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Influence of Compression and Stiffness Apparel on Vertical Jump Performance

Wannop, John W.; Worobets, Jay T.; Madden, Ryan; Stefanyshyn, Darren J.

The Journal of Strength & Conditioning Research: April 2016 - Volume 30 - Issue 4 - p 1093–1101
doi: 10.1519/JSC.0000000000001195
Original Research
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Wannop, JW, Worobets, JT, Madden, R, and Stefanyshyn, DJ. Influence of compression and stiffness apparel on vertical jump performance. J Strength Cond Res 30(4): 1093–1101, 2016—Compression apparel alters both compression of the soft tissues and the hip joint stiffness of athletes. It is not known whether it is the compression elements, the stiffness elements, or some combination that increases performance. Therefore, the purpose of this study was to determine how systematically increasing upper leg compression and hip joint stiffness independently from one another affects vertical jumping performance. Ten male athletes performed countermovement vertical jumps in 8 concept apparel conditions and 1 control condition (loose fitting shorts). The 8 apparel conditions, 4 that specifically altered the amount of compression exerted on the thigh and 4 that altered the hip joint stiffness by means of elastic thermoplastic polyurethane bands, were tested on 2 separate testing sessions (one testing the compression apparel and the other testing the stiffness apparel). Maximum jump height was measured, while kinematic data of the hip, knee, and ankle joint were recorded with a high-speed camera (480 Hz). Both compression and stiffness apparel can have a positive influence on vertical jumping performance. The increase in jump height for the optimal compression was due to increased hip joint range of motion and a trend of increasing the jump time. Optimal stiffness also increased jump height and had the trend of decreasing the hip joint range of motion and hip joint angular velocity. The exact mechanisms by which apparel interventions alter performance is not clear, but it may be due to alterations to the force-length and force-velocity relationships of muscle.

Human Performance Lab, Faculty of Kinesiology, University of Calgary, Calgary, Alberta, Canada

Address correspondence to John W. Wannop, b.wannop@ucalgary.ca.

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Introduction

Apparel that applies compression to the soft tissues of the body has been used for many different functions ranging from medical treatment of patients with vascular/lymphatic conditions (1,4,6,10,17) to augmentation of athletic performance (11,13). Over the past decade, the prevalence of research into sport apparel, specifically compression apparel, has increased exponentially (18). Previous studies have demonstrated that compression apparel has many positive benefits such as promoting blood flow (20,25,27), increasing skin temperature (14,15), increasing joint awareness, or proprioception in addition to a reduction in muscle vibration (23). In terms of direct measures of sport performance while wearing compression apparel during jumping, results have been inconclusive.

In the literature, only one study has shown a direct increase in maximum vertical jump height when athletes wore compression apparel as opposed to loose fitting shorts (13). Other studies have shown that wearing compression apparel can provide some positive influences during vertical jumping, with increased power production during a countermovement jump (22) in addition to athletes being able to maintain power production during a fatiguing protocol (23,24). Conversely, some studies have failed to show any effect of compression apparel on vertical jump height (2,5). It has been speculated that in these studies in which no performance benefit of compression was observed, that the amount of compression used in the study may have been insufficient. During compression studies, it is common for authors to select the amount of compression based on the manufacturer's guidelines, which usually consists of a combination of chest, hip, and thigh measurements (5,13,22–24). The size of the compression garments does have a significant influence on the amount of pressure that is exerted on the soft tissues (12). Therefore, the rationale behind this sizing practice or whether this method provides an adequate amount of compression required to elicit a performance benefit is not clear and its effectiveness remains to be demonstrated.

