Acute improvement of vertical jump performance after ...

Journal of Strength and Conditioning Research Publish Ahead of Print DOI: 10.1519/JSC.0000000000001328

Acute improvement of vertical jump performance after isometric squats depends

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on knee angle and vertical jumping ability

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Athanasios Tsoukos, Gregory C. Bogdanis*, Gerasimos Terzis, Panagiotis Veligekas

Athletics Laboratory, Faculty of Physical Education and Sports Science, University of

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Athens, Greece

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Running Head: Vertical jump after Isometric Squats depends on knee angle

* Corresponding author: Gregory C. Bogdanis, Ph.D. Postal: Faculty of P.E. and Sports Science, 41 Ethnikis Antistasis Str. Dafne, 172 37 Athens, GREECE Phone number: +30210 7276115 Email: [email protected]

Copyright ª 2016 National Strength and Conditioning Association

ABSTRACT This study examined the acute effects of maximum isometric squats at two different knee angles (90o or 140o) on countermovement jump (CMJ) performance in power athletes. Fourteen national level male track and field power athletes completed 3 main

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trials (two experimental and one control) in a randomized and counterbalanced order one week apart. CMJ performance was evaluated using a force-plate before and 15 s,

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3, 6, 9 and 12 minutes after 3 sets of 3 s maximum isometric contractions with 1 minute rest in between, from a squat position with knee angle set at 90o or 140o. CMJ performance was improved compared to baseline only in the 140o condition by 3.8 ± 1.2% on the 12th minute of recovery (p = 0.027), while there was no change in CMJ

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height in the 90o condition. 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. To determine the possible effects of baseline jump performance on subsequent CMJ performance, subjects were divided into two groups (“high jumpers” and “low jumpers”). The baseline CMJ values of “high jumpers” and “low jumpers” differed

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significantly (CMJ: 45.1 ± 2.2 vs. 37.1 ± 3.9 cm, respectively, p = 0.001). CMJ was increased only in the “high jumpers” group by 5.4 ± 1.4% (p = 0.001) and 7.4 ± 1.2%

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(p = 0.001) at the knee angles of 90o and 140o, respectively. This improvement was larger at the 140o angle (p = 0.049). Knee angle during isometric squats and vertical

jumping ability are important determinants of the acute CMJ performance increase observed following a conditioning activity.

Key words: postactivation potentiation; muscle fatigue; muscle length; power athletes; conditioning activity


INTRODUCTION Postactivation potentiation (PAP) has been defined as a temporary enhancement of muscle force and/or power output following 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

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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

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conditioning activity may also induce fatigue. Therefore, during the recovery period following the conditioning activity, the balance between fatigue and PAP determines whether the muscle performance is enhanced, reduced, or remains unaltered (30).

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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 as well as 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

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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

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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 (9,10,11,20,25,40). For Copyright ª 2016 National Strength and Conditioning Association

example, Mc Bride et al. (25), found a small improvement of 0.87% in 40 m sprint time following 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 with the total

muscle actions and the subjects' training background (9, 25).

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volume lifted during the conditioning squat exercise, the resting period between

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There is evidence to suggest that isometric exercise may be superior to concentric or eccentric dynamic muscle actions in order to elicit PAP (4,12,31). A recent study comparing the influence of contraction type on PAP has shown that 3

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sets of 3 s of maximal isometric half squat were the most effective in increasing jumping performance compared to 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 due to the possible influence of the length of the leg extensor muscles on PAP and fatigue. Previous studies have shown

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that potentiation and fatigue can coexist in skeletal muscle and their interplay determines the degree and timing of performance increase following a conditioning

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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 165o (shorter muscle length) compared with 90o (longer muscle length) and this was attributed to metabolic


factors due 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 two studies using electrical stimulation of the quadriceps (28) and triceps brachii muscle (34) reported greater twitch potentiation at shorter

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compared to longer muscle lengths, following submaximal muscle actions. Similarly, Miyamoto et al. (26) reported greater PAP when a 10 s maximal voluntary isometric

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conditioning contraction was performed with the ankle joint in plantar flexion (short muscle length) compared to dorsiflexion (long muscle length).

Taken collectively, the findings of the above studies (22,26,28,34) show that at

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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 two 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 140o (i.e. shorter muscle length) would cause less fatigue and greater PAP compared with a

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squat with a knee angle of 90o (i.e. longer muscle length). Furthermore, taking into account that both PAP and fatigue are influenced by muscle fiber composition

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(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.

