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Original Research

Effect of Functional Isometric Squats on Vertical Jump in Trained and Untrained Men

Berning, Joseph M1; Adams, Kent J2; DeBeliso, Mark2; Sevene-Adams, Patricia G2; Harris, Chad3; Stamford, Bryant A4

Author Information
Journal of Strength and Conditioning Research: September 2010 - Volume 24 - Issue 9 - p 2285-2289
doi: 10.1519/JSC.0b013e3181e7ff9a
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Several research studies have employed dynamic movements such as heavy squats or bench presses (6,9,10) as a means of eliciting a powerful postactivation potentiation (PAP) response. Others have successfully employed maximal isometric muscle actions (5,7,24,25). A third approach, functional isometrics (FIs), combines maximal dynamic and isometric muscle actions using weight loads in excess of the 1 repetition maximum (1RM) (4,14-19). It has been postulated that FI may most effectively enhance explosive performance via the PAP response by maximally overloading muscles and connective tissue while simultaneously hyperstimulating the nervous system for brief periods (3-5 seconds) (4,15,18,19). However, to our knowledge, no studies have assessed the effects of an FI stimulus on subsequent PAP response.

Postactivation potentiation studies have investigated the impact of various squatting protocols (e.g., 3RM, 5RM, and half-squats) on explosive performance (e.g., vertical and horizontal jump, sprinting) (1,3,6,12,13,20,22,26). Some studies have shown positive PAP responses, (6,13,26), but there also have been studies without significant results (3,10,12,20,22). An influential factor could be timing. The amount of rest between the squats and subsequent performance may be critical, with at least 4 minutes suggested as being effective, if not optimal (2,10). Another factor could be training status. Studies have shown that physically stronger and resistance trained subjects may demonstrate a greater PAP response than untrained subjects (1,3,6,26). A third factor could be the magnitude of overload employed (3,6,13,21), because the recruitment of high threshold motor units in the prior muscle action may enhance subsequent explosive performance (23).

The purpose of the present investigation was to examine the acute impact of FI squats on subsequent performance in the countermovement vertical jump (CMVJ) with regard to training status (resistance trained vs. untrained subjects) and timing (4 minutes vs. 5 minutes post-FI squat). A high degree of overload on the FI squat was emphasized (i.e., 150% of the 1RM) in an attempt to maximize hyperstimulation of the nervous system.

It was hypothesized that hyperstimulation of the nervous system caused by the FI squat would create a substantial PAP effect, leading to enhanced performance on the CMVJ. Second, the response in resistance trained subjects would be greater than in the untrained. Third, the greatest effect would be observed at 4-minutes post-FI squat when compared with 5 minutes.


Experimental Approach to the Problem

To determine the impact of FI squats on PAP and vertical jump performance with regard to training status and timing, a within-subjects design consisting of 2 sessions was used. Trained and untrained subjects participated. In session 1, subjects' maximal CMVJ was measured 4 minutes after a cycling warm-up. Next, 1RM in the back squat was assessed and150% of the 1RM was calculated. In session 2, subjects performed a cycling warm-up plus an FI half-squat with 150% of 1RM during which maximal isometric force was exerted for 3 seconds (15,17,19). After 4 minutes and 5 minutes post-FI squat, subjects performed a maximal CMVJ. This approach allowed assessment of the effect of adding an FI squat to a standard, low-intensity cycling warm-up.


Thirteen resistance trained men (age: 22.8 ± 3.2 years, mass: 90.0 ± 16.3 kg, height: 178.9 ± 7.1 cm; parallel back squat 1RM: 153.2 ± 31.1 kg) and 8 untrained men (age: 28.5 ± 5.9 years, mass: 101.5 ± 23.0 kg, height: 177.0 ± 4.8 cm; parallel back squat 1RM: 132.2 ± 35.9 kg) participated. Resistance trained subjects had been training in the squat exercise 2 times a week for at least 1 year before participating in the study and had tested their 1RM in the squat approximately every 3 months during this time. They also performed standard resistance training for the rest of their body and participated in recreational activities. None were currently involved in competitive sports. Untrained subjects did not have any resistance training experience and were not currently exercising. Their history included general recreational activity but no recent competitive sports. Each subject was instructed in proper form by a National Strength and Conditioning Association Certified Strength and Conditioning Specialist (CSCS). Subjects were asked not to perform any lower body exercises during the week they participated in this study. Diet and hydration were not controlled.

