Neuromechanical strategies employed to increase jump height during the initiation of the squat jump.

The maximal height attained in a vertical jump is heavily influenced by the execution of a large countermovement prior to the upward motion. When a jump must be executed without a countermovement, as in a squat jump, the maximal jump height is reduced. During such conditions, the human body may use other strategies in order to increase performance. The purpose of this research was to investigate the effects of two strategies employed during the initiation of the squat jump: the premovement silent period (PSP), and the small amplitude countermovement (SACM). Fifteen elite male volleyball players (20.6 +/- 1.6 years) and 13 untrained males (20.2 +/- 1.7 years) performed 10 maximal effort squat jumps from identical starting positions. The electromyographic activity of the vastus lateralis and biceps femoris was measured in conjunction with the vertical ground reaction force and vertical displacement. It was found that the presence of a PSP or a SACM of 1-3 cm did not increase maximal squat jump height significantly (p > 0.05), in neither the highly trained athletes nor the untrained individuals. These results suggest that these strategies do not play a major role in the determination of jump height. Researchers have assumed that a squat jump is purely concentric, and that there are no facilitating mechanisms present that may influence the performance of the jump. This study provides evidence to support this assumption.     

Methodological concerns for determining power output in the jump squat

The purpose of this study was to investigate the validity of power measurement techniques during the jump squat (JS) utilizing various combinations of a force plate and linear position transducer (LPT) devices. Nine men with at least 6 months of prior resistance training experience participated in this acute investigation. One repetition maximums (1RM) in the squat were determined, followed by JS testing under 2 loading conditions (30% of 1RM [JS30] and 90% of 1RM [JS90]). Three different techniques were used simultaneously in data collection: (a) 1 linear position transducer (1-LPT); (b) 1 linear position transducer and a force plate (1-LPT + FP); and (c) 2 linear position transducers and a force place (2-LPT + FP). Vertical velocity-, force-, and power-time curves were calculated for each lift using these methodologies and were compared. Peak force and peak power were overestimated by 1-LPT in both JS30 and JS90 compared with 2-LPT + FP and 1-LPT + FP (p 相似文献   

A proposed method for determining peak power in the jump squat exercise

In recent years a great deal of research has been published using peak power (PP) in the jump squat (JS) exercise as a measure of athletic performance. However, no standardized method for the determination of PP exists at this time to accurately evaluate this variable. Our proposed method (PM) for determining PP (PPPM) in the JS uses the product of vertical ground reaction forces and velocity of the center of mass of both the subject and the external resistance of a loaded Olympic bar. Fifteen male subjects with a mean age of 27 +/- 3 years, weight of 78 +/- 17 kg, and height of 175 +/- 10 cm participated in this study. PP was measured in the JS at five different testing loads (30%, 35%, 40%, 45%, and 50% body weight) based on methods commonly discussed in the literature to compare PP results of previous methods to those obtained using the PM. Paired t-tests at different load levels were used for statistical analysis with an overall alpha = 0.05. The average PP among five testing loads, measured by the PM, was 3782 +/- 906 W. PP derived from the product of force and velocity of the bar alone was 72% lower than PPPM at 1057 +/- 243 W (P < 0.0001). The PP estimated by the product of bar velocity and vertical ground reaction forces of the bar plus the subject was 8% higher than PPPM at 4100 +/- 844 W (P = 0.0001). Our results indicate that using the methods traditionally reported in the literature may cause an overestimation of PP during athletic performance. Using the PM in future research will facilitate test validity and enable the generalization of results outside the scope of specific research projects.     