METHODS 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 two different knee angles on subsequent vertical jump performance in national level power trained athletes. Following two familiarization sessions with isometric squats at different knee angles and two preliminary measurements, subjects performed three main trials

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one week apart. The main trials involved 3 sets of 3 s maximum voluntary isometric contractions (MVIC) with 1 minute rest between each set, from a squat position, with

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knee angle set at 90o or 140o, as well as a control condition. Counter-movement vertical jump (CMJ) performance was evaluated before and 15 s, 3, 6, 9 and 12 minutes after each main trial (Fig. 1). To determine the possible effects of baseline

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jump performance on PAP, the median-split technique (7) was used to divide the subjects into two groups (“high jumpers” and “low jumpers”), depending on whether their baseline CMJ performance was above or below the median CMJ value.

Subjects

Fourteen national level male track and field power trained athletes (jumpers and

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decathletes) volunteered to participate in the study (age: 27.1 ± 7.0 y, height: 179 ± 7 cm, body mass: 78.3 ± 7.3 kg, body fat: 10.2 ± 5.0%). Subjects had training

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experience of 9.4 ± 5.8 y, which included at least 6 years of resistance training, and took part in 6 to 8 training sessions per week. All athletes had no musculoskeletal injuries for at least 1 year prior to 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.

Procedures Before each preliminary and experimental test, subjects performed a

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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).

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Prior to each preliminary measurement, subjects were instructed to have a light training session in the previous day, while for the three main trials subjects abstained from training for 24 h prior to the test (16). Subjects were also instructed to replicate

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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

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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

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fully loaded with 25 kg weight plates and was further stabilized by two assistants. An experienced weightlifting coach ensured that body position was standardized, so that the torso was upright in both conditions. Due to 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, UK). Sampling frequency was set at 1000 Hz and instantaneous force data were low-pass filtered (4th order reverse Butterworth low pass digital filter) with a cut-off frequency of 20Hz. Peak and average force, as well as total duration of the isometric muscle action and total impulse (force-time integral, obtained by numerical integration using the trapezoidal

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rule) were then calculated using custom written routines (Labview version 8.0,

Measurement of CMJ performance

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National Instruments, Austin, TX, USA).

CMJ performance was assessed according to the protocol of Bosco et al. (6) by

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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 degrees and to takeoff and land maintaining the same body position. Three CMJ separated by 30 s of rest were performed at baseline, while one CMJ was performed at each recovery time point. Subjects remained seated between CMJ efforts to reduce

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fatigue. The ICC for the CMJ assessment was 0.98 (p0.137). 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

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d were classified as small (0.2), medium (0.5) and large (0.8). The intraclass correlation coefficient (ICC) was calculated using a two-way mixed model to determine test-retest reliability for all dependent variables. Statistical significance was

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accepted at p < 0.05.

RESULTS

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 90o compared with 140o (Table 1). In accordance with the

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experimental design, total duration of the isometric contractions was equal between conditions. Muscle fatigue, determined at the second preliminary visit as the percent

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drop of force during the maximal isometric fatigue test, was significantly higher when knee angle was 90o compared to 140o (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 Fig.2. The baseline CMJ performance was similar in all three main trials (41.2 ± 5.5, 41.9 ± 4.5 and 41.5 ± 5.1 cm for the control, 90o and 140o conditions,

respectively, p = 0.91). The 2-way ANOVA revealed a significant main effect for


condition (p = 0.003, η2 = 0.36), as well as a condition x time interaction (p = 0.001, η2 = 0.21). The post-hoc test for the interaction showed that CMJ performance was improved compared to baseline only in the 140o condition by 3.8 ± 1.2% on the 12th minute of recovery (p = 0.027), while there was no change in CMJ height in the 90o condition (Fig. 2). In the control condition there was a decrease in CMJ performance

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over time, reaching -3.6 ± 1.2% (p = 0.049) after 12 minutes of recovery (Fig. 2). Furthermore, the improvement in CMJ performance in the 140o condition was greater

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at the 6, 9 and 12 minutes time-points compared with the control condition and at the 9 and 12 minutes time-points, compared with the 90o condition (Fig. 2)

To determine the possible effects of baseline jump performance on PAP,

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subjects were divided into two 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 x condition

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x time; p = 0.017, η2 = 0.28) indicating that the two groups responded differently across the three main trials. The Tukey post-hoc tests showed that CMJ performance

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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 90o and 140o, respectively (Fig. 3). This improvement was larger at the 140o 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.


DISCUSSION 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 140o resulted in a delayed increase in vertical jump height, while there was no change in CMJ performance in the 90o

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condition. Another important finding was that the jumpers with the higher baseline vertical jump performance demonstrated an increased jump performance at both knee

did not show a PAP response.

