Performance and aerobic measures
There were only small differences at baseline between groups for peak speed and running economy (mL·kg−1·km−1) in men and women, for vV˙O2max and %V˙O2max in men, and for V˙O2max and running economy (mL·kg−1·min−1) in women. Mean improvements in peak speed of small to very large magnitude were observed in both groups for men and women, but PRT was clearly harmful relative to HRT (Tables 3 and 4). After the intervention period, male HRT showed small or moderate improvements in aerobic measures, whereas the effects from PRT on aerobic measures were trivial (Table 3). Both female groups showed small to moderate improvements in all aerobic measures (Table 4). Male and female HRT showed greater improvements in running economy compared with PRT. Differences between groups on all other aerobic measures were unclear.
In both training groups and in both sexes, changes in contact time were opposite to those of flight time. The direction of the changes were opposite in the two training groups, and overall, the changes with PRT were clearly positive and small-moderate in magnitude relative to those with HRT (Tables 3 and 4).
1RM improved in all groups, with male athletes improving by 20%–40% (Table 3) and female athletes improving by 30%–50% (Table 4). Improvements were greater with HRT. Changes in neuromuscular related measures from the five-jump test were small to moderate improvements with HRT and trivial or negative with PRT (Tables 3 and 4). Overall, PRT was associated with either unclear or negative effects on all neuromuscular measures in men and women. There was a moderate improvement in leg stiffness after HRT in men and women and unclear decrease (male) or possibly small improvement (female) after PRT (Tables 3 and 4, respectively).
The residual error in competition times calculated at the beginning of the season was ∼2.0% for the training and control groups, and at the end of the season, it was 1.3%–1.4% in the training groups and 1.5% in the control group. Figure 2 shows the least-squares mean performance times for men and women in the competitions that the training groups entered. The mean effects of the training interventions on performance at each competition were generally consistent from week 8 through the end of the season for male and female athletes. Overall, PRT resulted in possible harm to competition times (slower run times) by 0.8% (90% CI = ±1.5%) compared with control male athletes. HRT was also possibly harmful to competition performance (0.1%; ±1.3%). The men’s overall mean performance was worse (slower) than that of the control teams by 0.5% (±1.2%) after implementation of the two resistance training interventions. There was an unclear difference between PRT and HRT (−0.7% ±1.5%). There was a likely beneficial effect of PRT training for women, resulting in −1.1% (±1.3%) faster run times (compared with control female athletes). HRT was also likely beneficial to competition performance, −1.4% (±1.4%). The women’s overall mean performance was better (faster) than that of control teams by −1.2% (±1.3%). When compared with HRT, PRT was possibly harmful (0.3%; ±1.0%). Individual responses expressed as an SD for both treatments combined was 0.3% (90% CI = −1.2% to 1.3%) for men and −0.6% (−1.0% to 0.5%) for women.
Previous studies (17,36,43) have reported that various forms of resistance training may lead to improved endurance performance in trained subjects. However, the optimal prescription of resistance training to improve endurance running performance has yet to be firmly established. Accordingly, we investigated whether the combination of plyometric training and heavy-resistance training (PRT) may facilitate additional improvements in neuromuscular efficiency, strength, and running mechanics compared with HRT alone during the competition phase of a men’s and women’s collegiate cross-country season. Interestingly, our data revealed distinct differences between the prescribed training regimes in terms of performance gains and physiological adaptations and an apparent gender-specific response to resistance training.
To determine the effects of HRT and PRT on performance from competition data, the coefficient of variation (CV) representing typical variation in performance time for the faster male and female runners across the competition season was determined. The CV sets the benchmark for the smallest worthwhile change in an athlete’s performance and for the typical (standard) error of measurement of tests used to assess the smallest important or worthwhile change (24). Our CV of ∼2.0% at the start of the competitions and ∼1.5% at the end are in line with the 1.5%–1.7% reported by Hopkins and Hewson (23) and were the basis of using a ±0.5% threshold value for beneficial and harmful effects on performance (approximately 0.3 of the within-subject SD top athletes show between competitions) (23,24). Accordingly, there were substantial beneficial mean effects on competition performance for the female training groups compared with controls (−1.2% ± 1.3%), whereas resistance training for men proved to be possibly harmful (0.5% ±1.2%). This observation could be an indication that endurance-trained female athletes may have a greater requirement in terms of resistance training maintenance (38), whereas this type of training for men might be beneficial in general only during the preseason or build-up phase of training when there is less emphasis on competition and gains can be made in physiological measures without the risk of harm to competition performance. The differences in effects between men in women could also be due in part to differences in training intensity and competition distance. The proportion of training that occurred at ≥80% V˙O2max for women was moderately higher than that for men (Table 1), which might have translated into performance enhancement over the women’s shorter race distance (5–6 vs 8–10 km for the men). Although we observed an overall benefit in competition performance from either form of resistance training in women and harm in men, HRT was substantially better for women (0.3%; ±1.0%) whereas PRT was worse (−0.7% ± 1.5%).
