Running economy (RE) is a metric of efficiency and is commonly defined as submaximal oxygen consumption (milliliters per minute per kilogram) at a given velocity (2). As such, any improvement in the RE should translate into performance improvements because of decreased oxygen consumption for a given distance, sparing of muscle glycogen, decrease in fatigue, and the ability to sustain higher velocity.
Resistance and plyometric training have frequently demonstrated improvements in the RE. For example, Turner et al. (19) have shown that a 6-week plyometric training regimen improved the RE in recreational male and female runners at some but not all running speeds, without significant changes in the V[Combining Dot Above]O2max. Likewise, Johnston et al. (7) found a significant improvement in the RE (4%) after 10 weeks of resistance training in female distance runners. Paavolainen et al. (14) demonstrated that 9 weeks of sport-specific explosive training decreased 5k run time in elite male crosscountry runners despite there being no change in the V[Combining Dot Above]O2max. Similarly, Spurrs et al. (18) and Saunders et al. (17) also found improved RE in experienced male middle and long distance runners of 5.7% (as an average of 3 different velocities) and 4.1%, respectively, after plyometric training regimens of 6 and 9 weeks, respectively. The aforementioned studies have, however, primarily investigated the effects of training on the RE in experienced runners, with a preponderance of participants being male. Interestingly, not all the investigations have corroborated an increase in the RE with strength training. A study conducted by Kelly et al. (9) investigated the effect of 10 weeks of a thrice-weekly heavy strength training program on the RE in female recreational runners. Although measures of strength improved, there was no significant change in the RE.
The effect of resistance and plyometric training on V[Combining Dot Above]O2max however remains equivocal. For example, Johnston et al. (7) found that a 10-week strength training program conducted thrice weekly in female distance runners had no effect on the V[Combining Dot Above]O2max, despite there being a significant improvement in the RE (4%). A more recent study investigated the effect of a 12-week full-body progressive resistance training protocol on young, healthy, but previously untrained, women (3). No significant changes in the V[Combining Dot Above]O2max were noted. Although there is ample evidence that indicates that improvements in the RE are attributable to resistance and plyometric workouts, a demonstrated improvement of V[Combining Dot Above]O2max is absent from the literature.
Soccer is an intermittent, high-intensity sport. Nevertheless, soccer relies primarily on aerobic metabolism for energetics, and it has been suggested that up to 98% of the total energy expenditure during game play is derived from aerobic metabolism (1). Aerobic endurance performance in soccer is governed by 3 interrelated mechanisms: V[Combining Dot Above]O2max, lactate threshold, and RE (15). Although the intermittent nature of the sport allows for periods of recovery, the average intensity remains high, in the range of 75–80% of V[Combining Dot Above]O2max (16). Consequently, maximal oxygen consumption has been suggested as the most important component of aerobic endurance performance in soccer (1), and there is evidence that indicates maximal aerobic power correlates with soccer success (16,20).
Although there is a general consensus in the literature regarding the efficacy of strength-plyometric training to improve the RE, there remains a dearth of evidence pertaining to studies on females and, more specifically, on female soccer athletes. Therefore, the purpose of this study was to investigate the effect of a 10-week combined strength-plyometric training protocol on V[Combining Dot Above]O2max and RE in female collegiate soccer players. To our knowledge, this is the first study to investigate the effect of such a training program on V[Combining Dot Above]O2max and RE in Division I female soccer players. We hypothesized that the RE would improve in the absence of a significant change in V[Combining Dot Above]O2max.
