Ballistic training is often used to improve skeletal muscle function and athletic performance (15). In ballistic exercise, the athletes have to exert the highest strength in the shortest time to maximally accelerate their body mass (e.g., jumping) or an object (e.g., kicking or throwing a ball). Jump-squat is among the most used ballistic exercise to enhance mechanical power in lower limb muscles (15,25,30). Jump-squat has been shown to improve jump height (17,25,38) and sprint performance (15,16,38). However, since the increased role of change of direction (COD) in soccer (8), the effects of jump-squat training on COD were only recently investigated, reporting improvements in COD only after jump-squat training (26,27) or jump-squat added to a traditional strength training program (23). Importantly, jump-squat training was shown to improve physical ability in soccer players in preseason (27) and to counteract the decrease in speed and power performance due to the high endurance training load the players undergo before the season begins (28). In addition, jump-squat training was effectively added to traditional soccer training to elicit power inseason (35). Finally, to get meaningful adaptations, jump-squat training was conducted for 6 weeks or more (15,16,23,26,27,35).
Muscle architecture, encompassing muscle thickness, pennation angle, and fascicle length, is a strong determinant of muscle force generating capacity (5). Muscles with longer fascicles can develop force at a higher rate, whereas muscles with wider thickness and pennation angle have a larger physiological cross-sectional area, thus enhancing the maximal force produced (5). Muscle thickness, pennation angle, and fascicle length are known to increase after traditional resistance training (3,11,20,32). However, little is known about the effects of jump-squat training on muscle architecture. Previous studies have examined the effects of jump-squat training using quadriceps muscle as the target muscle because of its influential role in jumping tasks (19). However, inconsistent results, such as increases in pennation angle but not in muscle thickness in vastus lateralis (15) or increases in muscle thickness after a combined strength and jump-squat training in rectus femoris (35), have been recently reported. Such a discrepancy could have derived from the different targeted muscles and from the different protocols used. Indeed, given that some Olympic-lift exercises were included in the latter (35), the larger knee range of movement compared with the self-selected depth used in jump-squat training may have resulted in a greater work completed. Moreover, no change in fascicle length after combined strength/jump training (36) nor after combined jump/sprint training was observed (4).
Jump-squat training has been shown to improve lower limb isometric muscle strength (15)and increase squat 1RM (16,25,30). Given the important contribution of the quadriceps and hamstrings during both take-off and landing in jump-squat (19), training using jump-squat may have specific effects on the maximal strength of these muscle groups. A previous surface electromyographic study highlighted that a higher hamstrings activity in both concentric and eccentric phases occurred when jumps are performed without a stretch-shortening cycle (31). Because jump-squat does not include a fast countermovement or a plyometric action, the repetitive jumps may result in a noteworthy specific strength adaptation in the hamstrings. Interestingly, it was shown that quadriceps muscle activation was not affected by the load (21), leading to hypothesize that specific adaptations in the hamstrings-to-quadriceps strength ratio, an index to estimate hamstring injury risk (9), may be derived from jump-squat training. Interestingly, greater fatigue was shown in the hamstrings compared with the quadriceps after a standardized task (10) or after a soccer match simulation (9). Therefore, jump-squat training may be used to increase hamstrings strength, consequently increasing the hamstrings-to-quadriceps strength ratio (9,10), therefore decreasing the hamstrings strain injury risk.
