A warm-up protocol usually consists of general (e.g., cycling and stretching) and specific warm-up exercises. Recently, it has been recommended to include high-intensity resistance exercises (conditioning activity) in the warm-up protocol to benefit from postactivation potentiation (PAP) (16). Many studies have found positive effects of conditioning activity on subsequent dynamic ballistic performance. For example, Young et al. (25) reported enhanced jump performance after 5 repetition maximums (RMs) of squat exercises.
In most cases, preparatory exercises are performed before high-intensity squat exercises (e.g., 5RMs of squat exercises). It is widely accepted that a practical warm-up involves starting with a low-intensity squat exercise with a low risk of injury and gradually increasing the intensity of the exercise (i.e., an ascending intensity squat protocol). However, most of the previous studies employed a single-load protocol (3,8,13–15,20,22,24,25). Although some studies employed an ascending intensity protocol, a significant increase in jump height was not observed (6,18), or only a minimal increase in jump height (2.39%) was observed (10). Crum et al. (6) employed squats ranging from 30 to 65% 1RM in a study of the effects of conditioning activities. To benefit from PAP, type 2 (fast twitch) fibers need to be recruited during conditioning activities against the size principle (11). Regarding ankle plantar flexor, 180°·s−1 of maximal voluntary concentric torque increased after conditioning activity with ≥80% maximal voluntary isometric action (7). However, the intensity required to enhance exercise performance in an ascending intensity squat protocol remains to be clarified. Gourgoulis et al. (10) employed squats ranging from 20 to 90% 1RM as conditioning activities. In that study, jump performance was measured immediately after completion of the conditioning activities. However, this time interval is inadequate to monitor the influence of conditioning activities because of the powerful effects of fatigue (17,21).
The purpose of conditioning activity is to enhance subsequent performance while minimizing fatigue. Khamoui et al. (12) suggested that a higher number of repetitions of conditioning activity do not increase the potentiating effect. Therefore, practical warm-up protocols may include minimum-load conditioning activities with as few repetitions as possible for efficiency. Vandervoort et al. (23) reported that PAP occurs even after approximately 3 seconds of maximal muscle action. Given that 4–6 seconds are required to perform a single repetition of a squat exercise, the exercise may not necessarily have to be repeated several times to enhance subsequent jump performance.
Athletes who have the ability to exert high muscular power, such as weightlifters, may be able to recruit type 2 fibers with strong neural drive (19). Indeed, Young et al. (25) reported that an increase in vertical jump height after conditioning activity correlated significantly with 5RM squat strength. The effects of conditioning activity may therefore be pronounced in very powerful athletes. In this study, the effects of conditioning activity with an ascending intensity protocol consisting of a single-repetition squat exercise on subsequent vertical jump performance were examined. The intensity of conditioning activity required to enhance jump performance in an ascending intensity squat protocol with adequate rest interval was also investigated. It is hypothesized that conditioning activity with an ascending intensity squat protocol can enhance the subsequent jump performance.
Experimental Approach to the Problem
This study employed a 1-group experimental design with squat (SQ) and control (CON) conditions occurring in random order. These sessions were conducted on 2 separate days separated by a minimum of 48 hours. In the SQ condition, the subjects performed squat exercises with incremental loads (1 repetition of 20% 1RM, 40% 1RM, 60% 1RM, 80% 1RM back squat with eccentric and concentric phases, and a maximal isometric [MI] half-squat exercise) after a general warm-up. In the CON condition, the subjects rested on a chair (no squat exercise) after a general warm-up. In both the conditions, vertical jump height was measured before the session (baseline) and after 1 minute of each action (see Procedure section) to quantify dynamic ballistic performance.
Fourteen male college weightlifters (mean ± SD: age, 19.9 ± 1.4 years [18-22 years]; height, 165.2 ± 6.0 cm; weight, 71.5 ± 14.7 kg; 1RM of snatch, 106 ± 14 kg; 1RM of clean and jerk, 134 ± 19 kg; 1RM of snatch/body mass, 1.48 ± 0.22; 1RM of clean and jerk/body mass, 1.87 ± 0.27) voluntarily participated in the study. They were all familiar with squat exercises and vertical jumps. All subjects were informed of the experimental procedure, potential risk, purpose of the study, and their rights. They provided written consent to participate. The study was approved by the institutional ethical committee.