Experimental Approach to the Problem
To investigate the hypothesis of the study that a combined strength-plyometric training program would improve the RE in the absence of an increase in maximal oxygen consumption, a single-group pretesting-posttesting design was used. Fifteen Division I female soccer players were recruited and tested for V[Combining Dot Above]O2max, RE, time-to-fatigue during the V[Combining Dot Above]O2max protocol, speed of highest completed stage, and isometric strength of knee flexion and extension after the competitive season (PRE) and after a 10-week strength-plyometric-agility training program (POST). The training protocol was limited to conform to the constraints of the player's schedules and allow “spring league” training to occur on nonexercising days. Therefore, weekly training sessions were scheduled on a Monday-through-Thursday basis, leaving Fridays and Saturdays free for team play. The training regimen consisted of a resistance training program, conducted twice weekly on nonconsecutive days. Each session lasted approximately 60 minutes, was preceded by a brief warm-up/stretching session, and consisted of 9–10 exercises of 3 sets each. The plyometric-agility training portion was conducted twice weekly on different nonconsecutive days from the resistance training, and drills varied between days, with an emphasis on speed and quickness on 1 day followed by plyometric-agility training on a separate day.
The dependent variables were maximal oxygen consumption (V[Combining Dot Above]O2max), time-to-fatigue during V[Combining Dot Above]O2max protocol, interpolated maximal speed of highest completed stage during V[Combining Dot Above]O2max protocol, RE at 9 km·h−1, percentage of V[Combining Dot Above]O2max at 9 km·h−1, and isometric knee flexion and extension.
Fifteen National Collegiate Athletic Association Division I female soccer players participated in this investigation. All the subjects were between the ages of 18 and 20 years (19.0 ± 0.7 years; 1.67 ± 0.1 m) and were freshmen, sophomores, or juniors. Four subjects did not complete all testing because of withdrawal from the team (1), participation in a secondary university scholared athletic sport (1), injury (1), and failing to qualify for the team (1). Only the subjects who completed all the testing procedures were used in the statistical analysis. The study was approved by the local Institutional Review Board, and the subjects provided signed informed consent before their participation.
Height and mass were measured using a Detecto balance scale (Columbia, MD, USA) and body mass index was calculated. Body composition was estimated using a 3-site skinfold method (6).
A SensorMedics Vmax 29c metabolic cart (Yorba Linda, CA, USA) was used during pretesting and posttesting sessions to measure O2 consumption. The flow sensor was calibrated against a 3.0-L syringe and CO2, and O2 sensors were calibrated against known gases before each test. The mouthpiece and flow sensor were attached to a headpiece to collect expired air. Peak O2 consumption was calculated as the average of the 3 highest, continuous 20-second interval V[Combining Dot Above]O2 measurements. To more closely approximate the achievable maximal oxygen consumption during competition play, an incremental treadmill protocol that maintained a 0° incline was used throughout testing (4). The treadmill protocol began at a velocity of 9 km·h−1 for 5 minutes; thereafter, the speed was increased in increments of 1 km·h−1 every 2 minutes, until the subject was unable to maintain the pace. Running economy was calculated by averaging the V[Combining Dot Above]O2 values during the final minute of the first stage of 9 km·h−1. The respiratory exchange ratio (RER) was monitored throughout each test and averaged over 20-second intervals. A Polar heart rate (HR) monitor (Polar Electro, Finland) was used to collect resting and exercise HR measurements throughout all testing.
Maximal isometric strength measurements for knee flexion and extension were performed on the same day, after a period of rest, using a portable fixed dynamometer (BTE Evaluator, BTE Technologies, Inc., Hanover, MD, USA). Isometric testing followed the procedures used by Kollock et al. (10). Briefly, participants were fitted to a testing chair and secured via velcro straps across the torso and thigh. The participants were positioned such that the dynamometer was parallel to the direction of the isometric contraction and both the hip and knee were flexed to 90°. A nylon ankle strap was placed superior to the medial malleolus with the load cell fixed securely to the wall. Each subject performed 3 maximal muscular contractions for 5 seconds each, with a 10-second rest between trials (Table 1). The mean peak torque was calculated and normalized to body weight as
where force is in newtons, lever length is the tibial length from the lateral joint line of the knee to the apex of the lateral calcaneous (M) and body mass is in kilograms. To ensure reliability, maximal coefficient of variance was set at 10%. Strength measured with a portable fixed dynamometer has been shown to be reliable (10).