Several previous studies have investigated the effect of jump-squat training using the external load that maximized the power output (15–17,38). However, measuring such a load appropriately requires devices (i.e., force plates and linear transducers) that are often unavailable in the field setting. Notwithstanding, it was reported that the maximal power output usually ranges from 0 to 30% of the squat 1RM (14,18,30) and is also shown in a direct optimum load vs. body mass comparison (29). Jump-squat training is characterized by repetitive explosive concentric take-offs followed by repetitive eccentric landings. Both work and force developed during these phases are accounted for the external load used during the jump-squat. Particularly, compared with body mass squat jump, a greater inertia accumulated during a weighted jump results in a greater eccentric work completed, which was shown to be a key factor for inducing improvements in muscle performance (17). Previous studies have shown that irrespective of the exercise, an accentuated eccentric phase induced specific adaptations in muscle architecture after isokinetic or isoload knee extension training (11) or greater hypertrophic stimuli after a 6-week bench press training. (13). Finally, the repeated excessive braking load during landing could result in greater improvements in COD, which similarly requires the athletes to repetitively brake the inertia of their body mass and subsequently accelerate.
Therefore, the aim of the present study was to evaluate the effects of weighted (with 30% of squat 1RM) jump-squat training (WJST) or body mass squat-jump training (BMSJT) on quadriceps muscle architecture and lower limb lean mass (LM) in recreational soccer players. Change of direction, sprint, and jump performance were also evaluated. Last, both changes in hamstrings and quadriceps peak torque were measured, as well as the changes in functional Hecc:Qconc ratio were calculated.
Experimental Approach to the Problem
The present investigation was designed as a pre-post, parallel 3-group, randomized-controlled trial. Using a restricted blocks randomization (computer-generated sequence), the participants were randomly allocated into BMSJT or WJST or control group (CON). The allocation and the randomization were completed by one of the researchers without any contact or knowledge of the participants. Therefore, no allocation concealment mechanisms were necessary. To calculate the sample size, a statistical software (GPower; University of Dusseldorf, Dusseldorf, Germany) was used. Given the study design (3 groups, 2 repeated measures), the effect size = 0.25 (medium), α-error <0.05, the nonsphericity correction € = 1, the correlation between the repeated measures = 0.5, and a desired power (1−ß error) = 0.8, the total sample size resulted in 42 participants. To prevent the effect of any possible dropout on the statistical power, 48 participants were included.
Forty-eight male recreational soccer players (± SD age: 21 ± 3 years, age ranged from 18 to 25 years; body-mass: 73 ± 4 kg; height: 1.78 ± 0.10 m) volunteered to participate in the present investigation. The participants joined 2 Italian recreational soccer teams, which competed in a recreational soccer championship. The participants had a soccer history of at least 5 consecutive years in young or recreational soccer teams. Within the previous season, their typical training volume consisted of 3 training sessions (about 2 hours per session) plus one match per week, from September to May. Lower limb muscular or joint injuries in the previous 12 months, and cardio-pulmonary diseases, smoking, or drugs use, were listed as exclusion criteria. The present investigation was approved by the University of Verona and was in line with the Declaration of Helsinki (1975 and further updates) concerning the ethical standards in studies involving human subjects. Finally, the participants were carefully informed about any possible risks due to the investigation's procedures, and they signed a written informed consent.
To evaluate the lower limb muscle strength, squat 1RM, isokinetic concentric, eccentric, and isometric quadriceps peak torque and eccentric hamstrings peak torque were measured. To evaluate the quadriceps muscle architecture, muscle thickness, fascicle length, and pennation angle were measured on vastus lateralis muscle. To evaluate the lower limb (LM), dual-energy X-ray absorptiometry (DXA) scans were used. Finally, to evaluate their soccer abilities, COD, sprinting and, jumping abilities were measured.
The present investigation lasted 10 weeks and was conducted in the off-season (from May to July). The participants were instructed to avoid any other form of resistance training for the entire duration of the present investigation. In the first week, the participants were involved in 3 testing sessions. In the first session, the participants were familiarized with the squat technique, isokinetic strength testing procedures, COD, sprinting, and jumping ability testing procedures. Within the second session, muscle architecture, LM, and squat 1RM were measured, and the participants familiarized with the training protocols. Within the third session, isokinetic strength, COD, sprinting and, jumping abilities were measured. The intervention lasted 8 weeks. Finally, the posttraining testing measurements were assessed the week after the end of the intervention, and they were conducted over 2 sessions. In the first one, muscle architecture, LM, squat 1RM, and isokinetic strength were measured. In the second session, COD, sprinting, and jumping abilities were measured. Each assessment was performed by the same experienced operators and interspersed by 30 minutes of passive recovery. Change of direction, sprints, and jumps were measured indoor on a concrete surface.