The experimental procedure of this study is shown in Figure 1. All subjects attended 2 testing sessions: SQ and CON conditions. The subjects were instructed to avoid excessive physical activity before testing and to continue their normal routine between test days. In both the conditions, the subjects completed submaximal cycling and static stretching. The subjects completed a 5-minute submaximal warm-up on a cycle ergometer, cycling at 70 rpm at an intensity of 20 kW. The subjects then performed self-static stretching for 5 muscle groups in the legs and lower back: hip extensors (grasping 1 bent leg in a supine position), hamstrings (raising a straight leg in a supine position), hip flexors (extending the hip joint on 1 knee, i.e., pulling the knee upward), quadriceps femoris (grasping the foot behind the thigh in a side position), and lower back muscles (rotating the trunk in a supine position). To stretch the muscle tendon unit to the maximal point without causing pain, each stretch was held in position until the subject verbally indicated that he had stretched the muscle to a point of mild discomfort (5). Each stretch was held for 30 seconds with a rest interval of a few seconds between stretches (2). This procedure was repeated twice (2 sets).
In the SQ condition, for the conditioning activity, the subjects performed squat exercises at 20% 1RM, 40% 1RM, 60% 1RM, 80% 1RM, and MI half-squat (knee angle of 90° in the deepest position) after static stretching. The subjects performed 1 set of 1 repetition of each squat exercise for each load with a rest interval of 3 minutes between loads. For the MI squat, the force was gradually increased on the bar for 1 second and the maximal voluntary action was maintained for 6 seconds (23) in the half-squat position. A 1RM for the half-squat position had been determined for each subject ≥7 days before the experiments using the protocol outlined by Baechle et al. (1). The subjects performed maximal vertical jumps at the beginning of the sessions and after cycling, static stretching, and each squat exercise in the SQ condition. In the CON condition, vertical jump height was measured at the same time points as that in the SQ condition. Vertical jumps were performed 1 minute after each action. This time interval was based on the findings of Miyamoto et al. (17). Vertical jumps were repeated twice, and the average value was used for analyses. To perform the vertical jump, the subjects stood in an upright position with hands on hips, flexed their knees and hips into a half-squat position (countermovement), and then immediately extended the knees and hips to perform a vertical jump. The depth of the countermovement was controlled to correspond to that used by each athlete during the half-squat, thus increasing the specificity between the potentiating exercise and the test movement. Jump height was measured using a jump meter (Jump MD; Takei Scientific Instruments Co., Ltd, Niigata, Japan) fixed to the waist. When subjects jumped, a rolled string uncoiled to the same length as the height of the jump. The uncoiled string was then measured using a rotary encoder with a minimum measurement unit of 1 cm. One subject complained of lower back pain and discontinued the protocol before the MI squat (i.e., the subject completed 20% 1RM, 40% 1RM, 60% 1RM, and 80% 1RM squat and subsequent vertical jumps as well as the CON condition). The intraclass correlation coefficient for jump height was 0.98.
Descriptive data are expressed as mean ± SD. A Student's paired t-test was used to compare jump height in the first test trials in the SQ and CON conditions. Because no significant difference in jump height between the SQ and CON conditions was found in the first test trial, the following analyses were performed. Two-way repeated-measures analysis of variance (ANOVA) (2 conditions × 8 test trials) was used to determine changes in jump height in the 2 conditions. When a significant interaction between 2 factors was observed, 1-way ANOVA with repeated measures followed by the Bonferroni's multiple comparison test was used to examine the differences between the test trials in each condition. The level of significance was set at p ≤ 0.05. For the results of the Student's paired t-test and 2- and 1-way ANOVA, r or
values are shown as indices of effect size, and p values are provided.
No difference was observed in the first jump height between the SQ (63.1 ± 7.3 cm) and CON conditions (63.0 ± 6.8 cm) (p = 0.889, r = 0.040). Two-way ANOVA showed a significant interaction between conditioning and test trial (p < 0.001,
= 0.466). The follow-up ANOVA (p < 0.001,
= 0.729) and post hoc comparisons revealed that in the SQ condition, jump height increased by 4.3 ± 4.4% (2.6 ± 2.8 cm) after the 60% 1RM squat (p = 0.004, r = 0.701), by 6.7 ± 5.2% (4.0 ± 3.1 cm) after the 80% 1RM squat (p < 0.001, r = 0.800), and by 10.0 ± 6.1% (6.0 ± 3.4 cm) after the MI squat (p < 0.001, r = 0.877) compared with the first test trial (baseline values). The jump height after the MI squat was significantly higher than that after the 80% 1RM squat (p = 0.002, r = 0.763), which significantly surpassed the record set after the 60% 1RM squat (p < 0.001, r = 0.851). In the CON condition, no significant difference in jump height was observed across trials (p = 0.145,
= 0.110) (Figure 2).
Vertical jump height increased after performance of an ascending intensity squat protocol compared with baseline values. The main finding of this study was that 1 repetition of a squat exercise with a sufficient load (intensity and/or volume) enhanced subsequent vertical jump performance in subjects performing an ascending intensity squat protocol. These findings supported our hypothesis that conditioning activities with an ascending intensity protocol consisting of a single-repetition squat exercise can enhance subsequent jump performance. In addition, as the intensity of conditioning activities increased, the magnitude of jump performance enhancement also increased in an ascending intensity squat protocol.
