Balance, agility, and change of direction ability are associated with athletic performance (7), and also with fall risk in the elderly (15). Particularly, power training has been raised as an interesting approach for improving postural balance (8,13). Power training is conventionally executed with either plyometric training or resistance training (9) and the gains in jumping performance have been in the order of around 8% in 3 months (1,19), whereas little or no gains in change of direction performance have been attained (7).
Among nonconventional power training methods, excellent results have been obtained with hypergravity (i.e., weighted vest) training, where improvements of 10% and even more have been obtained in 3- to 10-week interventions in athletes (4–6,16,21,22). Furthermore, it is of note that the majority of the aforementioned hypergravity interventions were conducted on trained athletes, in whom reduced training responses are usually to be expected (18).
Weighted vest interventions have been shown to be effective also in sedentary and elderly people in longer interventions (2,10,14,17,23). Moreover, balance improvements have been reported in postmenopausal women after a weighted vest intervention (23). The interventions with sedentary subjects have used additional training programs in addition to the weighted vests.
Because of the functional nature of the loading, hypergravity training may have the potential to improve change of direction ability. Moreover, hypergravity training compares favorably to conventional power training, in terms of improving jumping ability (4–6,16,21,22). Khlifa et al. showed, in a 10-week intervention with adult male basketball players, that wearing weighted vests during plyometric exercises accentuates performance gains compared with a control group doing the same plyometric exercises without the hypergravity. Consequently, Khlifa et al. suggested that wearing the vests only during exercise suffices (16). The independent effect of imposing hypergravity during nonexercise activities remains unexplored, however.
To shed light on the training independent effect of weighted vests, the purpose of this study was to investigate if hypergravity alone, imposed by weighted vests in daily activities excluding sporting activities, is feasible and effective in improving neuromuscular performance in young untrained men. It was speculated that wearing weighted vest during daily activities without any specific training program could be a sufficient stimulus to improve neuromuscular performance in young untrained men.
Experimental Approach to the Problem
The question of whether hypergravity alone is effective in improving neuromuscular performance and agility in young untrained men was assessed by asking the subjects to wear a weighted vest suit (4- or 6-kg suit, Hasanen Power Wear, Helsinki, Finland) during waking hours excluding sporting activities for 3 d·wk−1 for 3 weeks. The weighted vest weighed 5.6% of the body mass of the subject on an average. The intervention was based on the literature reporting positive results between 5 and 10% extra weight worn a minimum of 3 d·wk−1 (4,5,10,22). To assess the effect of weighted vest independent of training, the subjects were asked to take off the weighted vest for the duration of possible sporting activities. Performance and agility were assessed by the figure-of-8 running test, 10-m running time, and countermovement jump (CMJ) at baseline and at the 3 weeks' follow-up. Any differences between the groups observed in the change over the follow-up were attributed to the intervention. The experiment was carried out during summer. The number of subjects recruited was based on preliminary statistical power analysis. Interventions where the control and intervention groups differ only based on the use of weighted vest were used as the basis for the power calculations (i.e., 15% improvement with 10% SD, yielding a large effect size (ES) of 1.5. (4–6,16). The statistical power was calculated using the equation N = 2 × (1.96 + 0.84)/ES. The value of ES = 1.5 was used for the calculations, which yielded N = 7 for both groups for 80% statistical power.
A total of 20 young sedentary male volunteers were randomized into intervention (N = 10) and control (N = 10) groups using a purpose made computer program (C++ source attached as a supplementary file). The subjects were inquired about their habitual physical activity and classified as either physically active or inactive. The subjects were classified as physically inactive if they reported up to 1 exercise session or an equivalent of up to 1 hour of exercise per week. To be included in the study, a maximum exercise frequency of 2 times per week was allowed. One subject from the control and 2 from the intervention group dropped out before completing the follow-up measurements. The group sizes of subjects completing the study were N = 8 for the intervention group and N = 9 for the control group. Descriptive characteristics for the subjects completing the study are given in Table 1. The study was conducted in agreement with the Helsinki declaration with the agreement of the local ethical committee. Written informed consent was obtained from all the subjects before the baseline measurements were taken.
Neuromuscular performance was assessed with the figure-of-8 running test, maximal running velocity test, and the CMJ. Body composition (muscle mass and fat mass) was measured with bioimpedance measurement (InBody 720, Mega electronics, Kuopio, Finland). The measurements were scheduled 3 weeks apart on the same time of the day on the same weekday for each subject, respectively. Nutrition and hydration before the measurements were not controlled. The root mean square coefficient of variation (CV) for fat mass was 6.2%, and the respective value for muscle mass was 1.4% in this study calculated from the control group baseline and end measurements 3 weeks apart. Corresponding 2-way mixed-model absolute agreement intraclass correlation coefficients (ICCs) for average measures were 0.98 and 0.99, respectively.