All the subjects participated in a combined resistance-plyometric-agility training program 4 d·wk−1 for 10 weeks during an off-season period. All pretraining values were obtained 2 weeks before the implementation of the training program. Posttesting was performed 4 days after the final training session. The 10-week training program took place over a period of 11 weeks, with a 7-day break during the seventh week to accommodate the university's spring break. All the subjects completed a minimum of 95% of scheduled training sessions. All the training sessions were directed and supervised by a certified strength and conditioning specialist. The resistance training portion of the program was conducted 2 d·wk−1 on nonconsecutive days and was preceded by a team warm-up of a 3-minute jog and individual stretching sessions. Each resistance training session was approximately 60 minutes and consisted of 9–10 exercises of 3 sets each (Table 2). The plyometric-agility training was conducted separately from the resistance training on different nonconsecutive days. Training sessions lasted approximately 60 minutes during the first 4 weeks of training and approximately 30 minutes during the final 6 weeks. All the sessions began with a dynamic warm-up of running form drills and range-of-motion exercises. All the drills emphasized maximum effort and power while maintaining proper body angles during acceleration and jumping. The focus of the plyometric-agility conditioning drills varied between days, with speed and quickness emphasized 1 day and plyometric and agility emphasized on alternate days. A representative sample is included in Table 3.
Descriptive statistics were compared using analysis of variance (ANOVA) with repeated measures. Four subjects did not meet the requirements of the study and were not included in the subsequent analysis. Statistical power analysis based on previous data demonstrated an n of 11 would yield a power ≥0.80 at an alpha <0.05 for the primary dependent variables (RE and V[Combining Dot Above]O2max) (7,12). A Bonferroni adjustment was used by dividing the p value by the number of ANOVAs used (n = 5). Eleven subjects participated in the PRE-POST training period. Data are presented as mean values ± SD. Significance for all the tests was set at p ≤ 0.05.
A significant increase in the V[Combining Dot Above]O2peak of 5.2 ml·min−1·kg−1 occurred after training (10.5%; p = 0.008,) (Figure 1) and a significant decrease in maximal RER during the V[Combining Dot Above]O2peak protocol (2.9%; p = 0.001). There was a significant increase in the interpolated maximal speed of highest completed stage (3.6%; p = 0.016) and time-to-fatigue during the V[Combining Dot Above]O2peak protocol (6.9%; p = 0.017). Although there was no significant effect of training on the RE at 9 km·h−1 (data not shown), there was a significant decrease in the V[Combining Dot Above]O2peak percentage (−5.6%; p = 0.02) at 9 km·h−1 (Table 4). There was no significant change in maximal isometric strength of knee flexion and extension (Table 1), and no significant differences were noted in body mass, body fat percentage, or maximum HR at baseline or posttesting (Table 4).
The purpose of this study was to determine the extent to which an off-season combined resistance-plyometric-agility training program would affect V[Combining Dot Above]O2max and RE in collegiate female soccer players. This investigation demonstrated a significant increase in V[Combining Dot Above]O2peak (10.5%), in the absence of a consistently significant increase in the RE at 9 km·h−1 after a 10-week training program. Furthermore, there was no significant change in maximal isometric strength of knee flexion or extension. Therefore, the present findings suggest that a combined resistance-plyometric training program implemented during the off-season may positively impact V[Combining Dot Above]O2peak in collegiate female soccer players. The effect on RE, however, was equivocal.
Why this investigation failed to demonstrate a clearly definable increase in the RE would seem contradictory to the majority of literature on the subject. The RE is best described as milliliters of O2 consumed per kilogram of bodyweight, and this was the technique used in the present investigation. However, when expressing the RE as a percentage of peak oxygen consumption, it becomes apparent that there may have been an effect on the RE. Additionally, when taking into consideration the significant changes in time to fatigue and interpolated maximal speed, it becomes clearer that there was likely an underlying effect. The nature of the inconsistent results could be the product of insufficient statistical power, increasing the risk of a sample size–related type 2 error. With a decrease of 5.6% of the V[Combining Dot Above]O2peak at 9 km·h−1 it is possible that the change in the RE, when expressed as milliliters per minute per kilogram, was inadequate to meet the threshold of significance.