The SQ and CON conditions were conducted on different days. No difference was observed in jump height in the first jump trial (baseline) between the 2 conditions. This fact supports the assumption that the 2 conditions were conducted under equivalent conditions.
As mentioned in the Introduction, no increase (6,18) or minimal increase (10) in vertical jump height was reported in the previous studies that employed an ascending intensity squat protocol. In contrast, in this study, the magnitude of jump performance enhancement gradually increased in the ascending intensity squat protocol, finally reaching a level of approximately 10%. This value is higher than that reported in previous studies that employed a single-intensity exercise for conditioning activity to increase the subsequent jump height (<5.5%) (3,8,13–15,20,22,24,25). The following factors may have contributed to this result: (a) adequate recovery time between the end of conditioning activity and the beginning of jump performance measurement; (b) greater strength of the subjects in this study compared with untrained or recreationally active subjects in other studies; (c) relatively high final intensity of conditioning activity in this study (i.e., a larger number of cross-bridges in the force-generating state); and (d) the cumulative effect of all components of the experiment.
During conditioning activity, accrual of both potentiative effect and fatigue occurs. Compared with the potentiative effect, fatigue dissipates at a faster rate during recovery time. Postactivation potentiation is expected to be generated during this gap between the 2 effects (16). Previous studies (9,17) have reported that PAP does not occur immediately after completion of conditioning activities. Gourgoulis et al. (10) measured jump performance immediately after completion of an ascending intensity squat protocol; in this study, a recovery time of 1 minute was allowed. This difference in the recovery time may be related to the difference in the magnitude of the potentiation effect in the 2 studies, which employed an ascending intensity squat protocol.
It has been reported that there can be interindividual differences in the magnitude of effects of conditioning activity on subsequent enhancement of exercise performance (4,8,15). College weightlifters were enrolled in this study as subjects and were expected to be able to recruit type 2 fiber with a strong neural drive (19) against the size principle. Indeed, Young et al. (25) reported that an increase in vertical jump height after conditioning activity correlated significantly with 5RM squat strength. The effects of conditioning activity may therefore be pronounced in such powerful athletes. Therefore, it remains unclear whether the findings of this study are valid for people with less strength.
In the SQ condition, the average values for jump height increased gradually with an increased intensity of the squat exercise and enhanced significantly after the 60% 1RM, 80% 1RM, and MI squat exercises. The magnitudes of the subsequent jump performance enhancement also increased in an order corresponding to 60% 1RM, 80% 1RM, and MI squat. It has been reported that the magnitude of PAP is greater in type 2 fibers than in type 1 fibers (11). As muscle action intensity increases, motor units are generally recruited in type 1 and type 2 fibers in that order (the size principle). These characteristics of PAP and muscle action could be one of the reasons why squat exercises of higher intensity resulted in greater jump performance enhancement in this study.
It has been reported that the effect of conditioning activity on dynamic ballistic exercise performance lasts for 3 minutes (17). In this study, the time interval between squat exercises was set at 3 minutes. Therefore, a cumulative effect may have influenced the greater enhancement of jump performance observed after higher-intensity squat exercise. In an additional experiment involving 7 male weightlifters, jump performance was enhanced only 2.5 ± 2.6% after MI squat exercise of a single intensity. Although the load of an individual squat exercise may not be enough to maximize PAP, the total volume of all squat exercises in the present protocol may be enough to elicit greater PAP than that observed in some previous studies.
Because no test was performed on the neuromuscular activation (e.g., electromyography or twitch torque) or muscular cells, the mechanisms of performance enhancement could not be assessed. However, morphological changes in the muscles are unlikely in such a short term. Therefore, enhancement of the contractile properties of the muscles is more likely to be related to the phosphorylation of light chain myosin (16,21).
In conclusion, conditioning activity involving single-repetition exercises with incremental loads can enhance jump performance in an ascending intensity squat protocol. In addition, magnitude of jump performance enhancement after conditioning activity was greater when the conditioning activity was performed with stronger muscle action in an ascending intensity squat protocol. These findings suggest that an ascending intensity warm-up protocol including conditioning activities with sufficient intensity may be useful for athletes who require high muscular power.
This study examined the effects of conditioning activity consisting of squat exercises with increasing loads (20% 1RM, 40% 1RM, 60% 1RM, 80% 1RM, and MI squat). The results confirmed that this protocol enhances jump performance and that squat exercises should be performed from lower level to the maximal level of muscle action to gain greater benefit from PAP. The protocol presented here may be useful for warm-up before competition or training, particularly for athletes who require high muscular power.
The author thanks Mr. K. Tsuchiya and Dr. R. Akagi for their advice. The author also thanks Enago (www.enago.jp) for the English language review. The results of this study do not constitute endorsement of the product by the authors or NSCA.
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