Performance Measurement Protocol
Body composition and anthropometric variables were measured first. Next, the subjects were asked to execute a standardized warm-up comprising (a) 10 minutes of jogging at a preferred pace, (b) six 20-m sprints with progressing intensity from 50 up to 90% of maximum effort, (c) 30 seconds of balancing on a balance board twice, and (d) 10 heel raises on a stair edge twice. Performance measurements were then conducted in the following order: (a) figure-of-8 running test, (b) running velocity test, and (c) CMJ. The subjects were instructed to rest until they felt recovered. In practice, the recovery was 2–5 minutes.
Figure-of-8 Running Test
Agility was assessed with the figure-of-8 running test (12,24). Two poles were set 10 m apart, and the subject was asked to run 1 lap as fast as possible starting on the start-finish line next to one of the poles. The subject was then asked to run to the other pole, circle the pole clockwise, and to return to the first pole circling it counterclockwise before finishing. It was emphasized that full speed should be maintained through the start-finish line. The running was time measured with photocells (University of Jyväskylä, Finland) positioned at chest height on the start-finish line. Three to five trials were measured, and the shortest running time is reported. The 2 best trials were required to be within 5% of each other. The CV for running time was 1.3% in this study calculated from the control group baseline and end measurements 3 weeks apart. The corresponding ICC was 0.98.
Running Velocity Test
Running velocity was measured from a 10-m flying start maximal running velocity test. A 10-m run up was allowed from standing start, and the subjects were asked to run as fast as possible. Running time was measured using photocells (University of Jyväskylä, Finland). The photocells were positioned 10 m apart at hip height. Running velocity in meters per second was calculated as the distance covered (i.e., 10 m) divided by the running time. Two trials were measured, and the faster running velocity is reported. The CV for the running velocity was 1.1% in this study calculated from the control group baseline and end measurements 3 weeks apart. The corresponding ICC was 0.98.
A continuous trace of vertical ground reaction force (GRF) (sampling frequency 2,000 Hz) was measured during maximal CMJ. The subjects were instructed to perform the CMJ with their hands on their hips with their preferred countermovement depth and speed on a custommade force platform (University of Jyväskylä, Finland). A few practice attempts were allowed during which the subjects were instructed to avoid excessive movement of the upper body in the sagittal plane. Three to five trials were measured. The testing was stopped when the 2 best trials had a jump height within 5% of each other, and the results from the highest jump are reported.
Countermovement Jump Data Analysis
The measured GRFs were low pass filtered at 20 Hz using a digital zerolag second-order Butterworth filter. Maximal power was extracted from the GRF curve following the principles reported by Runge et al (20). Briefly, the weight of the subject was subtracted from the recorded vertical GRF value. The remaining reaction force was then divided by the body mass of the subject to produce vertical acceleration. Thereafter, instantaneous vertical velocity of the center of mass was calculated as the sum of acceleration data points multiplied by the inverse of sampling frequency from the beginning of the countermovement until the corresponding time point. Instantaneous power was then calculated as the product of the corresponding instantaneous force (including body weight) and velocity values. Peak instantaneous power normalized with the mass of the subject was selected to represent power production of the lower body musculature. The GRF analysis was conducted using a custommade C++ program (compiled with the GCC 4.5.0 compiler [http://gcc.gnu.org/] under the mingw [http://www.mingw.org/] environment, the source code is submitted to the journal as a supplementary file). Filter coefficients for the digital filtering were obtained using interactive digital filter design (http://www−users.cs.york.ac.uk/fisher/mkfilter/). Zerolag filtering was achieved by first passing the signal through in forward direction and then the resulting filtered signal again in the reverse direction. The CV for peak normalized power was 4.2% in this study calculated from the control group baseline and end measurements 3 weeks apart. The corresponding ICC was 0.95.
Results are given as mean (SD). At the baseline, the groups were compared with each other using independent t-tests. Body mass normalized peak power in the CMJ, running velocity in the 10-m running test, and running time in the figure-of-8 test were selected as outcome variables. The results for the other variables are reported without statistical comparisons. Association of the outcome variables at the baseline was tested with Pearson correlation coefficient for both the groups pooled. The associations between the changes of the outcome variables over the intervention were tested for the intervention group only. For the overall intervention results, the outcome results of the intervention group were compared to the respective control group results with multivariate analysis of variance (MANOVA) using measurement time (baseline and end) as a within-subject factor and group (intervention or control) as a between-subjects factor. For univariate comparisons, the follow-up values of the groups were compared with each other with analysis of covariance with the baseline value as a covariate. Statistical analyses were conducted with PASW Statistics 18.0.0 (SPSS Inc., Armonk, NY, USA) software and the significance level was set at p ≤ 0.05.