Although research studies have demonstrated improvements in aerobic endurance and RE with resistance and plyometric training, previous trials that investigated the effect of resistance and plyometric training on maximal aerobic capacity have been equivocal and typically demonstrated a null effect of training on V[Combining Dot Above]O2max (14,18,19). Present findings, however, indicate a significant improvement in V[Combining Dot Above]O2peak/V[Combining Dot Above]O2max after training. To the investigators' knowledge, this is the first study to note significant improvements to V[Combining Dot Above]O2peak in female athletes after resistance and plyometric training. The noted improvements in V[Combining Dot Above]O2peak while being statistically significant are, more importantly, also clinically relevant. The increase of 10.5% is the equivalent of a gain in relative oxygen consumption of 5.2 ml·min−1·kg−1. Previously, it has been shown that Division 1A female soccer players have exhibited a significant decrease in the V[Combining Dot Above]O2max over the course of a competitive season (12). Therefore, it is possible that our investigation, which began at the end of a competitive season, initially assessed measurements of subjects that had incurred a deficit in the V[Combining Dot Above]O2max throughout their competitive season. Consequently, the relatively small amount of aerobic training (spring league) that occurred concurrently with the training program could have contributed to the improvement in the V[Combining Dot Above]O2peak.
The relationship between maximal aerobic capacity and soccer performance is of paramount importance (5). It has been previously documented that there are positive correlations between the V[Combining Dot Above]O2max, total distance covered during matches, and the amount of high-intensity running during competitive play in elite female soccer players (11). Moreover, it has been demonstrated that a 6.2 ml·min−1·kg−1 improvement in the V[Combining Dot Above]O2max was associated with a 1,700-m increase in distance covered during game play, a 20% increase in total distance covered, a 24% increase in ball contacts and a 100% increase in the number of sprints in elite male soccer players (5). Although there is ample evidence linking soccer performance and maximal oxygen consumption there is, more importantly, evidence suggesting an association between maximal oxygen consumption and soccer success. For example, there is a significant relationship between the V[Combining Dot Above]O2max and team success in an elite men's soccer league (20). Furthermore, it was suggested that an average team difference of as little as 6 ml·kg−1·min−1 would be the equivalent of having 1 extra player on the field during game play (20). Similarly, there were significant improvements in performance variables among male Norwegian junior elite teams attributable to a training protocol, which increased the V[Combining Dot Above]O2max by 6.2 ml·kg−1·min−1 (5). With this in mind, the demonstrated difference in the present investigation of 5.2 ml·kg−1·min−1 may constitute a significant contribution to performance and conferring a likely advantage in team success.
Soccer is characterized by intermittent activity ranging from walking to sprinting; however, it has been reported that the average intensity during matches for elite female soccer players is approximately 77% of the V[Combining Dot Above]O2max or, 87% of the HRmax (11). It was also reported that mean peak HR and peak oxygen consumption values for elite female soccer players during match play were 97% of HRmax and 96% of the V[Combining Dot Above]O2max (11), respectively, indicating that they each fall within the parameters of the lactate threshold for trained subjects (8). It was also demonstrated that elite female soccer players performed an average of 125 high-intensity runs and 26 sprints per game (1.31 and 0.16 km, respectively), which accounted for 14.3% of the total distance covered in match play (11). This suggests that soccer is a sport of varying intensity; however, the cumulative impact on the cardiovascular system is indicative of an intensity level that equates to, or frequently exceeds, the lactate threshold. Consequently, fatigue plays an important role in competitive soccer. For example, a study involving professional soccer players found significant decrements in low- and high-intensity running, and number of sprints between the first and second halves of game play, highlighting the role of fatigue in elite soccer players (13). Similarly, another investigation found high-intensity running distances covered in the final 15 minutes of each half, compared with the first 15 minutes of each half, decreased by 30 and 34%, respectively, for elite female soccer players (11). As such, any training modality that reduces effort and increases efficiency should translate into a competitive advantage.