Recently, sport companies have begun to incorporate stiffness elements into their compression apparel (e.g., adidas Techfit PowerWeb shorts). Simply wearing compression apparel has been shown to increase the hip joint stiffness (Doan et al. 2003); therefore, these hybrid apparel products not only compress the soft tissue but also potentially alter the hip joint stiffness of the athlete substantially. Joint stiffness has been believed to be important for running and jumping performance (3,16,26); however, no studies have examined the effect that augmenting joint stiffness with apparel may have on performance. Similarly, with compression apparel, it is not known whether it is the compression of the soft tissues, the increase in hip joint stiffness, or some combination of these 2 variables that leads to increases in performance. At present, what amount of compression and/or stiffness is required for optimal performance is unknown. For athletes striving to attain peak performance information on the influence of compression and stiffness, apparel is lacking but is needed to assist both athletes and coaches in making informed decisions on what they wear during both training and competition. Therefore, the purpose of this study was to determine how systematically increasing upper leg compression and hip joint stiffness independently from one another affects vertical jumping performance.

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Methods

Experimental Approach to the Problem

To determine the influence that upper leg compression and hip joint stiffness independently have on vertical jump performance, 2 apparel conditions were selected: one apparel condition consisted of alterations in the amount of upper leg compression while minimally altering the hip joint stiffness while the other apparel condition altered the hip joint stiffness while minimally altering the compression of the soft tissues. The influence of each apparel condition on vertical jumping performance was determined as athletes performed countermovement jumps in each condition, whereas concurrent kinematic measurements provided insight into the mechanism by which each apparel condition influences performance.

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Subjects

Ten adult male competitive, recreational athletes participated in the study (mean ± SD: height = 1.80 ± 0.07 m, mass = 76.5 ± 8.2 kg). All participants were 18 years of age and older. To be included in the study, athletes had to be familiar with a countermovement jump and regularly involved in a recreational jumping sport (volleyball or basketball). Before testing, informed written consent was obtained from each subject in accordance with the University's Ethics Committee.

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Procedures

Eight concept apparel conditions and 1 control condition were tested during 2 separate testing sessions. The eight apparel conditions consisted of 4 that specifically altered the amount of compression exerted on the thigh. This was done by using compression shorts of different sizes (from smallest to largest, size XS, S, M, and L) composed of 75% Nylon and 25% Spandex. The remaining 4 apparel concepts specifically altered hip joint stiffness by using elastic thermoplastic polyurethane (TPU) bands (Figure 1). This was done using a harness that was anchored to a waist belt and by leg straps attached to the distal thigh of each leg, just above the knee. The belt and leg straps were connected by adjustable TPU bands that originated at the posterior midline of the waist belt and inserted on the posterior midline of each leg strap. To alter the hip joint stiffness, the number of TPU bands connecting the waist belt and leg straps was altered. The 1X harness had 1 band, 2X had 2 bands, the 3X had 3 bands, and the 6X had 6 bands, all attached in parallel to each leg strap. The control condition consisted of loose fitting adidas Climalite shorts.

Figure 1

Figure 1

Jump performance testing was broken into 2 testing sessions, with one session testing the control and the compression apparel and the other session testing the control and stiffness conditions. The sessions were separated by a minimum of 2 days to reduce the effect of fatigue on the athlete, and the order of the sessions was randomly assigned. During testing, athletes executed a countermovement squat jump, which was defined with the athlete starting from an erect position, performing a downward movement by flexing the knee and hip joint, before starting to push off and extending the knee and hip joint (8). The goal of the countermovement jump was for the athlete to reach as high on the Vertec jump height measurement system (Sports Imports, Hilliard, OH, USA) as possible. After a warm-up of practice jumps, 2 trials in each condition were recorded. During all testing sessions, the athletes were given a minimum of 1-minute rest between trials and the order that the conditions were tested was randomly assigned for each athlete. Kinematic data of the hip, knee, and ankle joint were collected with a high-speed camera (Casio EX-FH25) collecting at 480 Hz. The hip joint was represented by placing markers on the lateral aspect of the knee joint, defined as the axis of rotation of the knee joint for flexion and extension, the greater trochanter, defined as the axis of rotation of the hip joint for flexion and extension and along the most lateral aspect of the upper body, which was placed 20% of the way from the iliac crest to the humeral head. These markers were placed such that when the athlete was standing fully erect, the 3 markers formed a straight vertical line. Additional markers were placed on the lateral malleolus and the base of the fifth metatarsal, with the knee joint being defined using the 3 markers of the greater trochanter, the lateral aspect of the knee and the lateral malleolus, and the ankle joint was defined using the 3 markers of the lateral aspect of the knee, the lateral malleolus and the base of the fifth metatarsal. The upper body marker was marked with a permanent felt pen to ensure accurate placement of the marker for the second testing session. The high-speed camera was placed lateral to the athletes to capture the movement of these markers in the sagittal plane, and the coordinates of the markers defining the hip, knee, and ankle joint were tracked using Dartfish software (Dartfish, Fribourg, Switzerland). In addition to joint angles, jump time was analyzed, defined as the time of initiation of the countermovement to the time when the athlete left the ground, which was determined by visual inspection.