At baseline, the groups did not differ from each other (MANOVA between groups for all the reported variables, p = 0.828) in terms of descriptive characteristics (Table 1) or neuromuscular performance (Table 2). For the outcome variables, MANOVA indicated statistically significant group by time interaction (F = 5.1, p = 0.015), whereas the groups (F = 2.4, p = 0.110) or the time points (F = 3.0, p = 0.069) did not differ from each other. In the univariate analyses, the intervention improved the figure-of-8 running time (−2.2 vs. 0.5%), whereas normalized peak power (0.0 vs. 1.6%) and running velocity (1.3% vs. 0.1%) were unaffected by the intervention (Table 3). The measured values for variables not tested statistically are given in Table 3.
Association Between Outcome Variables
The outcome variables for both the groups pooled were strongly associated at baseline (r = 0.40–0.57, p ≤ 0.006). The changes in the outcome variables over the intervention were not associated to a statistically significant extent (r = 0.02–0.43, p ≥ 0.076).
The main finding of this study was that hypergravity alone, imposed by weighted vests in daily activities was effective in slightly improving agility (as measured with the change of direction figure-of-8 test) in young men. However, the improvement was relatively small and, in contrast to that in previous studies (4–6,16), not seen in jumping performance. Previous studies using training programs in addition to imposing hypergravity by wearing a weighted vest have shown that wearing a weighted vest is an effective way of augmenting the training stimulus (2,4–6,10,14,16,17,21–23).
The purpose of this study was to investigate (a) whether simply wearing weighted vest during daily activities excluding sporting activities is a sufficient stimulus for performance improvements and (b) what is the independent role of hypergravity. Apparently (close to), the minimal effective stimulus for triggering adaptations leading to performance enhancement was identified in this study. Furthermore, the independent effect of hypergravity in triggering adaptation is apparently limited. As suggested by Khlifa et al., we also speculate that most of the substantial performance gains reported in the literature (4–6,16,21,22) are probably mediated by the augmented load during the regular training curriculum of the athletes (16). Consequently, it may be questioned whether much benefit is reaped by imposing hypergravity during daily activities other than exercise.
Combining the literature results and the results of this study it appears that there may be a dose-response relationship between the intensity of physical activity while wearing the vest and the change in performance. In the only reported intervention, where no gain was achieved by wearing weighted vest, the vest was worn for 2 h·d−1, 4 d·wk−1 for 27 weeks by ambulatory elderly subjects (11). In their 20-week pilot study, Greendale et al. had found indications that the studied population is responsive to weighted vest augmented training conducted once a week for 1 hour per session (10). The improvement in performance observed in this study is in the lowest end of the spectrum among studies reporting improved performance (2,4–6,10,14,16,17,21–23), whereas arguably, also the physical activities done wearing the vest were also from the lightest end of the spectrum, because the vest was removed for the duration of any sporting activities.
This study has some limitations that should be considered when interpreting the results. The daily physical activity of the participants was not monitored, and consequently it is also not known, whether the participants' physical activity patterns changed during the intervention. However, the literature on body weight and spontaneous physical activity seems to indicate that hypergravity is likely to decrease spontaneous physical activity, which would lead to performance detriments instead of gains. Overweight people use, in absolute terms, similar amounts of energy in physical activities compared with lean peers, but because of the increased energy cost of moving their excess weight spend the energy in a shorter time and consequently are less active (3). Furthermore, the compliance of weighted vest suit use was not monitored. The intervention group was simply asked to wear the suit as described in the Methods section, but no attempt was made to find out whether they wore the suit or not. There is a relative paucity of weighted vest intervention results available in the literature, making the results of the intervention unpredictable a priori. The nonathletic study population was therefore used in this study. Typically, the performance gains after an intervention differ between nonathletes and trained athletes (18) and consequently the results may not be generalizable to athletic populations.
In conclusion, wearing weighted vests in daily activities excluding sporting activities was effective in slightly improving agility-related performance in young men.
Conventional power training interventions have been shown to improve change of direction performance to limited extent, whereas interventions including change of direction specific exercises have produced better results (7). Hypergravity training with sufficient volume and intensity has been shown to be effective in improving jumping ability (4–6). The present hypergravity intervention improved change of direction performance slightly, and therefore, it is speculated that with proper combination of volume and intensity both the jumping performance and the change of direction ability may be improved with weighted vest augmented training. Because only slight performance improvements were observed, the suggestion that wearing the vest only during exercise sessions suffices (16) seems to have been confirmed. The additional benefit on change of direction ability compared with conventional power training could be expected to be especially beneficial for athletes participating in ball games. Based on the existing literature, it appears that a proper volume and intensity could be in the order of 5–10% body weight vest worn during training sessions for a period of 3–4 weeks. It is to be noted that overuse injuries have been reported in female athletes (22).
This study was financially supported by the Academy of Finland (grant #138574). Dr. Rantalainen is supported by a grant from the Finnish Cultural Foundation given by the Foundations' Post Doc Pool. All the authors have no conflicts of interest. The authors would like to thank Mr. Pertti Hasanen from Hasanen Power Wear for supplying the weighted vest suits used in the present study.
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