Acute effects of augmented eccentric loading on jump squat performance

The purpose of this study was to determine the acute effects of a spectrum of eccentric loads on force, velocity, and power during the concentric portion of maximal-effort jump squats utilizing a repeated measures design. Thirteen resistance-trained men (age = 22.8 +/- 2.9 years, weight = 87.1 +/- 11.8 kg, 163.5 +/- 28.6 kg squat 1 repetition maximum [1RM]; mean +/- SD), who routinely incorporated back squats into their training, participated as subjects in this investigation. Jump squat performance was assessed using 4 experimental conditions. The first of these conditions consisted of an isoinertial load equal to 30% of back squat 1RM. The remaining conditions consisted of jump squats with a concentric load of 30% 1RM, subsequent to the application of experimental augmented eccentric loading (AEL) conditions of 20, 50, and 80% of back squat 1RM, respectively. All subjects performed 2 sets of 1RM of maximum-effort jump squats with all experimental conditions in a counter-balanced sequence. Forty-eight hours after completing the first testing session, subjects repeated the experimental testing protocol to establish stability reliability. Peak performance values for the reliable variables of force, velocity, and power, as well as force and power values obtained at 20-ms intervals during the initial 400 ms of the concentric jump squat range of motion, showed no statistical difference (p > 0.05) across the experimental AEL loads. These results suggest that load-spectrum AEL prior to a 30% 1RM jump squat fails to acutely enhance force, velocity, and power.     

Investigation of vertical jump performance after completing heavy squat exercises

The purpose of this study was to determine whether vertical jump height was influenced by completing a half squat or quarter squat exercise prior to jumping. Ten male subjects experienced with the squat exercise performed 4 warm up squat sets followed by 1 repetition with the weight of 90% of 1 repetition maximum of the half squat or quarter squat. No difference in jump heights after any of the 3 conditions, including a control group (F = 3.096, p = 0.070), was found. Correlations between the relative strength ratio and the difference in averaged jump heights before and after the half and quarter squat conditions were also tested, and no correlation was found (r = -0.128, p = 0.724, and r = -0.189, p = 0.601, respectively). Although statistical significance at the 0.05 level was not observed for the comparison of jump heights between conditions, we did observe a trend (i.e., p = 0.07). Therefore, we examined the individual responses to the exercises and determined that 5 of the subjects did increase their vertical jumps after both squat exercises. It may be that the influence of prejump exercise on jump performance may be individualized. Nevertheless, the use of a strength ratio does not appear to predict who will benefit from posttetanic potentiation in this type of exercise situation.     

Acute effects of heavy-load squats on consecutive squat jump performance

Postactivation potentiation (PAP) and complex training have generated interest within the strength and conditioning community in recent years, but much of the research to date has produced confounding results. The purpose of this study was to observe the acute effects of a heavy-load back squat [85% 1 repetition maximum (1RM)] condition on consecutive squat jump performance. Twelve in-season Division I male track-and-field athletes participated in two randomized testing conditions: a five-repetition back squat at 85% 1RM (BS) and a five-repetition squat jump (SJ). The BS condition consisted of seven consecutive squat jumps (BS-PRE), followed by five repetitions of the BS at 85% 1RM, followed by another set of seven consecutive squat jumps (BS-POST). The SJ condition was exactly the same as the BS condition except that five consecutive SJs replaced the five BSs, with 3 minutes' rest between each set. BS-PRE, BS-POST, SJ-PRE, and SJ-POST were analyzed and compared for mean and peak jump height, as well as mean and peak ground reaction force (GRF). The BS condition's mean and peak jump height and peak GRF increased 5.8% +/- 4.8%, 4.7% +/- 4.8%, and 4.6% +/- 7.4%, respectively, whereas the SJ condition's mean and peak jump height and peak GRF decreased 2.7% +/- 5.0%, 4.0% +/- 4.9%, and 1.3% +/- 7.5%, respectively. The results indicate that performing a heavy-load back squat before a set of consecutive SJs may enhance acute performance in average and peak jump height, as well as peak GRF.     