Previous research demonstrated significant improvements in the RE in trained and untrained populations after resistance and plyometric training; the present investigation provides inconsistent data to corroborate this finding. Past research has not, however, consistently demonstrated improvements in the maximal oxygen consumption mediated primarily through resistance and plyometric training. This investigation diverges from previous research in that regard, indicating a significant increase in the V[Combining Dot Above]O2peak in division 1A female soccer players after a strength-plyometric training program. Considering the deficits of performance noted in the latter stages of a competitive soccer game and the importance of maximal oxygen consumption on soccer performance and success, future research should examine the impact of resistance and plyometric training on the attenuation of fatigue-induced performance and potential improvement of the V[Combining Dot Above]O2max in female athletes.
As such, the implications for soccer are potentially significant. First, it should be noted that the present finding of a V[Combining Dot Above]O2peak at the end of the competitive season in collegiate-level female soccer players of 49.1 ml·min−1·kg−1 is in accord with previously reported values of 49.4 ml·min−1·kg−1 in elite female Danish soccer players (11). The present investigation demonstrated a significant increase in the V[Combining Dot Above]O2peak in collegiate female soccer players in the absence of significant aerobic training. These results mirror the results of Helgerud et al. (5), in which high-intensity interval-style training was used to significantly improve V[Combining Dot Above]O2max (6.2 ml·min−1·kg−1) in elite male soccer players after 8 weeks of training. Unlike Helgerud et al., however, the present investigation realized similar increases in maximal aerobic capacity (5.2 ml·min−1·kg−1) primarily through a resistance-plyometric training program. Also of significance is the possibility of a detraining effect on the maximal oxygen consumption through the course of a competitive season for collegiate female soccer players. Indeed, it was recently demonstrated that there was an 8.9% reduction in the V[Combining Dot Above]O2max over a competitive season in Division 1A female soccer players (12). Implementation of a similarly designed resistance-plyometric training program may help attenuate this loss of aerobic capacity throughout the competitive season.
An acknowledged limitation of this investigation was the failure to achieve a physiologically verifiable V[Combining Dot Above]O2max among all the subjects; hence, the use of V[Combining Dot Above]O2peak. The V[Combining Dot Above]O2max is typically defined as reaching a minimum of 2 out of 3 variables (plateau in O2 consumption, 95% of age-predicted HR max, and an RER of ≥1.1). A possible source for this discrepancy was the use of a protocol, which relied solely upon speed, and not elevation, to manipulate exercise intensity. Nevertheless, it is noteworthy that there was a significant decrease in the RER despite there being a significant increase in the V[Combining Dot Above]O2peak from PRE to POST. Importantly, however, it should be noted that the V[Combining Dot Above]O2 protocol adopted for this investigation was chosen specifically to more closely approximate the effort achieved under game conditions. Therefore, although the present investigation did not achieve the generally accepted definition of the V[Combining Dot Above]O2max during testing, the results should be interpreted in light of the specificity of the protocol. A second limitation of this investigation was the absence of a control group. However, given the small subject population available, it was deemed prudent to pool all the participants into 1 group.
The result of this study offers a practical application for soccer. We have provided evidence that an off-season resistance-plyometric training program may increase peak oxygen consumption and RE in competitive female soccer players. This is of practical benefit to the strength and conditioning professional as both an off-season conditioning program and a buffer against in-season decreases of the V[Combining Dot Above]O2max. Considering the data linking second half reductions in total distance, high-intensity runs, and sprints, it would be of considerable competitive importance to implement a resistance and plyometric training program during the off-season to improve RE and potentially improve V[Combining Dot Above]O2max or attenuate the season-induced decline of V[Combining Dot Above]O2max.
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Keywords:© 2012 National Strength and Conditioning Association
maximal oxygen consumption; weight training; V[Combining Dot Above]O2max