The back squat 1RM was measured using an Olympic bar. After a standardized warm-up consisting of 30 weight-free squats, the 1RM attempts started from 80% of the body mass. Thereafter, additional 5% was added until failure. Each set was separated by 3 minutes of passive recovery. A standard time under tension (2 seconds for the concentric and eccentric phase, 1 second for the isometric phase) was used, and the participants had to lower the bar until the thighs were parallel to the ground. Strong standardized encouragements were provided to the participants to maximally perform each trial. Squat 1RM/body mass was calculated and inserted into the data analysis. Last, 30% of squat 1RM was used as overload for WJST.
An isokinetic dynamometer (Cybex Norm; Lumex, Ronkonkoma, NY, USA) was used to measure quadriceps' and hamstrings' strength. The procedures followed previous recommendations (11). Briefly, the device was calibrated according to the manufacturer's procedures, and the center of rotation was aligned with the tested knee. The participants were seated on the dynamometer's chair, with their trunks slightly reclined backward and a hip angle of 95°. Two seatbelts secured the trunk and one strap secured the tested limb, whereas the untested limb was secured by an additional lever. The strength measurements were preceded by a standardized warm-up, consisting of 3 sets × 10 repetitions of weight-free squats. Quadriceps peak torque was measured in concentric (1.05 rad·s−1) and eccentric (−1.05 rad·s−1) modalities (12). Hamstrings peak torque was measured in eccentric (−1.05 rad·s−1) modality. Each testing modality consisted of 3 maximal trials and was separated by 2 minutes of passive recovery. Strong standardized encouragements were provided to the participants to maximally perform each trial. The peak torque was then calculated and inserted into the data analysis. Finally, the hamstrings-to-quadriceps strength ratio, defined as the ratio between eccentric hamstrings-to-concentric quadriceps peak torque (i.e., functional Hecc-Qconc ratio) (9) was also calculated. Excellent test-retest reliability was found for all the isokinetic measurements (from α = 0.915 to α = 0.963).
Vastus lateralis muscle architecture was measured using an ultrasound device (Acuson P50; Siemens, Munich, Germany) at the 39% of the distal length of the thigh (12). The participants lay supine and the 4 cm ultrasound transducer was oriented perpendicularly to the skin surface of the vastus lateralis and longitudinally to the muscle's fascicles. Two images were scanned and then analyzed using a free imaging analysis software (ImageJ; NIH, Bethesda, Maryland, USA). Images were obtained at 50% of the muscle width defined as the midpoint between the fascia separating the vastus lateralis and rectus femoris and fascia separating the vastus lateralis and biceps femoris muscles. Muscle thickness was defined as the distance between the superficial and deep aponeurosis. Pennation angle was defined as the angle between the fascicles and the aponeurosis. Finally, fascicle length was calculated according to the formula (5):
where y is the angle between the superficial and the deeper aponeurosis, PA is the pennation angle, and MT is the muscle thickness. The same experienced operator performed the data collection and data analysis, and the operator was blinded to the participants' allocation. Excellent reliability was found for muscle thickness (α = 0.917) and pennation angle (α = 0.902) and good reliability for fascicle length (α = 0.876).