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angles after the conditioning activities, whereas the jumpers with lower performances

The finding that only isometric "quarter squats" (knee angle 140o) resulted in an

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increase in CMJ performance may be explained by possible differences in muscle length between the two conditions (knee angle 90o and 140o). When knee angle is 90o 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

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knee extension (1,3,12). According to the knee angle-torque relationship during isometric knee extension, peak torque is almost twice as high when knee angle is 90o

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compared to 140-150o (3,21). Studies examining muscle fatigue during isometric knee extension have clearly shown that fatigue is much greater at a knee angle of 90o (longer muscle length) compared with 165o (shorter muscle length), and this has been

attributed to metabolic factors due to differences in cross bridge interactions (14,35). Similarly, another study using electrical stimulation with 5 s tetanic isometric contractions at long, compared to a shorter muscle length (knee angle: 90o vs. 150o) demonstrated a steeper torque decline and thus greater fatigue when knee angle was 90o (29). This knee angle-specific fatigue may also explain the findings of the present Copyright ª 2016 National Strength and Conditioning Association

study, where the decline in force during the 15 s fatigue test was 19.2% when the knee angle was 90o compared to a non significant change at the knee angle of 140o. 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 compared to long muscle length. Smith et al. (34) examined the effects of PAP of the triceps brachii

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muscle at short vs. long length (elbow angle: 120o vs. 60o) and found an almost 3-fold higher twitch torque potentiation at short (216 ± 169%) than at long muscle length (77

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± 33%). In addition, Place et al. (28) reported a large twitch potentiation (32%) following isometric fatiguing knee extension exercise only when knee angle was 145o compared with a lack of PAP when knee angle was 105o. They also argued that the

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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), while 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 s maximal isometric knee extension was performed at short vs. longer muscle length.

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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

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entire course of recovery when the isometric conditioning contractions were performed at a knee angle of 90o, possibly due to the prevalence of fatigue over PAP. In contrast, CMJ enhancement in the later part of recovery following exercise with the knee angle at 140o 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 x 3 s) as in our study, that were performed at a knee angle of 90o (15,23,36). In contrast, studies where isometric exercise was performed

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at wide joint angles and thus shorter muscle length, reported improvements of similar magnitude and timing as in the present study (12). Thus, the direct comparison of two

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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 140o (quarter squat) than 90o (half squat). To our knowledge, there are only two other

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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 five of the subjects improved CMJ after both squat depths, but could not explain this finding from their data. In the second study (13), the effects of a 3-repetition maximum

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parallel squat (knee angle: 60-70o) or quarter-squat (knee angle: 135o) on CMJ performed 5 after minutes were compared. Both conditions induced PAP, but parallel

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squat induced slightly better results. The authors suggested that the deeper squat increased gluteus maximus activation and work produced, due to the longer duration and vertical displacement of each repetition. In the present study, the duration of contraction was kept the same between the two conditions and this enables a more direct comparison of the effects of the other characteristics of the two squat depths (i.e. muscle length). It is thus possible that during dynamic squat exercise, deeper squats may be more effective in inducing PAP due to the greater work done (13). 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 due to 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. that the increase in performance was evident in the condition when the work done was higher.

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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

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highest force-time integral (i.e. total impulse) was observed in the angle of 140o, due to the higher external force generated at this extended position, since the duration of both conditioning muscle actions was the same at 90o and 140o (Table 1). Thus, one

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possible explanation for the lack of a PAP effect at a knee angle of 90o 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

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that the best CMJ performers exhibited improvements in performance at both knee angles, with a higher improvement observed at the 140o knee angle. The enhancement

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of CMJ only in the "high jumpers" could be related to fiber type distribution, since 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 and a possibly higher percentage of fast twitch fibers (5,17) may exhibit


a higher degree of phosphorylation of regulatory myosin light chains following a conditioning activity (18,19,27), while having 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

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isometric conditioning activity and initial CMJ performance. Notably, careful examination of the data from previous studies examining the effect of conditioning

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contractions on CMJ, suggest 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,

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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", while there was no improvement in recreationally trained individuals. In contrast, Batista et al. (2) found no effect of one or three 5 s maximal isometric leg press efforts on subsequent CMJ in subjects with different muscle strength. Interestingly, the

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baseline CMJ performance was below the median value used in the present study (i.e. 42.6 cm) in all three groups of subjects examined and this may partially explain the

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lack of PAP following that maximal isometric leg press exercise protocol. Moreover, the knee angle during isometric leg press was 80-90o 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.


Practical applications The findings of the present study have practical applications for power-trained athletes. Repeated maximal isometric squat exercise of brief duration (3 x 3 s with 1 min rest in between) may be used as a conditioning activity or as a part of a warm-up routine in order to acutely increase leg muscle power. This conditioning activity is

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effective when knee angle is 140o, while performance of a deeper isometric squat (knee angle of 90o) may cause muscle fatigue which counteracts the effects of PAP.

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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

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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

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conditioning activity may be applicable mainly for high level power athletes.


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FIGURE LEGENDS

D

Figure 1. Schematic representation of the study protocol. CMJ:

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countermovement jump

Figure 2. Time course of changes in countermovement jump (CMJ) performance. Values are expressed as percent changes compared to baseline. *:

C EP

p