Postactivation potentiation (PAP) has been defined as a temporary enhancement of muscle force and/or power output after a voluntary muscle action performed typically at maximal or near maximal intensity (37). This muscle action is known as the conditioning activity. The proposed physiological mechanisms involved in PAP are the phosphorylation of regulatory myosin light chains, an increased recruitment of high-order motor units and likely changes in muscle fascicle angle (37). However, the conditioning activity may also induce fatigue. Therefore, during the recovery period after the conditioning activity, the balance between fatigue and PAP determines whether the muscle performance is enhanced, reduced, or remains unaltered (30). Factors affecting the balance between fatigue and PAP include the characteristics of the conditioning activity (volume, intensity, type of contraction), the length of recovery time between the conditioning activity and muscle performance, and the subjects' characteristics (muscular strength, training level, and fatigability) (8,18,31,32,37,39). Interestingly, muscle fiber type may influence PAP and fatigue in opposite directions. Hamada et al. (19) showed that subjects with predominantly fast twitch muscle fibers (71.8% type II) elicited a greater PAP response compared to subjects with predominantly slow twitch muscle fibers (61.4% type I). However, they also exhibited greater fatigue during a series of maximal voluntary contractions (19). These findings would suggest that subjects with a high percentage of fast twitch fibers may exhibit greater PAP, but also be susceptible to greater fatigue during conditioning muscle actions (37).
Several studies have examined the effect of heavy dynamic squat exercise (3–5 repetitions at a load of 90–93% of 1 repetition maximum) on explosive performance, such as sprinting and vertical jumping, with conflicting results (9–11,20,25,40). For example, Mc Bride et al. (25) found a small improvement of 0.87% in 40-m sprint time after a 3RM parallel squat, but no effect on 10 and 30 m performance. On the other hand, Chiu et al. (9) found no effect of 5 single squats performed at 90% of 1RM on loaded jump performance, but reported a possible influence of fatigue and training status. The reasons for those discrepancies may be related to the total volume lifted during the conditioning squat exercise, the resting period between muscle actions, and the subjects' training background (9,25).
There is evidence to suggest that isometric exercise may be superior to concentric or eccentric dynamic muscle actions to elicit PAP (4,13,31). A recent study comparing the influence of contraction type on PAP has shown that 3 sets of 3 seconds of maximal isometric half squat were the most effective in increasing jumping performance compared with concentric and eccentric squats, when the impulse of the ground reaction force of the conditioning exercise was equated (4). However, the effects of the depth of the isometric squat on subsequent jumping performance have not been examined and may be important because of the possible influence of the length of the leg extensor muscles on PAP and fatigue. Previous studies have shown that potentiation and fatigue can coexist in skeletal muscle and their interplay determines the degree and timing of performance increase after a conditioning muscle action (30).
The limited data on the effects of muscle length during conditioning exercise on subsequent muscle performance suggest that the magnitude of both potentiation and fatigue may depend on joint angle and thus muscle length (22,26,29,34). Lee et al. (22) observed lower fatigue during isometric contractions induced by electrical stimulation of the quadriceps, when the knee joint was set at 165° (shorter muscle length) compared with 90° (longer muscle length) and this was attributed to metabolic factors owing to differences in cross-bridge interactions (14). In addition to the lower fatigue at the shorter muscle length, there is evidence showing that PAP is also enhanced when the conditioning contractions are performed with the muscle at shorter length (26,28,34). Data from 2 studies using electrical stimulation of the quadriceps (28) and triceps brachii muscle (34) reported greater twitch potentiation at shorter muscle lengths compared with longer muscle lengths after submaximal muscle actions. Similarly, Miyamoto et al. (26) reported greater PAP when a 10-second maximal voluntary isometric conditioning contraction was performed with the ankle joint in plantar flexion (short muscle length) compared with dorsiflexion (long muscle length).