This study was approved by the University's Institutional Review Board on Human Subjects. Subjects were verbally informed of procedures in detail with opportunity to address any questions. After this, an informed consent document was read and signed by each subject. A physical activity readiness questionnaire also was completed.


This study required 2 sessions, each lasting 30-60 minutes and separated by 48-72 hours. The following describes activities for each session.

Session 1

Demographic data, resistance training experience, leg segment heights, maximal vertical jump height, and 1RM in the parallel back squat were determined for each subject. In addition, subjects practiced FI squatting.

Leg segment heights were determined to establish appropriate squat height used in the FI squat (14,18). The subject assumed a standing position and the distance from the greater trochanter to the floor was measured. The subject then squatted down until the upper thigh was parallel to the floor and a second measure was taken from the greater trochanter to the floor. The difference between standing and parallel squat values was determined and divided by 2 to establish the midpoint or the position for the half-squat.

The subject then stood in the squat rack with an Olympic bar positioned on the upper back. A tape measure was placed on the greater trochanter and stretched to the floor. The subject squatted until the computed half-squat distance measured from the floor to the greater trochanter was reached. Pins were inserted into the squat rack above and below the level of the Olympic bar. The pinholes were approximately 4 in. apart. From this position, the subject would perform the FI squat during the second session.

A 5-minute, low-intensity cycling warm-up at 1-kg resistance and self-selected pedaling rate was performed (Monark Ergomedic 894E Peak Bike, Vansbro, Sweden). Next, instructions on how to effectively perform the CMVJ were provided, and subjects practiced jumping. A 4-minute seated rest followed, and then subjects were asked to perform their maximal CMVJ. Three attempts were made, and the best of the 3 jumps was used as baseline data. Countermovement vertical jump heights were determined using the OPTO-Jump (Microgate Engineering, Bolzano, Italy).

Next, the 1RM in the parallel back squat was determined. A safety squat rack was used, and the 1RM was determined using standard methods (11). Results were multiplied by 150% to determine the appropriate weight for each subject on the FI squat. For example, if the 1RM in the parallel back squat was 300 lb, the subject would perform the FI squat with 450 lb.

Subjects practiced the FI squat technique with an unloaded Olympic bar until comfortable with the procedures. When subjects signaled they were ready, they were instructed to squat the bar upward as fast and forcefully as possible, impacting the bar into the upper cross pins (a movement of approximately 4 in.). The upper cross pins prevented the bar from traveling upward past the half-squat position. Although the subject pushed as forcefully as possible against the immoveable cross pins and rack (the squat rack was bolted to the floor preventing any movement), the investigator started a stop watch and yelled out “1 second, 2 seconds, 3 seconds, stop.” At this point, the subject lowered the bar and recovered.

Session 2

Subjects reported to the test facility having rested a minimum of 48 hours since their first visit. Subjects performed the same 5-minute, low-intensity cycling warm-up described above. Next, subjects performed 2 warm-up sets of parallel squats at 50 and 75% of the 1RM. After the warm-up, the squat rack was set up, so the cross pins were at the appropriate predetermined height, and the Olympic bar was loaded with 150% of the subject's 1RM. This process permitted the subject to rest 5 minutes before the actual FI squat began.

Subjects took their position in the squat rack. Because of the extreme loads, for added safety, all subjects were required to wear a weight lifting belt to support the lower back. When ready, subjects lifted the loaded bar and made contact with the upper cross pins for 3 seconds of maximal pushing effort. When the lift was completed, subjects removed the weight belt and immediately sat down on a bench where they remained for 4 minutes.

At 4 minutes after the FI squat, subjects performed a maximal CMVJ in the same manner as they did on day 1. After completing the first jump, subjects stood still for 1 additional minute and then performed a second maximal CMVJ (i.e., at 5-minutes post).

Statistical Analyses

A 2 × 3 (group × CMJ) repeated-measures analysis of variance was used to determine interaction between trained vs. untrained and CMVJ assessed 4 minutes after a cycling warm-up vs. Countermovement vertical jump assessed 4 and 5 minutes after a cycling plus FI warm-up. Bonferroni post hoc tests were employed to determine differences within groups and CMVJ heights.


Figure 1 illustrates that no significant difference (p > 0.05) in CMVJ was detected in the untrained group between the 2 warm-up conditions after 4 or 5 minutes. In contrast, resistance trained subjects significantly (p < 0.05; observed power: sphericity assumed equals 0.851; Greenhouse Geisser equals 0.735) increased CMVJ heights by 2.4 cm (+ 5.1%) 4 minutes after warm-up when an FI squat was added to cycling. This increase was maintained when subjects were retested at 5 minutes post-FI squat (+2.6 cm, + 5.5%).