During the stiffness testing sessions, the lower leg straps were attached to the distal thigh superior to the knee joint. The lower leg straps were marked with a felt pen to ensure that no movement occurred when altering between stiffness conditions. The TPU straps on each leg were tightened such that when the athlete hyperextended their hip joint to the maximum amount of their range of motion, the straps were taut.

After each jumping session in each condition, the passive hip joint moment of the athlete was measured using a Biodex II dynamometer (Biodex Medical Systems, NY, USA). The athlete stood fully erect with their hip joint center at the same height and parallel with the rotational axis of the dynamometer. A leg attachment was adhered to the athlete's thigh, slightly superior to the knee joint. The placement of the leg pad was marked with a felt pen to ensure that placement was exactly replicated in each apparel condition. The athletes then flexed their hip joint to an angle of 90°. The Biodex was locked in this position, and the athlete was told to completely relax their flexed leg. The mean passive hip moment or the mean moment that the flexed leg imparted on the hip joint was recorded by the dynamometer. This procedure was conducted for each condition with 3 trials per condition being performed. This represented the passive stiffness or additional moment that each different apparel condition provided to the hip joint.

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Statistical Analyses

Pilot testing was performed to obtain data for a sample size calculation, comparing vertical jump height of 5 athletes when jumping in a compression apparel (sized medium) compared with a loose-fitting short (control). From these data, the mean difference in condition means was 2.286 and the SD of the differences was 2.087. Therefore, at an alpha level of 0.05, and power of 0.8, sample size calculations determined that 9 athletes (9 pairs) would be sufficient for the study.

For data analysis, the average of the 2 jump trials in each condition was calculated and comparisons were made between each compression or stiffness condition and the control using a paired t-test with a Holm-Bonferroni sequential correction (19) to account for multiple comparisons (α = 0.05). Therefore, after sequential correction was applied to the ranked (lowest to highest) p values, significant differences were determined if α1 < 0.0083, α2 < 0.01, and α3 < 0.0125 and α4 < 0.0167, α5 < 0.025, and α6 < 0.05.

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Results

Reliability data of jumping in each condition for both the compression and stiffness testing session is shown in Table 1. All conditions during the countermovement jump were highly repeatable.

Table 1

Table 1

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Compression

Across the compression conditions, there was no single condition that provided optimal performance for all athletes. Consequently, we performed a secondary analysis in which we regrouped athletes based on their best performance and compared this with the control condition.

The performance and passive hip moment results of the athletes wearing the compression apparel are shown in Table 2. There was no mechanical effect of the athletes wearing the compression apparel at the hip joint, as the passive hip moment did not significantly change when compared with the control condition. There was a trend of an increase in the jump time while the athletes were wearing the medium and small compression apparel as well as when grouped based on the compression apparel with which they performed best compared with the control (p = 0.054, 95% CIDiff = −0.0014 to 0.1198 for the medium and p = 0.076, 95% CIDiff = −0.0055 to 0.0885 for the small both compared with the control). Significant jump and reach height increases were only observed when the athletes were grouped based on the compression apparel in which they had their best performance (308.7 cm compared with 306.8 cm in the control, p = 0.005, 95% CIDiff = 0.58–3.27).