Lower Limb Lean Mass
Total body and regional composition were evaluated using DXA, a total body scanner (QDR Explorer W; Hologic, Marlborough, MA, USA; fan-bean technology, software for Windows XP version 12.6.1), according to the manufacturer's procedures. The DXA body composition approach assumes that the body consists of 3 components that are distinguishable by their X-ray attenuation properties: fat mass, LM, and bone mineral (34). The scanner was calibrated daily against the standard supplied by the manufacturer to avoid possible baseline drift. Whole-body scanning time was about 7 minutes. Data were analyzed using standard body region markers: upper and lower extremities, head, and trunk (pelvic triangle plus chest or abdomen). All scanning and analyses were performed by the same operator to ensure consistency. The whole lower limb LM amount was reported in data analysis.
Squat-Jump and Countermovement Jump
The peak heights of squat jump (SJ) and countermovement jump (CMJ) were investigated using an infrared device (OptoJump, Microgate, Bolzano, Italy). In the SJ, the participants were instructed to stand, flex the knees to approximately 90° and jump. The participants had to avoid as much as possible any countermovement, and they were instructed to stop for 2 seconds at each phase. In the CMJ, the participants were instructed to stand, lower themselves to a self-selected knee flexion and immediately jump. Arms were placed on the hips in both SJ and CMJ tests. The participants were instructed to avoid any knee flexion before landing in both SJ and CMJ, and the operator visually checked for it. Three attempts were performed for each jump, and the peak height was inserted into the data analysis. Two minutes of passive rest separated each jump. A good reliability was found for both SJ (α = 0.876) and CMJ (α = 0.861).
Sprint and Change of Direction
The time trials of 10- and 30-m dash and agility T-test (7) were separately investigated using an infrared device (Polifemo, Microgate, Bolzano, Italy). The participants were placed 30 cm behind the starting line, with the preferred foot in the forward position and autonomously started each trial. An excellent reliability was found for 10- and 30-m sprints (α = 0.945 and α = 0.921, respectively).
The agility T-test was performed turning to the right or left as first, and the sum of the 2 trials was inserted in the data analysis. Four cones were arranged in a T-shape, with a cone placed 9.14 m from the starting cone (photocell gates 2 m apart) and 2 further cones placed 4.57 m on either side of the second cone. The participants had to sprint forward 9.14 m from the start line to the first cone and touch the cone with their right hand, shuffle 4.57 m left to the second cone and touch it with their left hand, then shuffle 9.14 m right to the third cone and touch it with their right hand, and shuffle 4.57 m back left to the middle cone and touch it with their left hand before finally back-pedaling to the start line. The trials were not considered if participants failed to touch a designated cone or failed to face forward at all times. Only one timing gate placed on the start-finish line was used for timing the T-test. Each test was repeated 3 times, and the best performance was calculated and inserted into the data analysis. Two minutes of passive rest separated each trial. The agility t test showed a good reliability (α = 0.818).
Both BMSJT and WJST sessions involved a warm-up consisting of 5 minutes of cycling followed by 20 weight-free squats. Training volume load was calculated as a number of repetitions × load, assuming a similar time under tension and distance covered (13), particularly, load referred to body mass, resulting in 1 A.U. (=body mass only) in BMSJT and 1.2 A.U. in WJST (as shown in Table 3). To equalize the training volume over the whole intervention, BMSJT performed 5 sets × 10 repetitions (n = 50), and WJST initially performed 4 sets × 10 repetitions (n = 40). After 4 weeks, only in WJST, the load was increased to 1.25 A.U. and WJST performed 2 sets × 10 and 2 sets × 11 repetitions (n = 42). The sets were separated by 3 min of passive recovery. Both groups were instructed to maximally jump and finish the landing phase of each jump at a knee angle corresponding approximately to 90°. BMSJT were instructed to keep their hands on their hips for the full duration of each jump. In WJST, the overload consisted of a bar grasped on the shoulder in a back squat position for the whole duration of each jump. The weight used as the external load in WJST was tailored according to the individual squat 1RM results. The participants received strong standardized encouragements to maximally perform each jump. The intervention lasted 8 weeks, 2 sessions per week, separated by at least 2 days, during which CON did not perform any training.