Reliability and factorial validity of squat and countermovement jump tests

The primary aim of this study was to determine reliability and factorial validity of squat (SJ) and countermovement jump (CMJ) tests. The secondary aim was to compare 3 popular methods for the estimation of vertical jumping height. Physical education students (n = 93) performed 7 explosive power tests: 5 different vertical jumps (Sargent jump, Abalakow's jump with arm swing and without arm swing, SJ, and CMJ) and 2 horizontal jumps (standing long jump and standing triple jump). The greatest reliability among all jumping tests (Cronbach's alpha = 0.97 and 0.98) had SJ and CMJ. The reliability alpha coefficients for other jumps were also high and varied between 0.93 and 0.96. Within-subject variation (CV) in jumping tests ranged between 2.4 and 4.6%, the values being lowest in both horizontal jumps and CMJ. Factor analysis resulted in the extraction of only 1 significant principal component, which explained 66.43% of the variance of all 7 jumping tests. Since all jumping tests had high correlation coefficients with the principal component (r = 0.76-0.87), it was interpreted as the explosive power factor. The CMJ test showed the highest relationship with the explosive power factor (r = 0.87), that is, the greatest factorial validity. Other jumping tests had lower but relatively homogeneous correlation with the explosive power factor extracted. Based on the results of this study, it can be concluded that CMJ and SJ, measured by means of contact mat and digital timer, are the most reliable and valid field tests for the estimation of explosive power of the lower limbs in physically active men.     

Static block jump techniques in volleyball: upright versus squat starting positions

The difference in maximal jump height between static block jump starting from an upright position (upright BJ) and static block jump starting from a squat position (squat BJ) was determined in 10 division II collegiate women volleyball players. Also determined was the difference in take off time for quick block jump, to a constant point above the net, between upright BJs and squat BJs. An AMTI force plate and a video camera (60 Hz) were used to collect the data. Each subject performed three maximal upright BJs and three maximal squat BJs, and five quick upright BJs and five quick squat BJs, randomly. The highest jump for maximal upright BJs and squat BJs, and the fastest jump for quick upright BJs and squat BJs were recorded. There was a significant difference (p < 0.03) between maximal upright BJs and squat BJs in height jump; maximal upright BJ (33.2 cm) was higher by 1.2 cm. No significant difference (p > 0.5) was found for the fastest take off time (approximately 0.7 s) between quick upright BJs and squat BJs. These results suggest that college women volleyball players can jump higher from the upright, compared with the squat, position. They can take off to the same block position equally quickly from either the upright or squat starting position. These data may suggest that conditioning coaches should identify their players' preferred BJ position and incorporate a specific training program to enhance the players' power. Furthermore, the coaches may need to incorporate more specific squat endurance exercises.     

Comparison of weighted jump squat training with and without eccentric braking

The purpose of this study was to investigate the effect of weighted jump squat training with and without eccentric braking. Twenty male subjects were divided into two groups (n = 10 per group), Non-Braking Group and Braking Group. The subjects were physically active, but not highly trained. The program for Non-Braking Group consisted of 6 sets of 6 repetitions of weighted jump squats without reduction of eccentric load for 8 weeks. The training program for the Braking Group consisted of the same sets and repetitions, but eccentric load was reduced by using an electromagnetic braking mechanism. Jump and reach, countermovement jump, static jump, drop jump, one repetition maximum half squat, weighted jump squat, and isometric/isokinetic knee extension/flexion at several different positions/angular velocities were tested pre- and posttraining intervention. The Non-Braking Group exhibited greater improvement in peak torque during isokinetic concentric knee flexion at 300 degrees/s [Non-Braking Group: (mean +/- SD) 124.0 +/- 22.6 Nm at pre- and 134.1 +/- 18.4 Nm at posttraining, and Braking Group: 118.5 +/- 32.7 Nm at pre- and 113.2 +/- 26.7 Nm at posttraining]. Braking Group exhibited superior adaptations in peak power relative to body mass during weighted jump squat [Non-Braking Group: (mean +/- SD) 49.1 +/- 8.6 W/kg at pre- and 50.9 +/- 6.2 W/kg at posttraining, and Braking Group: 47.9 +/- 6.9 W/kg at pre- and 53.7 +/- 7.3 W/kg at posttraining]. It appears that power output in relatively slow movement (weighted jump squat) was improved more in the Braking Group, however strength in high velocity movements (isokinetic knee flexion at 300 degrees/s) was improved more in Non-Braking Group. This study supports load and velocity specific effects of weighted jump squat training.     