Table 2

Table 2

The kinematics of the hip, knee, and ankle joint are shown in Table 3. During the countermovement jump, the athletes had a significantly greater peak hip flexion angle while wearing the medium compression apparel compared with the control (89.1° vs. 79.7°, p = 0.003, 95% CIDiff = 3.50–15.26). Additionally, when grouped based on the compression apparel with which they had their best performance, a significant increase in their peak hip flexion angle was observed (88.7° compared with 79.7° for the control, p = 0.004, 95% CIDiff = 3.20–14.80). No differences were seen in regard to the peak hip extension angular velocity or in regard to the angle or angular velocity of the ankle or knee joint.

Table 3

Table 3

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Stiffness

Similar to the compression apparel, across the stiffness conditions, there was no single condition that provided optimal performance for all athletes. Consequently, we performed a secondary analysis in which we regrouped athletes based on their best performance and compared this with the control condition.

The performance results of the athletes while wearing the stiffness apparel are shown in Table 4. All stiffness apparel resulted in a significant increase in the passive hip moment compared with the control condition (p values were as follows: 1X = 0.004, 95% CIDiff = 3.60 to 6.02, 2X = 0.003, 95% CIDiff = 5.88 to 9.30, 3X = 0.004, 95% CIDiff = 6.92 to 11.78, 6X = 0.004, 95% CIDiff = 12.74–21.32). As the stiffness of the apparel increased, the mechanical effect on the hip joint increased, with the 6X apparel having the largest effect, increasing the passive hip joint moment by 17 Nm or ∼30% compared with the control on average. The stiffness apparel did not have any influence on the time taken by the athletes to complete their jump.

Table 4

Table 4

The kinematics while jumping in the stiffness apparel are shown in Table 5. While wearing the stiffness apparel, athletes had a trend of reducing the peak hip flexion angle in the 1X, 2X, and 3X conditions (p = 0.052, 95% CIDiff = −13.31 to 1.49 for 1X, p = 0.049, 95% CIDiff = −17.78 to −1.80 for 2X, and p = 0.032, 95% CIDiff = −20.02 to −0.65 for 6X), whereas the stiffest apparel (6X) significantly reduced the peak hip flexion angle compared with the control (72.4° compared with 84.1°, p = 0.003, 95% CIDiff = −19.36 to −4.11). As the stiffness of the apparel was increased, there was the trend of a reduction in the peak hip joint extension angular velocity, specifically in the 3X and 6X conditions (p = 0.087, CIDiff = −3.07 to 98.32 for 3X and p = 0.015, CIDiff = 17.38 to 113.94 for 6X). No other differences were seen at the knee joint or the hip joint.

Table 5

Table 5

When the athletes were grouped based on the stiffness apparel with which they had their best performance, a significant increase in their vertical jump and reach height was achieved (307.6 cm compared with 306.5 cm, p = 0.005, CIDiff = 0.33–1.96). In their best stiffness condition, there were trends toward a reduction in the peak hip flexion angle (p = 0.022, CIDiff = −22.12 to −0.33) and a reduction in the peak hip extension angular velocity (p = 0.018, CIDiff = −95.47 to −5.12).

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Discussion

In recent years, compression apparel that incorporates stiffness elements has begun to increase in popularity. However, the influence of both compression and stiffness apparel on athletic performance has not been thoroughly examined. Specifically, the amount of compression and/or joint stiffness that is required to elicit potential performance benefits has not been examined in detail. The goal of this study was to determine how systematically altering the compression and stiffness of lower extremity apparel independently influences vertical jumping performance.