Statistical analysis was performed using statistical software (SPSS 22; IBM, Armonk, NY, USA). The normality of the distribution was checked using Shapiro-Wilk's test. The sphericity assumption was calculated using Mauchly's test. The test-retest reliability was measured using an intraclass correlation coefficient (ICC, Cronbach-α) and interpreted as follows: α ≥ 0.9 = excellent; 0.9 > α ≥ 0.8 = good; 0.8 > α ≥ 0.7 = acceptable; 0.7 > α ≥ 0.6 = questionable; 0.6 > α ≥ 0.5 = poor (37). The variations of the dependent parameters were analyzed by separate mixed-factors ANOVA (time × group) for repeated measurements. In addition, data were log-transformed and analyzed using an ANCOVA, considering baseline values as covariates. Post hoc analysis using Bonferroni's correction was then performed to calculate the main effect for group (3 levels: BMSJT, WJST, and CON) and time (2 levels: pre- and posttraining). Significance was set at α < 0.05. Data are reported as mean with SD. Changes are reported as % change with 95% of confidence intervals (95% CI) and effect size (ES) with 95% CI. ES was interpreted following Hopkins's recommendations (24): 0.0–0.2 = trivial; 0.2–0.6 = small; 0.6–1.2 = moderate; 1.2–2.0 = large; >2.0 very large.
The compliance rate for BMSJT and WJST was 94 and 96%, for a total of 16 and 11 missed training sessions, respectively. No injury occurred during the intervention period.
Time × group interactions were found for muscle thickness (p = 0.013), pennation angle (p = 0.023), and fascicle length (p = 0.003). However, despite the similar increases in muscle thickness (BMSJT = moderate and WJST = small), pennation angle moderately increased only in BMSJT, whereas greater increases in fascicle length were found in WJST compared with BMSJT (+8%, CI 95%, 2–15). Finally time × group interaction was found for lower limb LM (p < 0.001), and greater increases in LM were found in WJST compared with BMSJT (+7%, CI 95%, 5–10). The control group did not show any change (Table 1).
Significant time × group interaction was found for the agility T-test (p < 0.001). Very large decreases in the agility T-test time were observed in WJST, whereas no change occurred in BMSJT. Significant time × group interactions were found for 10 m (p = 0.001) and 30 m (p = 0.012) performance. Moderate decreases in 10- and 30-m sprint time occurred in WJST and not in BMSJT. Significant time × group interactions were found for SJ (p = 0.003) and CMJ (p = 0.001). Although both BMSJT and WJST increased SJ and CMJ height, greater increases occurred in BMSJT than WJST in SJ (+5%, CI 95%, 2–8) and in CMJ (+6%, CI 95%, 1–11). CON did not show any change (Table 2).
Time × group interactions were found for squat 1RM (p = 0.021), concentric (p < 0.001), eccentric (p < 0.001) peak torque, and hamstrings' eccentric peak torque (p < 0.001). Both BMSJT and WJST similarly increased quadriceps' and hamstrings' muscle strength over time. Similarly, time × group interaction was found for functional Hecc to Qconc ratio (p < 0.001). Only BMSJT moderately increased it. CON did not show any change (Table 3).
The present investigation highlighted that (a) despite the similar increments in vastus lateralis muscle thickness, pennation angle widened only after BMSJT, whereas fascicle length increased more after WJST than in BMSJT; this was accompanied by greater increases in lower limb LM in WJST compared with BMSJT; (b) only WJST improved COD and sprint performance, whereas BMSJT improved jumping ability more than WJST; and (c) similar increases in hamstrings and quadriceps muscle strength occurred in both BMSJT and WJST, even if the functional Hecc to Qconc ratio increased in BMSJT but not in WJST.
