Effects of Unilateral Versus Bilateral Plyometric Training on Endurance Running Performance

The purpose of the study was to investigate the effects of an 11-week unilateral versus bilateral plyometric training intervention on maximal isometric voluntary (MVC) knee extensor torque, countermovement jump height (CMJ), running economy (RE) and 3-km time trial (TT) performance. Twenty-seven recreationally trained endurance runners (12 females and 15 males) were randomly assigned to one of three groups: unilateral plyometric training (UPT; n = 9), bilateral plyometric training (BPT; n = 9) and control (CON; n = 9). RE, VO 2max , 3-km treadmill TT, isometric MVC (bilateral and unilateral) and CMJ (bilateral and unilateral) were measured prior to and after 11 weeks of training (UPT and BPT; volume equated, 20-40 minutes, 2-3 days/week). Separate two-way repeated measures ANOVAs were used to assess within and between group differences in RE, VO 2max , 3-km TT, maximal isometric knee extensor torque and CMJ. Following 11 weeks of plyometric training there were significant improvements in RE (UPT 5.6%; BPT 4.9%, p < 0.01) and 3-km TT performance (UPT 2.4%; BPT 2.5%, p < 0.01) in addition to CMJ (UPT 12.5%; BPT 14.5%, p < 0.01) and maximal isometric knee extensor torque in the unilateral group (14.0%, p < 0.01). No significant differences in VO 2max or anthropometric measures were detected ( p > 0.05). No statistically significant differences between training interventions ( p > 0.05) were detected in any measure. These data demonstrate that UPT and BPT result in similar improvements in RE and 3-km TT run performance in recreational distance runners.


INTRODUCTION
Distance running performance is dependent on the complex interaction of numerous physiological and biomechanical factors. Physiological determinants include maximal oxygen uptake (VO2max), lactate threshold and running economy (RE) (3,15). RE is the submaximal rate of oxygen consumption (2) or energy cost of running (11) at a given running velocity, with lower values indicating better RE (20). RE can vary by as much as 30% in trained endurance runners who are homogenous with respect to VO2max (8). Although aerobic factors largely underpin endurance performance, biomechanical characteristics are known to influence RE, which can be improved with plyometric training (4,5,7). Several biomechanical parameters can affect RE including stride length, lower limb joint angles and neuromuscular factors associated with mechanical stiffness of the musculotendinous system (20,29). Improvements in the latter have been associated with plyometric training (27). Plyometric training involves an eccentric muscle contraction followed immediately by a concentric contraction to allow the muscle to store and return elastic energy (5,31). Studies have demonstrated a 4-8% enhanced RE following a structured plyometric training program (5,22,27,28) in addition to concomitant improvements of 2.6-5% in 3-km and 5-km time trial (TT) runs (5,22,23,28). Furthermore, an increase muscle activation of the leg extensors from strength and plyometric training have been moderately correlated with increases in running speed and decreases in oxygen consumption at submaximal speeds (9,18,30).
Various mechanisms have been suggested to underpin the potential of plyometric training to influence changes in muscletendon interaction associated with better RE and endurance performance. Fletcher & MacIntosh (11) have proposed that an optimal tendon stiffness may exist which reduces the velocity and magnitude of muscle shortening thereby allowing muscle fibers to optimize their length and operate in a more isometric state. Isometric contractions require less metabolic energy than contractions that result in an active shortening or lengthening for a given force production (11). Furthermore, a reduction in muscle fascicle shortening velocity decreases motor unit activation and therefore energy cost (12). It is these mechanisms that have been suggested to reduce the energy cost of running following a structured plyometric training program.
Numerous studies have successfully used unilateral and bilateral exercises in a concurrent plyometric and endurance training intervention to improve distance running performance and RE. Bilateral drop jumps have reportedly had a positive effect on RE and time trial performance (5,25). In addition, combined unilateral and bilateral jumps, hops and bounds have demonstrated significant improvements in RE (22,23,27,28,31). However, it is not clear from the available literature whether each plyometric training method individually may offer superior benefits in optimizing muscle tendon unit stiffness in distance runners.
Running is a bipedal sport with phases of single leg support within the athlete's stride therefore the force generated to move the runner forward comes from the propulsive leg individually in each stride (17,19 (6) reported UPT to be more effective than BPT at increasing the sum of the right and left leg CMJ height and rate of force development. Collectively, these data are suggestive that plyometric training should be reflective of the specific unilateral or bilateral patterns that occur within the sport. However, to date no research has directly compared unilateral and bilateral strength and/or plyometric training on a range of strength, power, and endurance running performance variables. Therefore, the aim of the present study was to compare the effects of UPT and BPT on RE, endurance performance, maximal isometric voluntary (MVC) knee extensor torque, and CMJ height, in recreationally trained runners.

Experimental Approach to the Problem
This study was designed to address how a short-term, high frequency (2-3 sessions/week) plyometric training program using a high volume affects explosive strength, RE and 3-km TT run performance. Prior to the commencement of all baseline measures, participants were familiarized with all testing and training procedures on a separate day and were habituated to treadmill running in their regular training routines. Following initial baseline measures (anthropometric measures, maximal isometric knee extensor torque, CMJ, RE, VO2max, [day 1] and 3-km TT [day 2]) participants were randomly assigned to one of three groups: UPT (n = 9), BPT (n = 9), and control (CON; n = 9). Experimental groups undertook either an 11-week UPT or BPT program alongside regular running training whereas the CON group undertook their regular run training only. All tests were undertaken at the same time of day to avoid diurnal variation in performance with individuals instructed to have a light meal and be well hydrated approximately 3-4 hours prior to testing.