Wearing the compression apparel had no significant mechanical effect on the hip joint of the athletes; however, as the size of the apparel was decreased, there was the trend of a slight increase in the passive hip joint moment. This is in contrast to the results of Doan et al. (13), who found that compression apparel crossing the hip joint caused a substantial increase in the passive hip joint moment throughout a range of angles of hip flexion and extension. Although the amount and type of compression used in the 2 studies were undoubtedly different, further differences may be due to the fact that Doan et al. (13) completed all hip moment measurements on a mannequin outfitted with the compression apparel. The absence of any musculature or connective tissues on the mannequin resulted in increases of 53–285% of the hip joint moment by merely adding the compression apparel. When the musculature and connective tissue were taken into account by performing measurements on the athletes, the addition of the compression apparel only increased the passive stiffness of the hip joint by, at most, 13%.

Athletes were able to achieve their highest vertical jumps while wearing compression, with 7 of the 10 athletes having their best performance while wearing the medium-sized apparel, and although not statistically significant, displayed the trend of an increase in vertical jump performance. Although the majority of athletes had their best performance in this condition, not all athletes performed optimally in the same apparel condition. Some athletes performed best with higher or lower amounts of compression. It is important to take note that although the majority of athletes (9 of 10) had their best performance in one of the compression apparel conditions, the majority of athletes also had their worst performance in compression apparel as well (6 of 10). This indicates that not only can an optimal amount of compression increase performance, but suboptimal compression can reduce performance, lowering it below that of a loose-fitting short. It should be noted that although both S and XS had the trend of increasing the passive hip joint moment, these 2 compression conditions were the majority of athletes' worst apparel conditions. Therefore, even if the athletes were to receive a 13% increase in the passive hip joint moment by wearing the apparel (which was the mean increase while wearing the XS), their performance was still hindered.

When comparing the athletes' best compression condition with the control condition, an increase in jump height of ∼2 cm was seen, whereas no significant effect on performance was observed in the athletes' worst compression condition. These results indicate that the ability of compression apparel to supplement performance is dependent on the specific amount of compression for the specific athlete. It is speculated that the inconclusive results of previous compression performance studies may have been due to very individualized athletic responses to the magnitude of the compression (2,5). This notion is supported by the fact that the amount of pressure by which compression apparel exerts on the soft tissues is dependent on the sizing, posture of the athlete, and type of compression tested (12). These results indicate that compression apparel can either supplement or be a detriment to performance depending on its individual fit; therefore, future work should focus on determining how to properly classify athletes depending on the amount of compression required to enhance performance.

Although optimal compression apparel can enhance performance, the mechanisms by which compression acts to aid performance remain unknown. When the athletes were performing in their optimal compression apparel (best compression), the peak hip joint flexion angle was significantly increased, whereas in their worst compression condition, the peak hip joint flexion angle was unchanged from the control. The absence of a linear increase in peak hip joint flexion angle with increasing compression supports the notion of the individual-specific compression fit and subsequent performance benefits. Previously, studies have shown that increasing the depth of the squat or having a lower position of the body's center of mass during the initiation of the push off phase during jumping leads to an increase in vertical jump height (8,21). Additionally, this increase in squat depth during a countermovement jump (peak hip flexion angle) while wearing compression has been shown previously where a performance increase was also recognized (13). Therefore, it seems that increasing the depth of the squat (hip joint range of motion) is associated with an increase in vertical jump height, with optimal compression apparel providing the means to accomplish this.

It is currently not clear how increasing the depth of the squat leads to an increase in vertical jump height. It has been postulated that the action of the countermovement allows the athlete to build up the muscles' active state and force before shortening (8), which would permit the hip joint moment to start at a larger magnitude during the initiation of the concentric push-off phase of the jump, and a corresponding increase in the magnitude of the vertical ground reaction force at the start of push-off (21). Interestingly, although the athletes' hip joint range of motion was increased in the optimal compression condition, the peak hip joint angular velocity remained similar to the control. In addition, there was the trend of an increase in the time to complete the jump when the athletes wore their optimal compression apparel. The athletes were not flexing or extending their hip at a faster velocity, they were simply increasing the time to complete their jump to accommodate for the increase in the range of motion that occurred while wearing the optimal compression.

