Increased muscular activity can result in decreased neuromuscular force generation (19). However, it can also enhance subsequent force generation and improve strength and power performance (3,8,11,16,21,23), a phenomenon termed postactivation potentiation (PAP). Postactivation potentiation has been attributed to 3 possible mechanisms: regulatory light chain phosphorylation, increased recruitment of motor units, and muscle fiber pennation angle change (for review, see Tillin and Bishop (22)). In the first mechanism, Ca2+ release from the sarcoplasmic reticulum increases and so does the sensitivity of the actin-myosin interaction, which alters the structure of the myosin head and results in a higher force generation state of the cross-bridges (19). For the second mechanism, motor unit recruitment can be increased by increased excitation potential because of previous muscular contractions. This excitation, which can last for several minutes, results in enhanced force generation (12). Finally, previous muscular contractions can reduce the muscle fiber pennation angle, allowing more faithful force transmission in subsequent contractions (14).
Numerous studies have used the back squat as a stimulus to induce PAP and assess its effects on strength and power performance (3,8,16,21,23) because of its wide use by practitioners and researchers. However, different variations of the exercise have been used depending on the depth of the squat. For example, Esformes et al. (8) compared half squats with plyometric exercises as a stimulus for inducing PAP. Kilduff et al. (16) used full squats to examine optimal recovery time after the stimulus. Furthermore, Smilios et al. (21) used both half and jump squats to examine the effect of load on PAP. Finally, Witmer et al. (23) used parallel squats (PSs) to examine their effect on subsequent jump performance.
The use of strength-power potentiating complex pairs for enhancing power performance often entails the use of a heavy load squat exercise followed by a jumping ballistic activity (8,15). However, in addition to the load (11,21), the variation in the depth to which the squat is performed can affect the performance outcome (17). For example, Caterisano et al. (1) reported higher gluteus maximum activation with increasing squat depth. However, this muscle also plays an important role in countermovement jump (CMJ) performance (10). Therefore, the various squats that have been used in research and are used in practice to allow for higher sport specificity and loading patterns could have an effect on subsequent jumping performance. It is plausible that higher activation of this muscle could induce higher PAP, resulting in better power performance during subsequent jumping performance. In addition, the deeper squat would also require more work to be performed, potentially resulting in greater muscle excitation. Therefore, the aim of the current study was to compare the effect of prior execution of PS or quarter squat (QS) on subsequent CMJ performance variables.
Experimental Approach to the Problem
The aim of the present study was to compare the effect of PS or QS, routinely used in training practice, as a PAP stimulus on subsequent CMJ performance. Twenty-seven, semiprofessional, male rugby union players performed a CMJ followed by a 10-minute rest. Subsequently, they performed 3 repetitions of a PS or QS at each squat’s respective 3-repetition maximum (3RM) load. Subjects then rested for 5 minutes and performed another CMJ. To avoid any order bias, a counterbalanced randomized order design was employed. Pairwise comparisons between baseline and postconditioning contraction stimulus values were carried out on all CMJ performance variables (jump height [JH], impulse [I], peak power [PP], and flight time [FT]) and delta values, to provide an indication of any PAP effects.
Twenty-seven, semiprofessional, male rugby union players (mean ± SD, age, 18 ± 2 years; body mass, 87.2 ± 5.4 kg; height, 180.7 ± 5.1 cm) agreed to participate in the study. The subjects were in the competition phase of their annual training cycle. Their sport training program included a minimum of 3 sessions of resistance training per week, with training loads ranging from 40 to 90% of 1RM. All subjects had experience of resistance training before the study and were free from any upper-body injuries at the time of the study for at least 1 year. Subjects were asked to refrain from eating 2 hours before examination and from drinking coffee and alcohol 24 hours before each visit to the laboratory. Subjects were allowed to consume water ad libitum before and during the exercise task. Approval from Cardiff Metropolitan University’s Ethics Committee was granted, and written informed consent was obtained from all subjects.
Subjects initially visited the laboratory to be familiarized with the experimental protocol, and the subjects’ weight and height were measured. Height was measured to the nearest 0.1 cm using a stadiometer (Harpenden, Holtain Ltd, Crosswell, UK), and weight was measured to the nearest 0.1 kg using a calibrated balance beam scale (Seca, Birmingham, UK). Subsequently, each subject’s 3RM for the PS and the QS were determined according to the guidelines set by the National Strength and Conditioning Association (13). Three repetition maximum was defined as the load that caused failure on the third repetition but without loss of proper exercise technique. To establish the 3RM load, subjects attempted 3 repetitions of a load and, if successful, increased the loading. A 5-minute rest interval was allowed between trials, with 3–5 trials typically required for determining each subject’s 3RM. Parallel squat was deemed to be successful if the subject could descend until the inguinal fold was lower than the patella (60–70° knee joint angle; full extension = 180°), whereas QS when the subjects’ knees flexed to approximately 135° knee joint angle. Subjects had to rise without help for both squat types.
After the first visit, subjects returned to the laboratory on 2 separate occasions for the experimental sessions. At the start of each experimental session, the subjects were required to complete a standardized warm-up of 5 minutes of light-intensity cycling and a number of dynamic stretches specific to muscles involved in the relevant exercises. A 5-minute rest interval was allowed after the end of the warm-up.