Taken collectively, the findings of the above studies (22,26,28,34) show that at short muscle length fatigue may be lower and PAP may be greater compared with longer muscle length. Thus, the purpose of the present study was to investigate the effects of maximum isometric squat exercise at 2 different knee angles on countermovement jump (CMJ) performance in well-trained power athletes. It was hypothesized that maximum isometric squat exercise with a knee angle set at 140° (i.e., shorter muscle length) would cause less fatigue and greater PAP compared with a squat with a knee angle of 90° (i.e., longer muscle length). Furthermore, taking into account that both PAP and fatigue are influenced by muscle fiber composition (26,37), together with the fact that there is a high correlation between the proportion of fast twitch fibers and vertical jump performance (5,17), it was hypothesized that individuals with higher CMJ performance would exhibit a greater PAP response.
Experimental Approach to the Problem
A randomized and counterbalanced repeated measures design was used to examine the effects of maximum isometric squat exercise performed at 2 different knee angles on subsequent vertical jump performance in national level power trained athletes. After 2 familiarization sessions with isometric squats at different knee angles and 2 preliminary measurements, subjects performed 3 main trials 1 week apart. The main trials involved 3 sets of 3 seconds maximum voluntary isometric contractions (MVIC) with 1-minute rest between each set, from a squat position, with knee angle set at 90 or 140°, as well as a control condition. Countermovement vertical jump (CMJ) performance was evaluated before and 15 seconds, 3, 6, 9 and 12 minutes after each main trial (Figure 1). To determine the possible effects of baseline jump performance on PAP, the median-split technique (7) was used to divide the subjects into 2 groups (“high jumpers” and “low jumpers”) depending on whether their baseline CMJ performance was above or below the median CMJ value.
Fourteen national level male track and field power trained athletes (jumpers and decathletes) volunteered to participate in the study (age: 27.1 ± 7.0 years, height: 179 ± 7 cm, body mass: 78.3 ± 7.3 kg, body fat: 10.2 ± 5.0%). Subjects had training experience of 9.4 ± 5.8 years, which included at least 6 years of resistance training, and took part in 6–8 training sessions per week. All athletes had no musculoskeletal injuries for at least 1 year before the study. None of the athletes were taking any nutritional supplements or drugs during the study. Written informed consent was obtained from each participant after a thorough explanation of the testing protocol, the possible risks involved, and the right to terminate participation at will. The study was approved by the local Institutional Review Board, and all procedures were in accordance with the Helsinki Declaration of 1975, as revised in 1996.
Before each preliminary and experimental test, subjects performed a standardized warm-up, which consisted of 5 minutes of light jogging on a treadmill (∼60% of predicted maximal heart rate) and 5 minutes of dynamic stretching (4,38). Before each preliminary measurement, subjects were instructed to have a light training session in the previous day; whereas for the 3 main trials, subjects abstained from training for 24 hours before the test (16). Subjects were also instructed to replicate their dietary intake 24 hours before each main trial.
Force Measurement During Isometric Squats
Isometric squat strength testing was performed in a power rack bolted on the floor. Subjects had the bar on their shoulders and the depth of the squat was determined by the holes spacing on the side bars of the frame, where the adjustable bar catchers (rods) were placed. To ensure that muscle action was isometric, the bar was immobilized using straps tied on the frame side rods. Additionally, the bar was fully loaded with 25 kg weight plates and was further stabilized by 2 assistants. An experienced weightlifting coach ensured that body position was standardized, so that the torso was upright in both conditions. Because of the fact that no external work is done during an isometric action, the intensity and volume of the conditioning activity was quantified by measuring the time history of the ground reaction force applied on the subjects' feet. The vertical component of the ground reaction force was measured using a force plate placed on the floor inside the power rack under the feet of the subjects (Applied measurements Ltd., Reading, United Kingdom). Sampling frequency was set at 1,000 Hz and instantaneous force data were low-pass filtered (fourth order reverse Butterworth low pass digital filter) with a cut-off frequency of 20 Hz. Peak and average force and total duration of the isometric muscle action and total impulse (force-time integral, obtained by numerical integration using the trapezoidal rule) were then calculated using custom-written routines (LabVIEW version 8.0, National Instruments, Austin, TX, USA).