Although the means for the proposed increase in initial force development during the push-off phase of the jump is unknown, it could be related to the force-length (moment-angle) relationship of muscle. The increase in the peak hip joint flexion angle while wearing optimal compression may have shifted the athletes' hip extensors to a more advantageous region along the force-length curve, thereby allowing them to produce a greater amount of force during the initiation of the concentric phase of the push-off. Previous work has indeed shown that maximum voluntary hip extensor torque increases as the hip becomes more flexed (28). Further research is needed to determine the exact mechanism by which the increase in vertical jump height was accomplished for the optimal compression condition.

It is important to note, in regard to compression, that many other aspects may be a factor in terms of performance. Studies have shown that proprioception may be increased with compression, which may have contributed to the change in the range of motion (23). How the level of compression affects proprioception is unknown, and perhaps proprioception is optimized under a specific amount of compression. Furthermore, when examining compression, it is impossible to blind the participants regarding the condition in which they are performing. There is currently no placebo for compression as the participants can clearly feel the differences in the compression apparel compared with the control. The psychological effects of wearing the compression may have contributed to the increase in performance, as athletes may have performed best in the condition that felt best. Significant limitations were also present, specifically because of the fact that the compression apparel was not custom fit to each athlete. The different sizes of apparel fit the athletes differently based on their specific anthropometrics, and therefore each athlete would have experienced a different magnitude of compression, in addition to potentially different compression gradients. Unfortunately, the amount of compression that each garment applied to the leg was not measured in this study. Anthropometric data from the athletes were collected in this study; however, attempts to use these data to determine the required compression to enhance performance or a sizing metric was inconclusive, and therefore, these data were not presented.

Although from a mechanical standpoint, the stiffness apparel exerted a large influence on the hip joint, on average, no increase in jump height of the athletes was observed (there was a trend of increased jump height in the 6X condition). This was a surprising result because the stiffness apparel increased the passive hip moment as much as 30% (in the 6X apparel). The lack of performance increase may have been caused by the absolute passive hip moment increase of 17 Nm (in the 6X apparel) not being substantial enough to have a significant impact during the dynamic activity of jumping, in which hip joint moments can exceed 300 Nm (8). The passive hip moment added by the stiffest apparel would increase the hip joint moment by 5%, which would seem to be considerable enough to influence performance. Therefore, the inability of the athletes to systematically increase their maximum vertical jump height as stiffness increased may be due to the inability of the athletes to adapt their motor control activation patterns to take advantage of the additional mechanical output provided by the apparel (9). Simulation studies lend support to this notion, in which increases in leg muscle strength required a different optimal stimulation to realize the benefit of an increased jump height (9). Future studies may investigate whether supplementary training sessions in the stiff apparel result in a performance increase for the athletes, above and beyond the expected increase in jump height that may result from specific countermovement jump training.

Although the decrease in the peak hip flexion angle occurred while wearing the stiff apparel was not surprising, it was unexpected that this decrease in hip flexion did not negatively influence performance. As stated previously when discussing the compression apparel, previous studies have provided evidence that increasing the depth of the squat or having a lower position of the body's center of mass during the initiation of the push-off phase of the jump leads to an increase in vertical jump height (7,21). One major difference between this study and past studies, however, was in the time required to perform the jump. In the study by Bobbert et al. (7), when the athletes performed a jump initiated at a small hip flexion angle, a reduction of over 30% in the time required for the athletes to complete the jump was observed when compared with their self-selected jump. No difference in jump time was detected in this study, indicating that although the athletes were not flexing their hip to the same degree for the stiff apparel condition compared with the control condition, they were still performing the jump within a similar time frame. Interestingly, there was the trend of an increasing stiffness resulting in a decrease in the peak hip joint angular velocity.

It is currently unclear how altering the angular velocity of the hip joint could be beneficial to performance. The ability of the muscles to generate force dynamically is governed by the force-velocity relationship, which describes the fact that maximal force can be produced at the lowest concentric contraction velocities, and that peak power production occurs at an optimal combination of both parameters along the force-velocity curve (∼33% along the curve). Therefore, if power production in the control condition was not at the optimal amount, reducing the angular velocity of the hip joint may have shifted the athlete along the force-velocity curve to a more advantageous position, increasing hip extensor power output. Currently, this proposed mechanism is purely speculation; however, future work will aim to investigate its validity.

