Typically, a stretch-shortening cycle (countermovement jump vs. static jump) results in higher peak velocity, peak force, and jump height (10). During the squat, the stretch-shortening cycle can be removed by having the subject stop and sit on a box at the end of the eccentric phase (i.e., box squat). Various investigations have examined squats in comparison to Smith machine squats and front squats (6,12). In addition, investigations have examined the effect of stance width and the effect of stability and instability on muscle activity during the squat (9,11). One investigation examined the squat with and without a stretch-shortening cycle by performing a squat with an eccentric and concentric phase vs. a squat with a concentric phase only (13). However, no investigation has examined removing the stretch-shortening cycle from a squat by using a box on kinetic variables and muscle activity.
The stretch-shortening cycle has studied extensively during jumping (1,4,7). Investigations have shown that a stretch-shortening cycle enhances concentric performance during jumping (2,5). Countermovement jumps occur in a rapid fashion, which results in the storage and release of elastic energy and activation of the stretch reflex (4,5,8). Heavy resistance squatting follows a similar pattern to a countermovement jump; however, the time frame for completion of the movement is much longer. This could result in a loss of the potential benefits of a stretch-shortening cycle by extending the time of the amortization phase. One investigation examined an isokinetic squat with an eccentric and concentric phase to a concentric-only squat at a velocity of 0.4 m·s−1 (13). This velocity is significantly slower than a countermovement jump, which typically involves velocities of 3-4 m·s−1 (10). Walshe et al. (13) reported that a stretch-shortening cycle squat resulted in a significantly higher level of mechanical work and power in comparison to a concentric-only squat.
The current investigation attempted to determine the possible benefits of a stretch-shortening cycle by comparing a squat and box squat. Based on previous findings (13), it was hypothesized that removing the stretch-shortening cycle from the squat (i.e., box squat) would have a negative effect on kinetic variables and muscle activity. However, the hypothesis was tentative given the difference in velocity between a countermovement and static jump comparison in contrast to a heavy resistance squat with a significantly lower velocity.
Experimental Approach to the Problem
To investigate the effect of removing a stretch-shortening cycle from a squat (i.e., box squat), a comparison was performed between a standard squat movement pattern and a box squat. The box squat involved an eccentric phase followed by sitting on a box and then a concentric phase. Trials where performed at 60, 70, and 80% of each subject's 1 repetition maximum (1RM). Peak force and power during the concentric phase were measured along with muscle activity of the vastus lateralis (VL), vastus medialis (VM), biceps femoris (BF), and longissimus (L1). It was hypothesized that a box squat would have a negative effect on these variables.
Eight competitive male powerlifters with experience of at least 3 years in squatting participated in this investigation (Height: 179.61 ± 13.43 cm; Body Mass: 107.65 ± 29.79 kg; Age: 24.77 ± 3.22 years; and 1RM: 200.11 ± 58.91 kg). Written informed consent was obtained from all participants. Prior approval was given by the Institutional Review Board at Appalachian State University.
Subjects visited the laboratory on 2 occasions separated by at least 1 week. On the first day, subjects performed a 1RM in the squat. On the second day, the subjects performed 3 sets of 1 repetition in the squat and box squat using 60, 70, and 80% of their 1RM in a randomized fashion. The squat was performed with a descent and ascent with a quick transition between the 2 phases. The box squat was performed with a descent and then a 1-second pause while sitting on a box and then the ascent. Kinetic variables and muscle activity were measured for each trial using the best trial for comparison.
Squat One Repetition Maximum Testing
Previous squat 1RM mechanisms were used to determine warm-up loads. The warm-up protocol consisted of 10 repetitions at 50% of previous squat 1RM load, 2-4 repetitions at 70% 1RM, and 1 repetition at 90%. Subjects then completed up to 4 attempts to achieve their 1RM. All squats were performed to a 70° knee angle as measured by a goniometer. The 1RM was only performed for the squat and not the box squat. Three minutes of rest was allowed between each set of squats.
All subjects performed all trials on a force plate (FP) (AMTI, BP6001200, Watertown, MA, USA) with the barbell attached to 2 linear position transducers (LPTs) (Celesco Transducer Products PT5A-150, Chatsworth, CA, USA). As described previously (3), the 2 LPTs allowed for measurement of horizontal movement affecting vertical displacement. Through the use of trigonometry, vertical displacement was determined and combined with time to calculate vertical velocity, which was then coupled with the vertical force data to calculate power. Previous data indicate that a combination of kinetic (FP) and kinematic (LPT) equipment must be used to obtain the most valid representation of peak power generation during dynamic movements (3). Data were collected at 1,000 Hz using a BNC-2010 interface box with an analog-to-digital card (National Instruments, NI PCI-6014, Austin, TX, USA). All data were recorded and analyzed using customized software (LabVIEW, National Instruments, Version 7.1). Peak force and peak power were determined for each trial with the best value for each condition used for analysis. Reliability data for the measurement are reported previously (5).