Measurement of CMJ Performance
Countermovement vertical jump performance was assessed according to the protocol of Bosco et al. (6) by measuring flight time from the force plate data. The participants were asked to keep their hands on their hips (akimbo) throughout the entire jump, to bend the knees up to 90°, and to take off and land maintaining the same body position. Three CMJs separated by 30 seconds of rest were performed at baseline, whereas 1 CMJ was performed at each recovery time point. Subjects remained seated between CMJ efforts to reduce fatigue. The intraclass correlation coefficient (ICC) for the CMJ assessment was 0.98 (p < 0.01).
Familiarization and Preliminary Measurements
In the first 2 visits, subjects were familiarized with maximal isometric squat exercise at the 2 different knee angles (90–140°) in a squat rack. Subjects were also familiarized with countermovement jump (CMJ). Subjects were supervised and instructed for the correct lifting technique by an experienced strength and conditioning coach.
In the first preliminary visit, peak isometric force at 2 different knee angles was measured. These maximal isometric force values served as baseline measurements to ensure maximum effort by the subjects in the main PAP protocols. The order of force measurements at 2 different knee angles was randomized and counterbalanced. Each subject had a 3 seconds trial at the 2 angles and then repeated the isometric squats with the same order. The highest force at each angle was recorded. The rest interval between trials was 1 minute. The ICC for maximum isometric ground reaction force was determined for 2 knee angles in a separate session and was 0.93 and 0.97 (p < 0.01) for the knee angles of 140 and 90°, respectively. Knee angle, i.e., the angle between the thigh and shank, was measured using a digital camera (Casio Exilim Pro EX-F1, CasioComputer Co., Ltd., Tokyo, Japan) at 30 frames per second and the Kinovea video analysis software (v 0.8.15). Markers were placed on the lateral malleolus, femoral epicondyle, and greater trochanter of the right lower limb to enable calculation of the knee angle (full extension = 180°). The ICC for knee angle measurements was 0.96 and 0.98 (p < 0.01) for the knee angles of 140 and 90°, respectively.
Evaluation of Fatigue During Maximal Isometric Contractions at the Two Experimental Knee Angles
In the second preliminary session, a 15-second isometric fatigue test was performed. After a standardized warm-up (4,38), each subject performed a maximal voluntary isometric squat for 15 seconds at 90 or 140° angle in a random and counterbalanced order against the immovable barbell with 30 minutes of recovery between efforts. Body position was standardized, and force and knee angle were recorded as described above. Fatigue was quantified by calculating the percent of force decrease from the peak value to the end of the fatigue test.
In the main trials, 1 minute after the end of the standardized warm-up, subjects performed 3 CMJ, and the best was kept as the baseline value. After the baseline CMJ evaluation, subjects rested for 2 minutes and then performed a specific isometric warm-up, which included 3 sets of 3-second isometric contractions from a squat position with knee angle of 90 or 140° and 1 minute of recovery between sets. The intensity was at 50, 75, and 90% of each subjects' maximum voluntary isometric vertical ground reaction force that was determined during the preliminary measurements. The participants had a visual feedback during the specific isometric warm-up from a screen that was placed in front of them, so that they would not exceed the predetermined submaximal values. Subjects then rested for 3 minutes and performed 3 sets of 3-second maximal isometric voluntary contractions with knee angle set at 90 or 140° and a 1-minute rest between each set. After the experimental isometric conditioning activity, participants performed maximal CMJs at the following recovery time points: 15 seconds, 3, 6, 9, and 12 minutes. In the control trial, participants started the CMJ evaluation at all the above time points, 5 minutes after the standardized warm-up.
All data are expressed as means and SDs. Statistical analyses were carried out using the Statistica v.8.0 software (StatSoft Inc., Tulsa, OK, USA). The effects of maximum voluntary isometric contractions from a squat position with 2 different knee angles (90 and 140°), on the subsequent CMJ performance at different time points, were determined by a 2-way repeated measures ANOVA (condition-including control-x recovery time). To compare CMJ performance of the 2 groups (high and low jumpers) in the 3 main trials, a 3-way mixed between-within group ANOVA (group × condition × time) was used. When a significant main effect or interaction was observed (p ≤ 0.05), a Tukey's post hoc test was performed. Partial eta squared (η2) values were calculated to estimate the effect size for main effects and interaction. Effect sizes for partial eta squared were classified as small (0.01–0.059), moderate (0.06–0.137), and large (>0.137). Pairwise comparisons were made using paired t-test. Cohen's d was also calculated to obtain the effect size for pairwise comparisons. Effect sizes for Cohen's d were classified as small (0.2), medium (0.5), and large (0.8). The ICC was calculated using a 2-way mixed model to determine test–retest reliability for all dependent variables. Statistical significance was accepted at p ≤ 0.05.