The subjects performed a CMJ that served as baseline (BL-CMJ). Subjects kept their arms on their waist during the jump to isolate the leg contribution and the depth was self-selected. After the CMJ, a 10-minute rest was allowed, followed by one of the conditioning contractions. The conditioning contractions were 1 set of 3 repetitions of either PS or QS at 3RM. Each conditioning contraction was applied in a counterbalanced randomized order on separate days. All exercises were executed using a squat rack, a weightlifting bar, and free weight plates. Experienced spotters were present at all times to ensure safety of subjects and appropriate exercise technique execution. In addition, the spotters visually inspected and confirmed the depth of the squat and provided verbal feedback when needed. Finally, after a 5-minute rest, the subjects performed another CMJ (POST-CMJ). Performance variables assessed during the CMJ included JH, I, PP, and FT using a jump mat (Smartjump; Fusion Sport, Brisbane, Australia).
Testing took place on the same time of day for each subject and with a minimum of 24 hours intervening between testing sessions. Subjects refrained from any strenuous activities or resistance/plyometric training at least 48 hours before each testing session.
Normality of all raw data and delta values was examined and confirmed using the Kolmogorov-Smirnov’s test. Pairwise comparisons were made between presquat and postsquat CMJ values for all variables for each squat type. Furthermore, pairwise comparisons between squats were made for the BL-POST delta values for all variables. Finally, effect sizes (ESs) (4) were calculated for all comparisons. All data are presented as mean ± SD, unless otherwise stated. Significance was set at p ≤ 0.05, and all statistical analyses were conducted using SPSS v19.0.
Subjects’ 3RM load for PS and QS were 183.3 ± 17.3 and 200.7 ± 17.3 kg, respectively. All POST-CMJ variables improved significantly compared with BL-CMJ for both PS and QS (p < 0.05; Table 1). In addition, delta values for PS were significantly higher than QS for all variables examined (p < 0.05; Table 1).
Effect size ranged from 0.53 to 1.23 for the BL- to POST-CMJ comparisons (Table 2), indicating moderate to large effects (4). For the delta values, the effect was moderate as ES ranged from 0.38 to 0.67 (Table 2).
The aim of the present study was to examine if subsequent CMJ performance could be enhanced by variations of the back squat as a prior conditioning stimulus and whether a particular squat depth produced higher benefits. Our results show that both QS and PS increase CMJ performance variables and that PS achieves increased performance compared with QS.
The squat exercise has received substantial attention in the literature, either as the subject of research (6,7,9) or as the intervention used to induce PAP (3,8,16,21,23), because of its widespread use in the field and benefits it can offer to training (2,24). However, the high mechanical demands it poses to the body and subsequent likelihood of injury (2,9) and the specific flexibility requirements to allow for correct execution (18) has lead to practitioners using different variations of the back squat exercise, primarily by limiting the range of movement (2,6,24). This decreased range of motion allows the athlete to lift a higher load compared with full of PS, because of the higher mechanical advantage in the knee joint, and as a result overload the musculature. Indeed, analysis of our results showed that the QS 3RM was 9.5% (p < 0.001) higher than the PS 3RM load.
On the other hand, PSs have been shown to be more effective in engaging the gluteus maximus muscle (1), an important muscle in all hip extension movements (10). Training using reduced range of motion can reduce joint mobility (20), whereas the heavier load allowed by the QS presents a larger risk of back injury and the possibility of changing the power characteristics of the muscle (24).
It has previously been suggested that PAP affects primarily the rate of force development (RFD) (22). The current findings appear to support this notion. Although in the present study the depth of the downward phase of the CMJ was self-selected, and we are therefore unable to know whether the same depth was used for both presquat and postsquat CMJ, it is unlikely that the subjects would have changed the depth they used between the 2 jumps. Based on this hypothesis, the increased JH and FT are a result of the increased I, which is the main predictor of JH (17) and, consequently, of FT. The increase in I must have been produced by the muscles’ ability to generate higher force over the same period, suggesting an improvement in RFD after the conditioning contraction. Finally, the concurrent increase in power also points toward an increased RFD.
The increased CMJ performance after PS compared with QS found in our study can be attributed to the difference in range of movement between the 2 squat types and the impact it has on work produced by the gluteus maximus and the rest of the relevant muscles. The deeper squat position in PS increases the hip joint angle (5), resulting in a more stretched gluteus maximus, an important muscle in hip extension (10). This stretch increases the work required to produce the necessary torque to extend the hip. Furthermore, the mechanical disadvantage of the knee joint during the amortization phase of the PS dictates that more force is required to overcome inertia, further increasing the work performed. The additional work would require higher recruitment of motor units, therefore increasing the excitation potential of the muscle and augmenting its performance during the CMJ.
Both QS and PS are widely used in the training setting. Notwithstanding the reasons for selection of one over the other, the present study demonstrated that both squat types can provide sufficient stimulus to induce PAP. However, the PS produced superior power performance compared with QS. Therefore, practitioners can use the PS as a PAP conditioning contraction stimulus for acutely enhancing subsequent power performance, while allowing greater range of motion in the hips, knees, and ankles with less compressive loads in the lower back.
The authors would like to thank Ms. Annicka Jones for her help with the data collection.
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