Many sports require athletes to accelerate their own body (e.g., high or long jumps), an object (e.g., a ball in the shot put), or an opponent (e.g., martial arts). Speed-strength performance is present when the execution of a movement in a given time requires the development of large forces and, depending on the motor task, high movement speeds. Schmidtbleicher (58) referred to the ability of the neuromuscular system to produce the largest possible impulse in a defined period/available period as speed strength. Speed-strength performance consists of starting strength, rate of force development, and maximum strength components. Therefore, it also depends on muscle cross-sectional area, fiber type composition, and the discharge rate of the innervating motoneurones (20,49). Strength training-induced increases in speed strength seem indisputable (5,7,14,15,29,30). Several longitudinal investigations found increases in squat jump (SJ) and countermovement jump (CMJ) performance after strength training interventions using different exercises (5,7,14,15,19,24,31,33,42). However, results concerning effects in drop jump (DJ) performance are contradictory. Some studies confirm significant performance gains after a strength training intervention (27,57), but others report nonsignificant performance changes (41,43,63). However, medium to strong relationships between strength performance in different exercises of the lower extremities and DJ performances are measurable (10,25).
The most efficient training methods, periodization and selection of exercise require further study. However, positive effects on jump performance after different training interventions and different exercises have been detected (2,8,11–13,17,22,34,46,61,68). The different effectiveness of free-weight training compared with machine-based strength training has often been discussed (44,45). Both exercises train nearly the same muscles of the lower extremities, but in some aspects, they are different: The leg press has less requirements concerning balancing the weight, and therefore, less muscle activity contributes toward stabilization compared with the squat, but leg press allows more force to be applied in the linear path. This is why most individuals exhibit greater 1-repetition maximum (1RM) on machine-based equipment compared with free-weight exercises (18,62,67). Further, the squat movement keeps the individual in an upright position, but the leg press movement, for example, with a 45° leg press is performed with the individual in a nearly supine position. Compared with the squat, the 45° leg press spares the last 45° extension motion in the hip, and consequently, the hip extensors are not trained within this extension range. The same issue can be applied to the seated leg press—depended on the inclination of the back cushion. The seated leg press movement also has a horizontal/vertical movement pattern, whereas the squat requires a nearly vertical press. Therefore, the specific adaptations of the neuromuscular system have to be considered. The literature reveals numerous specific adaptations to strength training, primarily joint angle and contraction type-specific adaptations (23,40,54,59). Further, Wilk et al. (66) documented that not all closed kinetic chain exercises produce the same muscle recruitment pattern and tibiofemoral joint forces. Consequently, different morphological and neuronal adaptations result from different exercises.
Despite the maximal force production through many of the same muscles, squat and leg-press exercises are distinctly different and produce different specific neuromuscular adaptations because of diverse movement patterns. Therefore, we hypothesized that the same training regimen using 2 different exercises induces different adaptations to speed-strength performance. This study determined how the selection of training exercise influences the development of speed strength during an 8-week training intervention.
Experimental Approach to the Problem
This study provides information on the effects of different exercises within the training protocol on different speed strength and maximal strength parameters. Therefore, 78 students participated in this study. Thirty-nine subjects served as training group and 39 were controls. The pretest and posttests were conducted 3 days before and 3 days after the last training session. The following parameters were tested: 1RM, isometric maximum strength (MIF), CMJ and SJ height, and the DJ performance by the reactive strength index (RSI). The training period lasted 8 weeks. Training was conducted 2 times per week. Approval for this study was obtained from the institutional review board.
Seventy-eight students of the Institute of Sport Sciences volunteered for this study (Table 1). The participants were mostly athletes of track and field sports, soccer, hockey, or basketball. All participants had a strength training experience with a minimum of half a year. Therefore, the participants were used to the exercises squat and leg press because these exercises were part of their regular training routines. The participants were divided into 2 groups and further subdivided into 2 subgroups. Because the better trained participants did not want to refrain from strength training for 8 weeks, there is a tendency for weaker students to be in the control group (CON). Training groups were randomly divided into squat training group (SQ, n = 19; maximum strength performance in relation to body weight (REL [1RM/body weight]): mean of 1.2 REL; range of 0.8 REL to 1.5) and leg-press training group (LP, n = 20; mean of 3.1 REL; range of 1.8 REL to 5.1). The anthropometric data are shown in Table 1. The study conforms to the Code of Ethics of the World Medical Association (approved by the ethics advisory board of Swansea University) and required players to provide informed consent before participation.