During the main trials peak force (p < 0.001, d = −3.68), total impulse (p < 0.001, d = −2.42) and average force (p < 0.001, d = −3.23) were significantly lower when knee angle was 90° compared with 140° (Table 1). In accordance with the experimental design, total duration of the isometric contractions was equal between conditions. Muscle fatigue, determined at the second preliminary visit as the percent drop of force during the maximal isometric fatigue test, was significantly higher when knee angle was 90° compared with 140° (19.2 ± 8.0 vs. 0.3 ± 5.7%, p < 0.01, d = 2.82).
The time course of changes in CMJ performance during the main trials is shown in Figure 2. The baseline CMJ performance was similar in all 3 main trials (41.2 ± 5.5, 41.9 ± 4.5 and 41.5 ± 5.1 cm for the control, 90 and 140° conditions, respectively, p = 0.91). The 2-way ANOVA revealed a significant main effect for condition (p = 0.003, η2 = 0.36), and a condition × time interaction (p = 0.001, η2 = 0.21). The post hoc test for the interaction showed that CMJ performance was improved compared with baseline only in the 140° condition by 3.8 ± 1.2% on the 12th minute of recovery (p = 0.027), whereas there was no change in CMJ height in the 90° condition (Figure 2). In the control condition, there was a decrease in CMJ performance over time, reaching −3.6 ± 1.2% (p = 0.049) after 12 minutes of recovery (Figure 2). Furthermore, the improvement in CMJ performance in the 140° condition was greater at the 6, 9, and 12 minute time-points compared with the control condition and at the 9 and 12 minute time-points compared with the 90° condition (Figure 2).
To determine the possible effects of baseline jump performance on PAP, subjects were divided into 2 groups (“high jumpers” and “low jumpers”) based on the median of the baseline CMJ performance (42.6 cm) using the median-split technique. The baseline CMJ values of “high jumpers” and “low jumpers” differed significantly (CMJ: 45.1 ± 2.2 vs. 37.1 ± 3.9 cm, respectively, p = 0.001). The 3-way ANOVA using the best CMJ performance during recovery for each subject irrespective of time revealed a significant 3-way interaction effect (group × condition × time; p = 0.017, η2 = 0.28), indicating that the 2 groups responded differently across the 3 main trials. The Tukey's post hoc tests showed that CMJ performance was increased only in the “high jumpers” group by 5.4 ± 1.4% (p = 0.001, d = 0.99) and 7.2 ± 1.3% (p = 0.001, d = 1.60) at the knee angles of 90 and 140°, respectively (Figure 3). This improvement was larger at the 140° angle (p = 0.049, d = 0.62). In contrast, athletes in the “low jumpers” group did not show any improvement in performance at both knee angles.
The main finding of the present study was that the depth of isometric squats is an important determinant of the acute improvement in vertical jump performance. Isometric squats performed with a knee angle of 140° resulted in a delayed increase in vertical jump height, whereas there was no change in CMJ performance in the 90° condition. Another important finding was that the jumpers with the higher baseline vertical jump performance demonstrated an increased jump performance at both knee angles after the conditioning activities, whereas the jumpers with lower performances did not show a PAP response.
