
Aside from differences in the PSR, we also noted anthropometric differences between the HPSR and LPSR groups, with HPSR individuals being taller (10.3 ± 3.6 cm; ES, 1.68 ± 0.59, P < 0.001) and heavier (9.4 ± 6.1 kg; ES, 1.01 ± 0.66, P = 0.016) than those in the LPSR group. These differences in turn lead to a greater total jump displacement during both set 1 (0.96 ± 0.06 m vs 0.81 ± 0.08 m; ES, 1.78 ± 0.60; P < 0.001) and set 2 (0.96 ± 0.06 m vs 0.80 ± 0.06 m; ES, 2.30 ± 0.64; P < 0.001) versus the LPSR group. It is possible that this increased jump displacement may have increased the total jump time for the HPSR group; however, this, unfortunately, was not measured during the jumps. It is therefore possible that these between-group anthropometric differences, which in turn influenced total jump displacement (and possibly total jump time), may have influenced our results. For example, a greater jump displacement or total jump time may have exposed athletes in the HPSR group to greater amounts of friction in the Smith-type (PPS) device in which the jumps were conducted, which theoretically could have negatively influenced acceleration during the jumps (and in turn force, and thus power at a given velocity). Nevertheless, it should be noted that despite these anthropometric differences, the HPSR group had higher power output relative to their strength level (during the control condition), so it is unlikely that their anthropometric characteristics per se negatively influenced their responses to the contrast training protocol.
It has previously been demonstrated that absolute levels of maximal strength correlate poorly with peak height during counter-movement jumps (29), although there is also evidence to the contrary (45), particularly when relative strength is considered (46), and that to maximize power and explosiveness, specialized programs that also specifically train power and speed are necessary (4). This suggests that there are modifiable intrinsic factors, other than maximal strength, that contribute to peak power capacity. Because power is a product of both force and velocity, these intrinsic factors most likely enhance power production capacity via increasing the RFD and/or contraction velocity. The present data are in agreement with this notion and suggest that enhanced power output with contrast training is associated with positive changes in peak movement velocity in the absence of considerable changes in peak force, as demonstrated by the LPSR group. In support of this, Gossen and Sale (21) also proposed that contrast training may not enhance either peak force or unweighted contraction velocity, but instead may enhance performance by increasing acceleration (and hence the velocity attained) with a given external load. This is supported by data from the present study, given that participants presenting with a LPSR (whom had a substantial increase in peak squat jump power with the contrast training condition) also showed a substantial increase in peak squat jump velocity; this change that was not observed in the HPSR group.
Based on the present data, it is difficult to draw conclusions on the physiological mechanisms by which the PSR may modulate performance enhancement with contrast training. Nevertheless, improvements in muscular performance with contrast training may occur via mechanisms at both the spinal and muscular level. At the spinal level, PAP-induced increases in synaptic efficacy between Ia afferent terminals and α-motoneurons of the homonymous muscle (15) decreases presynaptic inhibition, thereby increasing central input to the motor neuron. Additionally, PAP has also been attributed to physiological events localized within the muscle, such as the phosphorylation of myosin light chains, which enhances the sensitivity of actin and myosin to Ca2+, thereby facilitating an increased rate of cross-bridge cycling (15,30,47). These factors, both at the spinal and muscular levels, may facilitate enhanced motor unit recruitment and better motor unit synchronization, leading to muscular performance enhancement (48). Based on the present results, it may be hypothesized the LPSR individuals may have intrinsically had greater basal levels of presynaptic inhibition and/or lower levels of myosin light chain phosphorylation in comparison to HPSR individuals, allowing the conditioning activity to have a greater positive influence on these factors, thereby facilitating an increase in contraction velocity and muscular power. However, further research is required to elucidate the underlying mechanisms responsible for the performance enhancement with CST in individuals with a low, but not a high, PSR.
Although long-term training studies are required, the increase in the average peak power output during the first set of jumps in the LPSR group suggests contrast training may benefit these individuals by providing a greater stimulus for chronic power development. However, it appears that this benefit is limited only to the first set of contrast training, and no added benefit is seen during subsequent sets. It is also possible, however, that longer recovery periods than those used in the present study may be required between successive squat jump sets to allow for any residual fatigue to dissipate, thus allowing for a possible PAP effect. Nevertheless, the observation of no change in the average peak power during either set 1 or set 2 with contrast training for the HPSR group, or indeed the entire cohort, suggests that there was no detrimental effect on performance with contrast training. This in turn indicates contrast training may still be a viable alternative to traditional power training for groups of athletes, such as those investigated in the present study.
As well as individual characteristics, factors related to the strength–power–potentiation contrast protocol itself also influence the ergogenic effects of contrast training (6). For example, most studies reporting an ergogenic effect of contrast training have used unloaded jumps as the performance test (e.g., 18,19,22,24,36), whereas weighted jumps have been less commonly investigated (9,11,32). The load used during squat jumps in the present study (30% 1-RM) was chosen as this has been suggested as the approximate load that allows for the optimal interaction between force production and movement velocity, therefore, maximizing mechanical power output and the potential for long-term power development (16). We chose a heavy load (85% 1-RM) for the conditioning activity, as this has been demonstrated as an appropriate load to achieve potentiation (18), and higher relative intensities have been shown to be more beneficial for potentiation than lower intensities (19,20). Six repetitions was used primarily as participants were able to perform six half squat repetitions at 85% 1-RM, and we aimed to mimic a chronic training program aimed at simultaneously developing strength and power in this level of athlete. It is possible that less repetitions would have induced less residual fatigue, and possibly altered the effects seen. Nevertheless, we believe our protocol represents an ecologically valid model for how contrast training may be implemented in practice with this level of athlete. Overall, the present data suggest that heavy back squats do not further enhance power output during a series of subsequent loaded squat jumps (performed at 30% 1-RM) in all individuals, and highlights the highly variable nature of performance benefits with complex training.
In conclusion, the major finding of this investigation was that contrast training did not enhance peak (nor average peak) power during multiple sets and repetitions of loaded squat jumps in a cohort of subelite Australian Rules Football players. However, consistent with previous investigations, there was a large degree of between-participant variability evident in the degree of performance enhancement, suggesting that individual characteristics may modulate the ergogenic effects of contrast training. Although there was no relationship between absolute or relative maximal strength levels and the potentiation of peak squat jump power with contrast training, there was a strong negative relationship between the individual PSR and the potentiation of peak squat jump power with contrast training. Subsequent analyses revealed that contrast training-induced improvements in peak power, average peak power and peak velocity during squat jumps were only noted for individuals with a LPSR, whereas those with a HPSR saw decreases in peak squat jump power, as well as peak force. These data suggest that an individual’s PSR is an important modulator of performance enhancement during contrast training. Those with a LPSR appear to benefit substantially from contrast training, whereas for individuals with a HPSR, small reductions in performance were observed.
The authors gratefully thank the participants for their efforts during the data collection for this study.
The authors received no external financial support for this study. The authors have no professional relationships with companies or manufacturers who will benefit from the results of the present study. The results of the present study do not constitute endorsement by ACSM. The results of the present study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.
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