The effects of carbohydrate loading on repetitive jump squat power performance

The beneficial role of carbohydrate (CHO) supplementation in endurance exercise is well documented. However, only few data are available on the effects of CHO loading on resistance exercise performance. Because of the repetitive use of high-threshold motor units, it was hypothesized that the power output (power-endurance) of multiple sets of jump squats would be enhanced following a high-CHO (6.5 g CHO kg body mass(-1)) diet compared to a moderate-CHO (4.4 g CHO kg body mass(-1)) diet. Eight healthy men (mean +/- SD: age 26.3 +/- 2.6 years; weight 73.0 +/- 6.3 kg; body fat 13.4 +/- 5.0%; height 178.2 +/- 6.1 cm) participated in 2 randomly assigned counterbalanced supplementation periods of 4 days after having their free-living habitual diet monitored. The resistance exercise test consisted of 4 sets of 12 repetitions of maximal-effort jump squats using a Plyometric Power System unit and a load of 30% of 1 repetition maximum (1RM). A 2-minute rest period was used between sets. Immediately before and after the exercise test, a blood sample was obtained to determine the serum glucose and blood lactate concentrations. No significant difference in power performance existed between the 2 diets. As expected, there was a significant (p 相似文献   

Comparison of four different methods to measure power output during the hang power clean and the weighted jump squat

Measurement of power output during resistance training is becoming ubiquitous in strength and conditioning programs, but there is great variation in the methods used. The main purposes of this study were to compare the power output values obtained from 4 different methods and to examine the relationships between these values. Male semiprofessional Australian rules football players (n = 30) performed hang power clean and weighted jump squat while ground reaction force (GRF)-time data and barbell displacement-time data were sampled simultaneously using a force platform and a linear position transducer attached to the barbell. Peak and mean power applied to the barbell was obtained from barbell displacement-time data (method 1). Peak and mean power applied to the system (barbell + lifter) was obtained from 3 other methods: (a) using GRF-time data (method 2), (b) using barbell displacement-time data (method 3), and (c) using both barbell displacement-time data and GRF-time data (method 4). The peak power values (W) obtained from methods 1, 2, 3, and 4 were (mean +/- SD) 1,644 +/- 295, 3,079 +/- 638, 3,821 +/- 917, and 4,017 +/- 833 in hang power clean and 1,184 +/- 115, 3,866 +/- 451, 3,567 +/- 494, and 4,427 +/- 557 in weighted jump squat. There were significant differences between power output values obtained from method 1 vs. methods 2, 3, and 4, as well as method 2 vs. methods 3 and 4. The power output applied to the barbell and that applied to the system was significantly correlated (r = 0.65-0.81). As a practical application, it is important to understand the characteristics of each method and consider how power output should be measured during the hang power clean and the weighted jump squat.     

Relationship between sprint times and the strength/power outputs of a machine squat jump