The finding that only isometric “quarter squats” (knee angle 140°) resulted in an increase in CMJ performance may be explained by possible differences in muscle length between the 2 conditions (knee angle 90 and 140°). When knee angle is 90°, the main leg extensor muscles operate at a longer length compared with the condition when knee angle is wider (22,29). The effects of knee angle on torque, muscle activation, and fatigue have been previously examined during monoarticular isometric knee extension (1,3,13). According to the knee angle–torque relationship during isometric knee extension, peak torque is almost twice as high when knee angle is 90° compared with 140–150° (3,21). Studies examining muscle fatigue during isometric knee extension have clearly shown that fatigue is much greater at a knee angle of 90° (longer muscle length) compared with 165° (shorter muscle length), and this has been attributed to metabolic factors because of differences in cross-bridge interactions (14,35). Similarly, another study using electrical stimulation with 5-second tetanic isometric contractions at long muscle length compared with a shorter muscle length (knee angle: 90 vs. 150°) demonstrated a steeper torque decline and thus greater fatigue when knee angle was 90 (29). This knee angle-specific fatigue may also explain the findings of the present study, where the decline in force during the 15-second fatigue test was 19.2% when the knee angle was 90° compared to a nonsignificant change at the knee angle of 140°. At the same time, there is evidence to suggest that force potentiation is also muscle length dependent, but in an opposite fashion, i.e., PAP is greater at short muscle length compared with long muscle length. Smith et al. (34) examined the effects of PAP of the triceps brachii muscle at short vs. long length (elbow angle: 120 vs. 60°) and found an almost 3-fold higher twitch torque potentiation at short muscle length (216 ± 169%) than at long muscle length (77 ± 33%). In addition, Place et al. (28) reported a large twitch potentiation (32%) after isometric fatiguing knee extension exercise only when knee angle was 145° compared with a lack of PAP when knee angle was 105°. They also argued that the mechanism behind this outcome is located in the level of muscle, i.e., phosphorylation of regulatory myosin light chains. Indeed, phosphorylation of regulatory myosin light chains has been considered as a main mechanism of PAP (37), whereas a muscle length effect on PAP is supported by the findings of Stuart et al. (35) who reported a greater potentiation and an increased myosin light-chain phosphorylation when a 10-second maximal isometric knee extension was performed at short vs. longer muscle length. Thus, the possible interplay between muscle potentiation and fatigue may explain the findings of the present study, where there was a lack of CMJ increase during the entire course of recovery when the isometric conditioning contractions were performed at a knee angle of 90°, possibly due to the prevalence of fatigue over PAP. In contrast, CMJ enhancement in the later part of recovery after exercise with the knee angle at 140° may be due to a larger muscle potentiation with minimal fatigue, as indicated by the lower percent drop in force.
Similar suggestions regarding the possible interactions between muscle potentiation and fatigue have been reported by Crewther et al. (11), who found a 3–4% improvement of CMJ after 4, 8, and 12 minutes of recovery from a 3RM squat conditioning activity. A possible predominance of fatigue over PAP may also explain the lack of CMJ improvement following maximal isometric knee extension exercise with similar characteristics (i.e., 3 × 3 seconds) as in our study that were performed at a knee angle of 90° (15,23,36). In contrast, studies where isometric exercise was performed at wide joint angles and thus shorter muscle length reported improvements of similar magnitude and timing as in the present study (13). Thus, the direct comparison of 2 different angles in the present study provides evidence to support that maximal isometric squat exercise is more beneficial when performed with knee angle set at 140° (quarter squat) than 90° (half squat). To our knowledge, there are only 2 other studies that have examined the effects of squat depth during dynamic, but not isometric, squat exercise. In the study of Mangus et al. (24), there was no effect of a single conditioning contraction (90% of 1RM) on vertical jump performance at both squat depths (half and quarter squat). However, the authors reported that 5 of the subjects improved CMJ after both squat depths, but could not explain this finding from their data. In the second study (12), the effects of a 3-repetition maximum parallel squat (knee angle: 60–70°) or quarter squat (knee angle: 135°) on CMJ performed after 5 minutes were compared. Both conditions induced PAP, but parallel squat induced slightly better results. The authors suggested that the deeper squat increased gluteus maximus activation and work produced because of the longer duration and vertical displacement of each repetition. In the present study, the duration of contraction was kept the same between the 2 conditions and this enables a more direct comparison of the effects of the other characteristics of the 2 squat depths (i.e., muscle length). It is thus possible that during dynamic squat exercise, deeper squats may be more effective in inducing PAP owing to the greater work done (12). On the other hand, our data suggest that when isometric, as opposed to dynamic squat is used as the conditioning activity, the quarter squat is more effective because of the possible influence of muscle length on PAP and fatigue. However, there may be a common factor that may partially explain the findings of both studies, i.e., the increase in performance was evident in the condition when the work done was higher. In the study of Esformes et al. (12), this was when the squat was deeper and its duration was longer, thus increasing the force–time integral. In the present study, the highest force–time integral (i.e., total impulse) was observed in the angle of 140°, owing to the higher external force generated at this extended position, because the duration of both conditioning muscle actions was the same at 90 and 140° (Table 1). Thus, one possible explanation for the lack of a PAP effect at a knee angle of 90° in the present study may be that there may be a minimum value of impulse that is required to elicit a PAP effect. This hypothesis was also put forward in a recent study, where it was concluded that during a dynamic conditioning activity, a minimum total work must be reached to trigger the mechanisms responsible for PAP (33).