The SQ completed an 8-week strength training protocol using the parallel squat. The LP used the same training protocol using the leg press (45° leg press). The CON (n = 39) was used as a control group and was randomly divided into 2 subgroups to control the maximum strength in squat (CON-SQ, n = 19; mean of 1.0 REL; range of 0.5 REL to 1.5) and leg press (CON-LP, n = 20; mean of 2.6 REL; range of 1.2 REL to 4.5). All other parameters were not statistically tested with the subgroups but with the whole CON group. The anthropometric data of CON are shown in Table 1.
All participants were allowed to continue their usual strength training programs with the exception of not exercising the lower extremities. They were also allowed to continue their sport-specific, regular training (e.g., soccer, track and field, hockey, or basketball). Before the study, all subjects were informed of the risks and potential benefits of the investigation. They gave informed written consent to participate in the study.
The pretest battery was performed 3 days after a familiarization test, which included the same tests described below in the same order. The same test battery was performed again 3 days after the last training session. All tests were realized in the order described below after a standardized warm-up. The warm-up consisted of 5 minutes of submaximal cycling on an ergometer and 2 to 3 sets of moderate (approximately 60% of 1RM) loaded squats with 6 repetitions each.
First SJ (intraclass correlation coefficient [ICC] = 0.88) then CMJ (ICC = 0.93) were measured (5 trials each) using a contact mat (Refitronic, Schmitten, Germany) that operates like a switch. The jumps were performed at a knee angle of 90° with the hands fixed on the hips. In the SJ, subjects had to hold a static position of 90° knee angle for 2 seconds, before they jumped (without momentum). In CMJ, subjects started in an upright position, then kneeling quickly to the turning point (90° knee angle) before they jumped. The correct movement execution was controlled visually by the investigators. The subjects had a pause of 2 minutes between jumps.
The DJ test (ICCs = 0.87–0.90) was carried out from different heights (24 cm [DJ24], 32 cm [DJ32], 40 cm [DJ40], and 48 cm [DJ48]). DJs were also measured (3 trials each) using the contact mat. The subjects started with the height of 24 cm. After 3 attempts, the height was raised. With an initial step, subjects “fell” from a box (of corresponding height) and were instructed to jump as high as possible after both feet had contacted the ground. The hands were also fixed on the hips. They were further encouraged to reduce ground contact time to a minimum. Therefore, we instructed them to avoid bending the knees. Shorter durations of ground contact (in milliseconds) and higher jumps (in centimeters) reflect better reactive power. The RSI was calculated from these data (RSI = jump height in millimeters/contact time in milliseconds × 100). The participants paused for 2 minutes between jumps.
Maximum Strength Tests
An isometric measurement (MIF) is often used in therapy and high-performance sports (36,47,48,64,65). The machine-based testing of MIF might favor a machine-based training compared with a free-weight training (66). Nevertheless, moderate to strong correlations to speed-strength performance are reported (47,48,65). Therefore, MIF was determined using a legwork machine (BAG, Wolf, Germany, ICC = 0.91), where subjects are in a seated position (hip angle of 60°) and have a knee angle of 120°. In addition, based on preliminary studies, subjects described knee joint angles of 90° or less as unpleasant. The force produced for 3 seconds in each trial was recorded using strain gauges (sampling rate: 50 Hz) and was plotted as force-time curves, indicating the force peak value. The MIF was tested 15 minutes after the jump tests. Subjects had 3 attempts with a break of 5 minutes between attempts. The joint angles were controlled through goniometer.
Dynamic maximum strength was measured using the 1RM in squat 15 minutes after determining MIF. The maximal load was determined in a series of 1RM, the loads between the attempts were increased by 2.5 kilogram (kg) and 5 kg for the squat and the leg press, respectively. Rest between attempts was at least 5 minutes. Determination of 1RM was achieved within a maximum of 5 trials, the subjects went to failure. The criterion for a successful attempt was a trial in which the leg was completely stretched. Attempts failed when the subjects rounded their back, lost the bar, or where not able to flex the knees to the desired depth. The knees in the squat had to be bent so that the thigh was parallel to the ground to complete a valid maximum test. The leg press was tested 15 minutes after determining 1RM in squat. In the leg press (Rowe & Kopp, Oberursel, Germany), the load was lowered to a knee joint angle of 90°. The lowering of the 1RM tests was monitored using a goniometer attached to a knee brace. Subjects had the knee brace on their right knee to supervise the movement range in the knee joint. At starting position (with both legs on the plate), a trigger was set. The ICCs for the leg press and for the squat were 0.95.