Figure 1:
Countermovement vertical jump (CMVJ) height (centimeters) for trained and untrained men at 4 and 5 minutes post-warm-up of cycling plus functional isometrics (FI) squat as compared to a CMVJ performed 4 minutes after a cycling only warm-up. Resistance trained men significantly increased CMVJ at both 4 and 5 minutes post-FI squat (*p < 0.05). Countermovement vertical jump results for untrained men at both 4 and 5 minutes showed a nonsignificant decrease.


The primary purpose of this study was to investigate the effect of a 3-second FI squat performed at 150% of the 1RM on CMVJ performance by resistance trained and untrained men. Although FIs have not been used previously in PAP studies, the theoretical basis of combining supramaximal dynamic muscle actions with maximal isometric muscle actions supports the potential for hyperstimulation of the neuromuscular system. This led to the hypothesis that there would be a substantial impact of FI to increase the PAP response that, in turn, would lead to enhanced performance on the CMVJ. This hypothesis was supported in the present study for resistance trained men who demonstrated a 5.1% increase in CMVJ after 4 minutes and 5.5% increase after 5 minutes compared with their CMVJ after a warm-up with no FI squat performed on a separate day. No significant effect was observed in the untrained men.

Because of several interacting factors that may affect PAP response, it is difficult to compare PAP studies (8,23). However, a comprehensive summary of PAP studies (23) suggests a 2-10% increase in performance because of prior muscle actions intended to stimulate a PAP effect. Past PAP studies have employed a dynamic movement such as heavy squats or bench presses (6,9,10,13) as a means to elicit a significant PAP effect. Others have used maximal isometric actions (5,7,20,24,25). To our knowledge, this is the first study that used FIs (i.e., a combination of dynamic and isometric muscle actions) to elicit a PAP response. Our results of a >5% increase in CMVJ post-FI compare favorably to results of other PAP studies (23). Tillin and Bishop (23) suggest that different types of muscle actions (e.g., isometric, dynamic) elicit different effects on subsequent explosive activities. Based on this observation, one may speculate that an additive effect may be possible when combining isometric and dynamic muscle actions with supramaximal loads in trained men. Unfortunately, a key limitation of this study is the lack of comparison groups for isometric and dynamic squats alone, making it impossible to confirm this speculation.

As previously stated, FIs in this study had no significant effect on CMVJs in untrained men. This supports our hypothesis and agrees with previous studies indicating that stronger, resistance trained individuals, when compared with those who are untrained and weaker have the ability to create a greater neuromuscular response after performing a brief but highly intense muscular activity (1,6,23). In this study, the trained subjects had an average parallel back squat of 1.7 times bodyweight vs. 1.3 times bodyweight in the untrained subjects. A parallel squat of 1.7 times bodyweight does not reflect the abilities of highly trained strength and power athletes, reflecting a limitation in generalizability of these results.

Timing is also considered to be an important element with regard to the impact of PAP on subsequent performance because of the complex interaction of fatigue and recovery (23). We hypothesized that resting 4 minutes would provide a greater effect than 5 minutes. Actually, in resistance trained mens, there was a slight, nonsignificant increase from minute 4 to 5 (+0.2 cm). Because of the unique nature of this FI study, and the wide variety of methods employed in other studies to stimulate PAP, it is difficult to make comparisons related to timing (23), but results due suggest that a favorable PAP effect was maintained for at least 5 minutes post-FI muscle action.

It is recommended that future studies expand the scope of the present investigation by examining systematically in resistance trained men and women-(a) the separate stimulatory influences of dynamic or isometric muscle actions and comparing these with the combined influence of both incorporated in FI on PAP and subsequent performance and (b) the impact of post-FI timing on PAP over an expanded time range (e.g., 1-8 minutes).

Practical Applications

In this study, weight trained men performing a FI squat significantly increased CMVJ height. These results suggest that FI squats may be beneficial in a couple of ways. First, it may be an advantage to incorporate FIs into a periodized training program as a means of eliciting a heightened PAP effect for subsequent explosive training work performed in that session. However, this potential application needs confirmation. Second, those who compete in explosive power sports may benefit from performing acute highly stimulating functional isometrics 4-5 minutes before competitive bouts that may lead to enhanced performance.


The authors acknowledge Dr. JP O'Shea for his contributions to this work. The results of this study do not constitute endorsement of any product by the authors or the NSCA. The authors have no conflict of interest related to this study.


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postactivation potentiation; power output; training status

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