When the subjects were divided into “high” and “low” jumpers, it was evident that the best CMJ performers exhibited improvements in performance at both knee angles, with a higher improvement observed at the 140° knee angle. The enhancement of CMJ only in the “high jumpers” could be related to fiber type distribution because CMJ performance is positively correlated with the percentage of fast twitch fibers (5,17). Support for this suggestion is provided by the study of Miyamoto et al. (26), who reported that muscles composed of predominantly fast twitch fibers exhibit high levels of PAP at a shortened position, but their performance can also be enhanced, albeit at a lesser degree, at a lengthened position. Therefore, subjects with higher CMJ performance could have a higher percentage of fasttwitch fibers (5,17) and therefore may exhibit a higher degree of phosphorylation of regulatory myosin light chains after a conditioning activity (18,19,27). These subjects may also have a greater number of higher order motor units which could be activated during a subsequent muscle action (16,37). These findings are in agreement with previous studies (16,18,19) but the present study is the first to show that CMJ performance enhancement depends on knee angle during the isometric conditioning activity and initial CMJ performance. Notably, careful examination of the data from previous studies examining the effect of conditioning contractions on CMJ suggests that an improvement of CMJ was observed only when the subjects had a high baseline CMJ (9,11). For example, in the study of Crewther et al. (11), where an improvement of CMJ was observed after a 3RM parallel squat, subjects had a baseline CMJ of 47.5 cm. Also, in the study of Chiu et al. (9), loaded jump squat performance after 5 sets of single squat repetitions at 90% of 1RM was improved only in the subjects characterized as “athletes in sports explosive strength,” whereas there was no improvement in recreationally trained individuals. In contrast, Batista et al. (2) found no effect of one or three 5-second maximal isometric leg press efforts on subsequent CMJ in subjects with different muscle strength. Interestingly, the baseline CMJ performance was below the median value used in the present study (i.e., 42.6 cm) in all 3 groups of subjects examined and this may partially explain the lack of PAP following that maximal isometric leg press exercise protocol. Moreover, the knee angle during isometric leg press was 80–90° in that study (2) and this may also contribute to the absence of a PAP effect. These observations lend further support to our finding that vertical jumping ability and knee angle are important determinants of the acute performance increase observed following a conditioning activity.
The findings of the present study have practical applications for power-trained athletes. Repeated maximal isometric squat exercise of brief duration (3 × 3 seconds with 1 minute rest in between) may be used as a conditioning activity or as a part of a warm-up routine to acutely increase leg muscle power. This conditioning activity is effective when knee angle is 140°, whereas performance of a deeper isometric squat (knee angle of 90°) may cause muscle fatigue which counteracts the effects of PAP. Athletes are advised to perform brief maximal isometric squats at a wide knee angle and expect an enhanced vertical jump performance 12 minutes after the conditioning activity. One important outcome of the present study was that only the athletes with greater vertical jumping ability exhibit a PAP response after isometric half and quarter squats, with a greater increase observed at the wider knee angles. Thus, this type of conditioning activity may be applicable mainly for high-level power athletes.
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