The training groups performed 5 sets of their 8–10RM during the first 3 weeks. Thereafter, training groups performed 5 sets of 6–8RM in the fourth to sixth week and 5 sets of 4–6RM in the seventh and eighth week. Subjects had always 5 minutes of rest between sets. The difference between the training groups was the selected exercise only (squat vs. leg press). Generally, bouncing the bar in the eccentric-concentric transition phase was not allowed in any training session. The subjects performed each set to momentary muscular failure in the last 2 repetitions of the targeted repetitions scheme (forced reps). This is the commonly used method to improve maximal strength and power in short-term interventions (21). The investigators of this investigation provided spotting and strong verbal encouragement. If necessary, the resistance was adapted for 2.5–10 kg for the next set or next training session so that the subject was able to perform in the particular repetition scheme.
The collected data were checked for normality using the Kolmogorov-Smirnov test. The ICC was calculated by the use of a 2-way mixed model (data of familiarization and pretest). The reliability (ICC) of each measurement is presented in context with the measurement. Two-factorial analyses of variance were performed using a repeated measures model for all group comparisons and comparisons between pretest and posttest results. The Mauchly sphericity test was performed before analyses of variance. The Greenhouse-Geisser correction was used if sphericity was calculated as significant. Scheffé's test was used for further post hoc analyses when there were significant F values. The 1RM changes were analyzed with the t-test for paired samples because the 2 different exercises cannot be compared directly. The correlation between the test parameters was calculated using Pearson's product-moment correlation coefficient. The significance level for all statistical tests was set at p ≤ 0.05. Subsequently, according to Cohen (16), the effect size of each variable was calculated
. In general, effect sizes higher than 0.5 are considered large (16). From 0.3 to 0.5 effect sizes are considered moderate and from 0.1 to 0.3 effect sizes are considered small (16). Effect sizes below 0.1 are trivial (16). The statistical analysis was performed using SPSS 17.0.
The data were normally distributed. Further, homogeneity of variance is assumed because of the lack of significance using Mauchly test.
The strength test data are shown in Table 2. Statistical analysis revealed a statistically significant result for the maximum isometric strength and the 1RM (Table 2).
Further, statistically significant group differences between CON and SQ, and CON and LP were found. No statistically significant difference in either 1RM or MIF was found between the SQ and LP.
Data of jump performance is shown in Table 3. The analysis of variance with repeated measures of SJ and CMJ revealed a statistically significant result for the repeated measures factor (Table 3).
The analysis of variance showed a statistically significant interaction of both parameters. Therefore, post hoc tests revealed statistically significant group differences between CON and SQ, and SQ and LP. The effect sizes of percentile performance enhancements of SQ and LP are calculated with d = 0.8–1.0, respectively.
Data of DJ performance are presented in Table 4. Analysis of variance revealed a statistically significant interaction for all RSI, except RSI24, but the post hoc test of RSI24 did not confirm statistical significance between the groups. The group differences are also displayed in Table 4. The effect sizes of percentile performance enhancements of SQ and LP are calculated in DJ24 with d = 0.4, in DJ32 with d = 0.7, in DJ40 with d = 0.6, and in DJ48 with d = 0.6.
Correlations between the maximal strength and speed-strength parameters are presented in Table 5.
The main finding is that the squat increased performance in SJ, CMJ, and DJ more effectively compared with the leg press in short-term strength training. Generally, these results are consistent with the literature regarding increases in strength and jump performance after a strength training intervention (5,7,14,15,19,24,27,31,33,42,51,57). However, to our best knowledge, no investigation compared the effectiveness of the LP and SQ to improve SJ, CMJ, and DJ in short-term interventions.
A statistically significant change occurred in the dynamic maximum strength and isometric maximum force in both training groups over the period of the study. The performance gains in training exercise were higher than for isometric testing (SQ: 1RM [+25.6%] vs. MIF [+7.7%]; LP: 1RM [+27.6%] vs. MIF [+2.4%]). Numerous investigations reported that the contraction type trained in an exercise exhibits the greatest increase in performance tests (22,52). The reason for this observation are the divergent strategies of the central nervous system (4,38,39,42). The increases in maximum strength during this investigation period are likely, primarily, due to neural adaptations, as for example enhanced motor unit activation, reduced neural inhibition, motor unit synchronization, and rate coding which were a result from changes at the central nervous level (1,3,26,35,40,49,50,53,56,60,64). Because of the dynamic training, it is likely that the neural adaptations could not be retrieved under isometric testing conditions (30,69). Hence, it is generally a problem if training exercise and test condition do not match. This is also problematic in diagnostics because data for strength variables have often been collected under isometric conditions. The authors expect that an analysis of the training progress through isometric testing after dynamic strength training would often not be sensitive enough to detect differences in performance (at least in the context of short-term training adaptations of a few weeks).