Strength testing is often used with team-sport athletes, but some measures of strength may have limited prognostic/diagnostic value in terms of the physical demands of the sport. The purpose of this study was to investigate relationships between sprint ability and the kinetic and kinematic outputs of a machine squat jump. Thirty elite level rugby union and league athletes with an extensive resistance-training background performed bilateral concentric-only machine squat jumps across loads of 20% to 90% 1 repetition maximum (1RM), and sprints over 10 meters and 30 or 40 meters. The magnitudes of the relationships were interpreted using Pearson correlation coefficients, which had uncertainty (90% confidence limits) of approximately +/-0.3. Correlations of 10-meter sprint time with kinetic and kinematic variables (force, velocity, power, and impulse) were generally positive and of moderate to strong magnitude (r = 0.32-0.53). The only negative correlations observed were for work, although the magnitude was small (r = -0.18 to -0.26). The correlations for 30- or 40-meter sprint times were similar to those for 10-meter times, although the correlation with work was positive and moderate (r = 0.35-0.40). Correlations of 10-meter time with kinetic variables expressed relative to body mass were generally positive and of trivial to small magnitude (r = 0.01-0.29), with the exceptions of work (r = -0.31 to -0.34), and impulse (r = -0.34 to -0.39). Similar correlations were observed for 30- and 40-meter times with kinetic measures expressed relative to body mass. Although correlations do not imply cause and effect, the preoccupation with maximizing power output in this particular resistance exercise to improve sprint ability appears problematic. Work and impulse are potentially important strength qualities to develop in the pursuit of improved sprinting performance.     

A simple method for measuring force, velocity and power output during squat jump

Our aim was to clarify the relationship between power output and the different mechanical parameters influencing it during squat jumps, and to further use this relationship in a new computation method to evaluate power output in field conditions. Based on fundamental laws of mechanics, computations were developed to express force, velocity and power generated during one squat jump. This computation method was validated on eleven physically active men performing two maximal squat jumps. During each trial, mean force, velocity and power were calculated during push-off from both force plate measurements and the proposed computations. Differences between the two methods were not significant and lower than 3% for force, velocity and power. The validity of the computation method was also highlighted by Bland and Altman analyses and linear regressions close to the identity line (P<0.001). The low coefficients of variation between two trials demonstrated the acceptable reliability of the proposed method. The proposed computations confirmed, from a biomechanical analysis, the positive relationship between power output, body mass and jump height, hitherto only shown by means of regression-based equations. Further, these computations pointed out that power also depends on push-off vertical distance. The accuracy and reliability of the proposed theoretical computations were in line with those observed when using laboratory ergometers such as force plates. Consequently, the proposed method, solely based on three simple parameters (body mass, jump height and push-off distance), allows to accurately evaluate force, velocity and power developed by lower limbs extensor muscles during squat jumps in field conditions.     

Analysis of biomechanical quantities during a squat jump: evaluation of a performance index

In this study, a procedure for evaluating the performance of an athlete in a squat jump has been developed. The athletes were divided into 2 categories according to their level of merit: elite athletes and non-elite athletes. In some of the subjects, the vertical component of acceleration during the squat jump was acquired with a uniaxial accelerometer. The acceleration-time curves obtained for each category of athletes were analyzed. Some analysis parameters suitable for establishing an athletic training level index were determined. A threshold value for this index that can be used to check gesture learning was also established, allowing the index to be used as a parameter for defining sport performance in a squat jump; thus it can also be used, during the training of an athlete, as the performance index to which reference should be made.     

Power-time, force-time, and velocity-time curve analysis during the jump squat: impact of load

The purpose of this investigation was to examine the impact of load on the power-, force- and velocity-time curves during the jump squat. The analysis of these curves for the entire movement at a sampling frequency of 200-500 Hz averaged across 18 untrained male subjects is the most novel aspect of this study. Jump squat performance was assessed in a randomized fashion across five different external loads: 0, 20, 40, 60, and 80 kg (equivalent to 0 +/- 0, 18 +/- 4, 37 +/- 8, 55 +/- 12, 74 +/- 15% of 1RM, respectively). The 0-kg loading condition (i.e., body mass only) was the load that maximized peak power output, displaying a significantly (p 相似文献   

Assessment of unloaded and loaded squat jump performance with a force platform: Which jump starting threshold provides more reliable outcomes?