In addition to actual competition data, we also observed a substantial increase in laboratory-derived peak running speed after HRT (4.6% and 4.4% in men and women, respectively) compared with PRT (1.0% and 2.2% in men and women, respectively). Peak running speed has been shown to be a good indicator of endurance performance in middle- and long-distance running events (4,34,35,41,47), and Noakes et al. (34,35) has suggested that peak running speed could be used as a measure of the “muscle power” factor in endurance runners. Muscle power is defined as an ability of the neuromuscular system to produce power during maximal exercise when glycolytic and/or oxidative energy production are high and muscle contractility may be limited (34). Indeed, in addition to the aerobic processes related to distance-running performance, the neuromuscular and anaerobic characteristics related to peak running speed are also strongly involved in distance-running performance.
In the present study, changes in physiological measures related to distance-running performance were consistent with performance data, indicating greater improvements after HRT than matched volume-load PRT (Tables 3 and 4). Specifically, the addition of HRT improved running economy by 1.7% and 3.4% in men and women, respectively, whereas PRT only improved running economy by 0.2% and 1.0% (Table 3 [men] and Table 4 [women]). Although both HRT and PRT results are in accordance with growing literature demonstrating that HRT or plyometric training improved the running economy of well-trained athletes (15,2533,36,42,43,46,48), the magnitude of enhancements was lower in our study compared with previous studies reporting effects after heavy-resistance (15,25,33,46) or plyometric training (36,42,43,48). This could be due to different phases of season that the studies were performed. Regardless, in both HRT and PRT, improvements in running economy occurred in the absence of any substantial change in V˙O2max, suggesting that improved running economy was a result of neuromuscular characteristics rather than improved cardiorespiratory fitness. This is a reasonable assertion because both HRT and PRT groups performed the same endurance training outside their respective resistance training programs. In further support, running economy improved in accord with many of the neuromuscular measures (Tables 3 and 4), which also coheres well with previous studies (10,33,36,42,43,46), reporting the importance of the neuromuscular characteristics in determining running economy and running performance after combined resistance and endurance training in runners.
With regard to changes in strength and neuromuscular measures that could be responsible for the greater improvements in running economy and peak running speed after HRT, it has been purported (3,26) that the nervous system plays an important role in regulating muscle stiffness and utilization of muscle elasticity during stretch-shortening cycle exercises, such as running, in which high contraction velocities are used. In the present study, small to moderate increases in leg stiffness occurred in the male and female HRT groups, and PRT training was associated with moderate negative effects on leg stiffness compared with HRT (Tables 3 and 4). Interestingly, the group with the smallest improvement in 1RM (male PRT) was the only group not to elicit a concomitant increase in stiffness. One of the most important roles of the muscle during running is to modulate the leg stiffness and the storage recoil of energy. The conversion of energy to motion involves recoil of some elastic energy in muscle and tendon; thus, a “stiffer” muscle or tendon would be better at transferring energy economically or without the need for additional oxygen consumption (7,10,43). Indeed, previous evidence has shown a negative correlation between leg stiffness and cost of running (1,2). Kerdok et al. (27) have shown changes in both muscle-tendon stiffness and running economy when manipulating the running surface, indicating that runners adjust the level of leg stiffness toward the most optimal degree to maintain consistent running mechanics on different surfaces. This could be important, particularly in cross-country runners like those in the present study where competitions often take place on a variety of undulating surfaces in a single competition. Conversely, the training-induced alterations in biomechanical measures support PRT training and therefore are not likely related to the changes in running economy, peak speed, or competition performance. Other studies have indicated that these biomechanical adaptations also occurred in response to plyometric training (36,42). Collectively, these findings suggest that HRT had a positive influence on cross-country running performance because of the improved running economy, peak speed, and neuromuscular characteristics.
Finally, it was not surprising to observe the magnitude of improvement in maximal strength (20%–40% in the leg press for most athletes) in our sample of distance runners with limited resistance training experience. The enhancements in 1RM strength from HRT were 30% and 50% greater than PRT in men and women, respectively, indicating a positive effect to HRT on strength parameters. The increased muscular strength due to resistance and/or plyometric training might primarily come from neural adaptations without observable muscle hypertrophy (16,39). The finding that no substantial change in body weight and small to moderate reductions in percent fat in both PRT and HRT groups, suggesting that little, if no hypertrophy occurred because of the resistance training interventions supports this suggestion. Increases in body mass are an undesirable side effect to resistance training that could be counterproductive to distance-running performance.
In conclusion, both HRT and PRT had a likely beneficial effect on competition times in women, whereas both treatments had possibly harmful effects in men. However, when comparing the two treatments, the addition of plyometric training to HRT was harmful to cross-country competition performance and most laboratory-based measures when compared with a matched volume-load HRT program. The greater improvements in competition performance and an enhancement in running economy and peak speed after HRT, compared with PRT, was probably a result of improvements in lower limb strength, leg stiffness, and utilization of stored elastic energy. Overall, our data indicate that women should include HRT in their programs, but men may want to implement such training in season with caution until more research establishes characteristics of positive or negative responders.
The authors thank Hope College for their generosity and the use of laboratory space and equipment, the Men’s and Women’s Cross Country teams at Hope College for participating in this research study, and the Salome Emmanuel and Kate Nelson-Nix for their assistance during data collection.
No funding was received for this study, and the authors have no professional relationship with a for-profit organization that would benefit from this study.
Publication does not constitute endorsement by the American College of Sports Medicine.
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Keywords:© 2013 American College of Sports Medicine
RUNNING ECONOMY; RESISTANCE TRAINING; PLYOMETRIC TRAINING; RUNNING PERFORMANCE; NEUROMUSCULAR CHARACTERISTICS; MIXED MODELING