Surprisingly, the SQ reached higher percentage increases in the MIF, although statistically significant level between both training groups was missed. This is an interesting finding because the higher conformity in posture, particularly of the hip joint angle between isometric and dynamic leg press, should have led to greater conformity in muscle recruitment pattern (66) and hence higher growth rates in MIF.
No statistically significant change was found for SJ, CMJ in LP, but a statistically significant increase in jumping performance occurred in the SQ. Therefore, a difference in the development of jump performance was observed in both strength training groups. The SQ group exhibited statistically significant enhancements in SJ (12.4%) and CMJ (12.0%). Contrary to SQ, the changes in LP did not reach statistical significance and amounted to 3.5% (SJ) and 0.5% (CMJ) on average. Therefore, the training of the lower extremities using the squat exercise was superior to the leg press (large effect sizes, d = 0.8–1.0) concerning improvements in jump performance. Differences in neural adaptations between the 2 exercises are expected to be responsible for the different changes found in speed-strength parameters. Different morphological adaptations due to different biomechanical demands could have further contributed (66). The benefit of the squat exercise is assumed to lie in the body position that corresponds closely to SJ and CMJ. The position of the body with respect to all joint angles (e.g., ankle, hip, and knee) in squat exercise is more similar to those 2 jumping types than in the leg press.
However, it was surprising that changes in jump performance in the LP group were so low that the level of significance was not reached, although the improvement of the dynamic maximum strength was high. Still, it can be assumed that at least part of the adaptations in this training exercise transferred to increase performance in SJ and CMJ (30,69) as strong correlations for both dynamic maximal strength tests with the SJ and CMJ (r = 0.665–0.696) were calculated.
Furthermore, the data suggest that SQ is more likely to effectively increase DJ performance. However, mostly slight increases (small effect size [d = 0.1–0.2]) were noted, whereas in LP a decrease (small to moderate effect size [d = 0.2–0.4]) was found. However, the repeated measurement factor or the post hoc tests were nonstatistically significant, probably because of the high SDs in the parameters. Nevertheless, the group comparisons show that SQ was superior to LP. The calculated effect sizes (d = 0.4–0.7) show a moderate to large effect comparing the performance enhancements of both groups. Again, the different underlying neural and morphological adaptations account for the differences in the performance changes in DJs. Further, the different effects of SQ and LP on DJ performance could result from differences in trunk muscle strength necessary to stabilize body posture (55). A stiff landing in DJ induces greater activity on lower limbs and trunk muscles, which shortens the duration of the landing phase (6,37). This leads to shorter ground contact and further to a greater vertical ground reaction force during the ascent phase, which contributes to a greater jump height (6,9,32). Therefore, weight-training exercises showing high trunk muscle activation (specifically M. rectus abdominis), such as during squats (28,52), are appropriate for training trunk and limb muscles. In general, core stability does not play a major role for DJ and speed-strength performance (38,55) but has to be considered because of the mentioned line of argument.
The correlation between performance in DJ and SQ was consistently higher (r = 0.362–0.514; p ≤ 0.05) than the nonsignificant correlations between DJ and LP (r = −0.100 to 0.039). However, it is likely that the smaller increase compared with the performance improvements of SJ and CMJ in SQ may be explained by the fact that increases in the strength of the knee, hip, and trunk muscles cannot contribute to further significant performance gains because the target muscles (M. triceps surae and M. tibialis anterior) were not explicitly trained.
This study reports 2 interesting findings for coaches and athletes. First, the data suggest a large influence of the selection of training exercise on performance improvement (at least short-term). The squat was shown to be the more effective training exercise. Therefore, it should be preferred to the leg press. This holds true especially in the special stage before competition because of the better transfer effects on jumping performance but must also be considered in the context of long-term development of speed-strength performance. Second, procedures for the collection of strength data should be carefully considered in the selection of test parameters. As in the data presented here, problems arise when measurements based on dynamic executions (when the joint angle varies over the range of motion) are compared with collected parameters of another contraction type and at a specific joint angle. Otherwise, there is an increased risk of assessing, for example, training and therapy successes in an incorrect way. Therefore, consequences should arise for the interpretation of normative data, for example in health care or high-performance sports, if isometric measurements were performed. From this point of view, it is also important to examine to what extent the relationship between isometric and dynamic functioning of the muscles of the selected weight-training exercise is affected.
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