This study aimed to explore the influence of different onset thresholds on the between-session reliability and magnitude of squat jump (SJ) performance. Twenty men were tested on two sessions separated by 48 h against external loads of 0.5, 30 and 60 kg. The initiation of the jump was defined as the first instant in which the vertical ground reaction force exceeded system weight by 10 N (10 N), 50 N (50 N), 1% of system weight (1%SW), 10% of system weight (10%SW) and five standard deviation of system weight minus 30 ms (5SDSW). The following variables were calculated from the force-time signal collected on a force platform: mean, peak and time to peak values of force, power and velocity, average rate of force development, peak rate of force development, rate of force development index, impulse, jump height, and push-off time. The 50 N, 10%SW and 5SDSW thresholds generally revealed a higher reliability than the 10 N and 1%SW thresholds (97 and 21 out of 252 comparisons for the coefficient of variation and intraclass correlation coefficient, respectively). The magnitude of most of the variables calculated using the 50 N and 10%SW thresholds significantly differed with respect to the 10 N, 1%SW and 5SDSW thresholds (P < 0.05). These results suggest that both the reliability and magnitude of SJ performance variables are influenced by the jump starting threshold. The 50 N, 10%SW and 5SDSW thresholds maximise the reliability of SJ performance variables, while the 5SDSW should be recommended since it considers more force signal than the 50 N and 10%SW thresholds.     

Comparison of loaded and unloaded jump squat training on strength/power performance in college football players

The purpose of this study was to explore the effects of 5 weeks of eccentrically loaded and unloaded jump squat training in experienced resistance-trained athletes during the strength/ power phase of a 15-week periodized off-season resistance training program. Forty-seven male college football players were randomly assigned to 1 of 3 groups. One group performed the jump squat exercise using both concentric and eccentric phases of contraction (CE; n = 15). A second group performed the jump squat exercise using the concentric phase only (n = 16), and a third group did not perform the jump squat exercise and served as control (CT; n = 16). No significant differences between the groups were seen in power, vertical jump height, 40-yd sprint speed and agility performance. In addition, no differences between the groups were seen in integrated electromyography activity during the jump squat exercise. Significant differences between the CE and CT groups were seen in Delta 1RM squat (65.8 and 27.5 kg, respectively) and Delta 1RM power clean (25.9 and 3.8 kg, respectively). No other between-group differences were observed. Results of this study provide evidence of the benefits of the jump squat exercise during a short-duration (5-week) training program for eliciting strength and power gains. In addition, the eccentric phase of this ballistic movement appears to have important implications for eliciting these strength gains in college football players during an off-season training program. Thus, coaches incorporating jump squats (using both concentric and eccentric phases of contraction) in the off-season training programs of their athletes can see significant performance improvements during a relatively short duration of training.     

A biomechanical comparison of the traditional squat, powerlifting squat, and box squat

The purpose of this study was to compare the biomechanics of the traditional squat with 2 popular exercise variations commonly referred to as the powerlifting squat and box squat. Twelve male powerlifters performed the exercises with 30, 50, and 70% of their measured 1 repetition maximum (1RM), with instruction to lift the loads as fast as possible. Inverse dynamics and spatial tracking of the external resistance were used to quantify biomechanical variables. A range of significant kinematic and kinetic differences (p < 0.05) emerged between the exercises. The traditional squat was performed with a narrow stance, whereas the powerlifting squat and box squat were performed with similar wide stances (48.3 ± 3.8, 89.6 ± 4.9, 92.1 ± 5.1 cm, respectively). During the eccentric phase of the traditional squat, the knee traveled past the toes resulting in anterior displacement of the system center of mass (COM). In contrast, during the powerlifting squat and box squat, a more vertical shin position was maintained, resulting in posterior displacements of the system COM. These differences in linear displacements had a significant effect (p < 0.05) on a number of peak joint moments, with the greatest effects measured at the spine and ankle. For both joints, the largest peak moment was produced during the traditional squat, followed by the powerlifting squat, then box squat. Significant differences (p < 0.05) were also noted at the hip joint where the largest moment in all 3 planes were produced during the powerlifting squat. Coaches and athletes should be aware of the biomechanical differences between the squatting variations and select according to the kinematic and kinetic profile that best match the training goals.