Electromyography (EMG) was used to obtain data from the VL, VM, BF, and L1 muscles. The skin was shaved, abraded, and cleansed with alcohol before placing a disposable bipolar surface electrode (Noraxon USA Inc., Scottsdale, AZ, USA; 2-cm interelectrode distance, 1 cm2 circular conductive area) over the muscle. The myoelectric signal was transmitted through the use of a telemetry transmitter (8-channel, 12-bit analog-to-digital converter, Noraxon USA Inc.). The amplified myoelectric signal, which was recorded during the exercises, was detected by the receiver-amplifier (Telemyo 900, gain = 2,000, differential input impedance = 10 MV, bandwidth frequency 10-500 Hz, common mode rejection ratio = 85 dB, Noraxon) and then sampled by an A/D card (National Instruments, NI PCI-6014 ) at 1,000 Hz. The signal was full wave rectified and filtered (6-pole Butterworth, notch filter 60 Hz, band pass filter 10-200 Hz). The integrated value (mV·s) was calculated and then averaged over the time of the eccentric or concentric phase to determine average integrated EMG (mV).(9) A custom designed program created in LabVIEW (National Instruments, Version 8.2) was used for recording and analyzing the data.
A general linear model multivariate analysis with Bonferonni post hoc tests was performed to test the significance of the difference between variables (peak force, peak power, VL, VM, BF, L1). An estimate of effect size η2 = 0.981, 0.948 at an observed power level of 1.0 for peak power and peak force, respectively. Statistical significance for all analyses was defined by p ≤ 0.05. All statistical analyses were performed through the use of a statistical software package (SPSS, Version 15.0, SPSS Inc., Chicago, IL, USA).
Peak force and peak power during the 60% of 1RM trials were not significantly different between the squat and box squat. Peak force was significantly higher in the box squat in comparison to the squat during the 70% of 1RM trials. Peak power was significantly higher in the box squat in comparison to the squat during the 80% of 1RM trials (Figures 1-3).
The BF muscle activity was significantly higher in the squat in comparison to the box squat during the 60% of 1RM trials. The VL muscle activity was significantly higher during the squat in comparison to the box squat during the 70% of 1RM trials. No significant differences in muscle activity were observed between the squat and box squat during the 80% of 1RM trials (Figures 4-6).
The primary findings in this investigation indicate that kinetic variables and muscle activity are similar between a squat and box squat. This is surprising given the reported contribution of the stretch-shortening cycle to concentric performance when comparing a static jump and countermovement jump (10). Walshe et al. (13) also reported increased work and power when comparing a eccentric-concentric squat to a concentric-only squat. It should be noted, however, that an eccentric phase was present in the box squat in this study although the amortization phase was lengthened considerably by having the subject sit on a box and then proceed to the concentric phase. It is possible that the benefit of an eccentric phase during the box squat was still capable of contributing to increased concentric performance; however, because of the extended amortization phase in both the squat and box squat, it is possible that neither lift effectively uses the stretch-shortening cycle.
As mentioned previously, the stretch-shortening cycle has been studied extensively during jumping (1,4,7), and investigations have shown that a stretch-shortening cycle enhances concentric performance during jumping (2,5). Walshe et al. (13) reported significant increases in concentric performance in a squat with a stretch-shortening cycle. This investigation examined an isokinetic squat at a velocity of 0.4 m·s−1 (13). The squat and box squat in this investigation occurred at a similar concentric velocity of 0.6-0.8 m·s−1. Thus, an increase in performance in the squat in comparison to the box squat was expected. This study, however, was different from the study performed by Walshe et al. (13) in that the box squat did involve an eccentric phase but a longer amortization phase in comparison to the squat. The allowable length of the amortization phase for an effective increase in concentric performance during a stretch-shortening cycle may be between 50 and 100 milliseconds (5). The amortization phase during the squat and box squat in this study fell outside this range given the load, which increased the transition time between the eccentric and concentric phases. However, another investigation involving a squat at a velocity of 0.4 m·s−1 did show enhanced concentric performance with a stretch-shortening cycle (13).
In conclusion, it appears that a box squat has neither a significant positive nor negative effect on squat performance. Peak force at 70% of 1RM and peak power at 80% of 1RM were significantly higher in the box squat, but muscle activity was significantly higher in the squat in various muscles at 60, 70, and 80% of 1RM. However, the mean values for all variables where very similar between the squat and box squat. Thus, in practical terms, very little difference existed between the 2 lifts.
The box squat is sometimes used in training as a substitute for the squat. This study found little difference in kinetic variables or muscle activity between the 2 lifts. Thus, the use of a squat or box squat provides a very similar stimulus to the leg and lower back musculature and therefore would most likely result in a similar amount of strength gain with training. The removal of the stretch-shortening cycle by using the box squat may be beneficial as a training tool for those athletes performing concentric-only actions in their sport. For most sport applications, a stretch-shortening cycle is a vital component, and thus, a squat including a stretch-shortening cycle may be of greater benefit.
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