Further support for the force-velocity optimization mechanism was found when examining the stiffness conditions in which the athletes had their best and worst performances. Comparing the best and worst stiffness conditions, the best stiffness condition resulted in an increase in jump height, the trend of reducing peak hip flexion angle, the trend of reducing the peak hip joint extension velocity, and an increase in the passive joint moment. Conversely, the worst stiffness condition showed no change in jump height, the trend of reducing the peak hip flexion angle, an increase in the passive hip joint moment, and no change in hip extension angular velocity. The only measured variable that was different between the athletes' best stiffness condition and the worst stiffness condition was the angular velocity of the hip joint. This indicates that the best stiffness condition potentially optimized or controlled the angular velocity of the hip joint to increase the force production/power output of the hip joint extensors by shifting the athlete to a region on the force-velocity curve where greater power could be produced.

From the current data, it is not known whether the added hip moment of the stiffness apparel was a major influence in altering vertical jump performance, whether it was the change in kinematics and angular velocity, or some combination of the 2. During the stiffness conditions, the stiffness elements may have assisted in the reversing of the movement of the center of mass. Therefore, although a similar jump time to the control may have been present during the jumps, the fact that the center of mass did not travel as low as in the control may result in less work required to move the center of mass back to its neutral point, and therefore potentially more work being used to increasing jump height (i.e., movement of the center of mass beyond its starting point). Further studies need to be completed to examine the effects of altering the kinematics of vertical jumping without the intervention of the stiffness apparel.

It is important to note that all passive hip moment measurements were taken at 90° of hip flexion. During the jump, as the hip joint angle changed, it cannot be determined what the magnitude of passive hip moment was being supplied from the apparel. Because no kinetic data were recorded during this experiment, the influences of the stiffness apparel on the timings and magnitudes of the ground reaction force and joint moments are unknown. Similar to the compression conditions, there was no placebo for the stiffness apparel as the participants could clearly feel the differences in the conditions compared with the control. The psychological effects of wearing the stiffness apparel may have had an influence as well.

In summary, both compression and stiffness apparel can have a positive influence on vertical jumping performance. The increase in jump height for the optimal compression apparel condition was due to increased hip joint range of motion and a trend of increasing the jump time. Optimal stiffness apparel also increased the jump height and had the trend of decreasing the hip joint range of motion along with the hip joint angular velocity, in addition to increasing the passive hip joint moment. Compression and stiffness apparel therefore seem to increase jumping performance through different means. At present, the exact mechanism by which these apparel interventions alter performance is unclear; however, the results observed in this study suggest alterations to the force-length and force-velocity relationships of muscle resulting in increased power output of the hip extensors in the optimal compression and stiffness conditions. In apparel that includes both compression and stiffness elements, it is not clear whether there will be a synergistic/additive effect or whether these alterations cancel each other. Further research is required to determine whether changes in performance were due to changes in the kinematics of the movement and could be accomplished without the compression or stiffness apparel, or whether the mechanical influence of the 2 apparel items were necessary for the mechanisms underlying the improved performance of the athletes.

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Practical Applications

Compression and stiffness apparel can influence individual vertical jumping performance; however, proper care must be used in selecting the correct apparel for the individual athlete. Altering the soft tissue compression on the lower legs, as well as the hip joint stiffness, can enhance athletic performance; however, inappropriate compression or hip joint stiffness can hinder performance, decreasing performance below that of a loose-fitting short. Coaches should ensure that athletes are matched to the optimal compression and stiffness apparel to enhance performance during competition by potentially optimizing an individual athletes' force-length and force-velocity properties of muscle.

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Acknowledgments

This project was supported by adidas Future Team. adidas had no involvement in data collection or in the interpretation of any of the results.

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Keywords:

Biomechanics; countermovement; muscle

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