The specific WJST vs. BMSJT training-induced adaptations in vastus lateralis muscle architecture is introduced here for the first time. The greater increases in fascicle length after WJST than in BMSJT may derive from the enhanced eccentric phase due to the greater external load used in WJST. Such a hypothesis is in agreement with the studies that have reported eccentric only (11,20) or enhanced eccentric training–induced (32) fascicle elongations. Indeed, as debated in the literature, it seems that eccentric exercise selectively affects fascicle length (1,11,20). Increments in fascicle length are reflective of serial sarcomere addition, which facilitates fastening in muscle contraction and larger range of movements (5). Consistently, combined jump/sprint training was able to induce vastus lateralis fascicle elongation in both distal and proximal sites to a large extent (4). On the other hand, increases in pennation angle do not seem to be induced after enhanced eccentric training. The present data highlighted that only BMSJT increased pennation angle, indicating that a greater eccentric work does not usually affect the in-parallel sarcomere number and consequent increases in pennation angle (1,11,20). Similar to the present study, increases in pennation angle were reported after body mass jump training (15). On the contrary, decreases in pennation angle occurred after combined jump/sprint training (4). Since inhomogeneous changes in vastus lateralis muscle architecture were reported (4,18), the lack of changes in WJST may have derived from the different sites on which the ultrasound scans were placed. Last, adaptations in muscle thickness can depend on adaptations in pennation angle, fascicle length, or both. The small and moderate increases (for WJST and BMSJT, respectively) in vastus lateralis muscle thickness are in contrast to previous studies that failed to show changes in muscle thickness after a jump-squat training performed at the load that elicited optimum power (15) or combined body mass jump/sprint training (4). One possible explanation for such an inconsistency may be the different populations involved. Both the above-mentioned studies recruited competitive athletes (4) or resistance-trained men (15), whereas the present population consisted of recreational soccer players. Given the greater training-induced effectiveness in structural muscle adaptations in untrained vs. trained populations (22), it may be hypothesized that the current participants were more prone to muscle enlargements. However, because the current increases in muscle thickness had small or moderate extent, it should be acknowledged that the traditional strength training could be more effective as previously reported (4,15). Aside, greater increases in lower limb LM were found in WJST than in BMSJT, although both increments were small. Increases in muscle size were previously reported (4), and they were shown to be specifically related to type-IIx fibers (40). The present results agree with a previous study that reported greater hypertrophy after eccentric vs. traditional training (13). On the contrary, no change in LM occurred in resistance-trained men (15), suggesting that the different initial fitness level may have led to different adaptations.
Very large improvements in the agility T-test time occurred only in WJST, with no changes recorded in BMSJT. The present results are in line with a previous study reporting improvements in COD after jump-squat training with the optimum power load (27). Consistently, jump-squat training added to traditional strength training resulted in gains in COD, as previously reported (23). Change of direction requires the athletes to rapidly brake and immediately accelerate their body in different directions. The greater external load in WJST than in BMSJT may have conditioned the participants to effectively perform both decelerations and accelerations required by the intervention (27). The increased capacity to rapidly accelerate the body mass is a key feature for sprint performance (39). The present results confirmed the effectiveness of WJST in improving sprint performance (15,39) and combined jump/sprint training (4) or strength/jump training (23). Unloaded jumps resulted in greater force at a given velocity within the force/velocity relationship (16). This may lead to argue that training with no external load may reduce transfer in power from training to performance. Such a transfer depends on the training intensity, frequency, and specificity, as previously reported (15). In addition, it may be expected that recreational soccer players may be accustomed to both sprint and CODs (8). Therefore, the absence of further improvements in BMSJT may be explained by the insufficient stimuli received during the training. Last, the greater eccentric load that WJST underwent may have greatly accounted for the increases in concentric/eccentric tasks as demanded in COD and sprints, as previously shown (17). Notwithstanding, the greater external load in WJST, greater increases in SJ, and CMJ were recorded in BMSJT. The increases in jump height after jump training have been largely reported (4,15–17,30,39). However, the training-testing specificity may have played a key role in the greater improvements in BMSJT, because both training and testing were performed without any external load. In line with the current result, adding an eccentric overload exercise did not lead to any difference in jump height gained compared with traditional training in handball players (33). In addition, it may be argued that BMSJT could have accustomed the participants to higher velocities developed during the vertical jumps, resulting in greater specific jumping adaptations (27).
To the best of the authors' knowledge, another novel aspect of the present investigation is the selective increment in functional Hecc to Qconc ratio in BMSJT but not in WJST. The functional Hecc to Qconc ratio can be used to evaluate the hamstrings strain injury risk, as the lower the ratio, the higher the risk (9). The different outcomes shown in BMSJT vs. WJST are mainly because of the greater, albeit not different, increases in quadriceps concentric peak torque in WJST than in BMSJT, with very similar increases in hamstrings eccentric peak torque. It could be speculated that the loaded jumps led to greater trunk flexion to maximize the jump height (2). Thus, higher forwarded load may have differently stimulated the forward vs. backward lower limb muscles. The increases in squat 1RM and quadriceps and hamstrings peak torque come with previous inconsistent literature. Indeed, no improvement in squat 1RM (15) or quadriceps concentric peak torque (4) was observed after jump-squat training. Conversely, increases in half squat 1RM (40) or in isometric maximal force (38) were previously reported. It can be argued that the current unaccustomed participants may have resulted in small but significant strength gains. Aside, the similar between-group adaptations in lower limb muscles strength may derive from the similar total training load volume, as already shown (11,13). Particularly, WJST resulted in overall greater but not significant increases in quadriceps strength, irrespective of the testing modality. In line with the present results, it was shown that volume-matched eccentric isoload vs. isokinetic training resulted in similar knee extensors strength gains (11). Interestingly, volume-matched but different training modalities resulted in similar increases in bench press 1RM (13).
The present investigation comes with some acknowledged limitations and some interesting perspectives. First, the unaccustomed population may have been sensitive to the training-induced adaptations. Therefore, further accustomed populations should be included for a more comprehensive evaluation of the jump-squat training-induced adaptations. Second, the present investigation has been conducted offseason. This may permit to isolate its training-induced adaptations, but it should be tailored to the weekly training load when performed pre- or inseason. Third, only the traditional lower and upper bounds of the external load that maximizes power were examined here. Therefore, further loads in between could provide more insights on this topic. Last, power output was not measured during the training or during the SJ and CMJ. The lack of the power measurement did not allow the correct use of the training load that elicits the maximum power. However, the present investigation was designed to have a strong practical impact because the device necessary to measure power output is often unavailable in the field practice.
In conclusion, specific training-induced adaptations were observed after BMSJT or WJST. Despite similar increases in vastus lateralis muscle thickness, greater increases in fascicle length occurred in WJST, whereas increases in pennation angle occurred only in BMSJT. In addition, greater increases in LM were shown in WJST than in BMSJT. Specific load-dependent performance improvements were shown because COD and sprint performance improved only in WJST, whereas greater increases in jump height were observed in BMSJT. Such adaptations were accompanied by similar increases in quadriceps and hamstrings strength and by increases in functional Hecc to Qconc ratio in BMSJT but not in WJST.
The present findings suggest that different external loads should be used to selectively improve COD, sprint, or jump performance in recreational soccer players. Because of the increased role of COD in soccer (8), trainers and conditioners may use WJST to improve such an ability. Similarly, the same training method may be recommended to improve sprints, whereas weight-free jump-squats should be proposed to improve jumping ability.
The functional Hecc to Qconc ratio is often monitored to reduce the hamstrings strain injury risk. Because it was seen to decrease with the advancement of a soccer match (9), specific training sessions should be dedicated to reinforce hamstrings eccentric strength. Although specific exercises have been proposed (e.g., Nordic hamstrings) (6), it can be suggested here that BMSJT could be included into a weekly routine, possibly coupled with specific hamstrings lengthening exercises because of the small effect reported here.
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