Subjects
Twenty-seven recreationally trained endurance runners (12 females and 15 males; age = 35  6 years; height = 1.7  0.1 m; body mass = 74.3  15.1 kg; VO2max: males = 53.5  6.3 ml·kg -1 ·min -1 , females = 42.8  4.5 ml·kg -1 ·min -1 ) with no history of plyometric training participated in the study. All participants met the criteria of the 'apparently healthy' category, as defined by the American College of Sports Medicine (1). Participants were informed of the benefits and potential risks associated with the investigation, completed a pre-test medical questionnaire and signed an informed consent prior to the start of the study. The study was approved by the University of Northampton's Research Ethics Committee.

Procedures
Unilateral and bilateral maximal isometric knee extensor torque. Participants initially completed a five minute warm-up on a motorized treadmill (HP Cosmos, Nuffendorf, Germany) at 4-6 km·h -1 followed by a dynamic stretch protocol (6 repetitions on quadriceps, hamstrings and gluteals). Following a three-minute recovery period, bilateral isometric contractions were performed on an isokinetic dynamometer (Biodex System 3 Pro, IPRS, Suffolk, UK) with the knee and hip at 90 o flexion to ensure consistent internal moment arms and muscle length properties, respectively. The dynamometer attachment was positioned proximal to the ankle with identical positioning used in all experimental testing sessions to ensure consistent external moment arms. Five sets were performed (3 s each) with one-minute rest between sets. The first two sets were 'warm-up' attempts performed at 50% and 75% perceived MVC followed by three maximal attempts. Three minutes later, unilateral contractions (dominant leg only as determine by the leg used to kick in sports) were performed with identical intensity and rest periods. Data were directed from the dynamometer to a high-level transducer (model HLT100C, Biopac, Goleta, CA) before analog-to-digital conversion at a 2000-Hz sampling rate (model MP150 Data Acquisition, Biopac). The data were then directed to a personal computer running AcqKnowledge software (v4.1, Biopac) and filtered using a zero lag, 6-Hz Butterworth low-pass filter. Maximal bilateral and unilateral isometric knee extensor torque was considered as the peak torques achieved during the three unilateral and bilateral trials, respectively. Participants were given verbal encouragement to give their maximal effort during all trials.
Unilateral and bilateral CMJ. Following a five-minute recovery period participants completed unilateral and bilateral CMJ tests using a portable electronic jump mat system (FSL JumpMat, Chester, UK) to estimate jump height. During the CMJ tests, participants were instructed to stand on one leg (right then left) for the unilateral test and with feet positioned shoulder width apart for the bilateral test. Participants were instructed to perform a fast-downward movement (approximately 45 o knee angle) followed by a maximal effort jump with arms to assist the performance. Participants were advised to land in the same place as take off in each test and had a 3-minute recovery period between the unilateral and bilateral tests. Each CMJ measure was executed 3 times (alternating right and left leg for the unilateral trials, with a 30-s recovery period between all unilateral and bilateral attempts) with the peak jump height for each test recorded. Participants were given verbal encouragement to give their maximal effort during all trials. RE and VO2max. Five minutes later, participants started the RE protocol on the same motorized treadmill. Participants ran at two submaximal speeds: 10 km·h −1 (RE speed 1: RES1) and 11.5 km·h −1 (RE speed 2: RES2) for females and 11.5 km·h −1 (RES1) and 13 km·h −1 (RES2) for males (gradient incline set at 0%) for 5 minutes at each speed with a 5-minute recovery period between bouts. RER of <1.0 was recorded during the RE trials, indicating that a VO2 slow component was not evident for each sample. Five minutes later the participants completed a VO2max test with an initial speed that corresponded to their final RE speed. The incline was set at 1% gradient throughout the VO2max test with treadmill speed increased by 1 km·h −1 at 150-s stages (1) throughout the VO2max test until volitional exhaustion was achieved (10:38 ± 1:37 min/s). VO2max was determined as the highest 30-second rolling average VO2 value in addition to respiratory exchange ratio >1.15 and heart rate within 5% of agepredicted maximum. Breath-by-breath VO2 (Cortex Metalizer, Cranley, Birmingham, UK) was collected throughout each of the RE velocities and VO2max stages. The average VO2 over the final 2 minutes at each RE speed (RES1 and RES2) was taken as the steady-state VO2. Steady-state VO2 was defined as less than 100 ml min -1 change during the last 2 minutes at each RE speed (10). RE at each speed was expressed as energy cost (kJ·kg -1 ·km -1 ) (10) thereby allowing for individual comparisons between sex and running speeds. Energy cost was calculated as: where VO2 is measured in L min -1 , caloric equivalent is in kcal L -1 of O2, speed (S) is measured in m min -1 , body mass (BM) is measured in kg and 1,000 is m km -1 .

3-km TT endurance run test.
During a second laboratory visit (48-96 hours later), participants undertook a 3-km treadmill TT (PRECOR TRM833/P30, Camberley, UK) to assess running performance. Following the same warmup and dynamic stretch protocol undertaken on day 1, participants were given 1-minute to establish a comfortable running speed before the start of the TT. Participants had autonomy over the treadmill speed control and could view distance only on the console display. Participants were given verbal encouragement during the test to give their best 3-km performance and instructed to exert a consistent effort over the fixed distance. Time was measured to the nearest second.

Training Interventions
Endurance training. All participants (UPT, BPT, CON) maintained their regular running training (determined by training diary logs) throughout the intervention period. The training intervention was carried out during the general preparation period of the endurance running season.
Plyometric training. Table 1 shows the 11week UPT and BPT programs. The progressive overload principle was incorporated into the training plan by increasing total foot contacts each week in a 4-week step loading manner and varying the complexity of exercises undertaken. Participants attended one supervised session each week to ensure consistency of exercise execution. The remaining weekly training sessions for each experimental group were unsupervised but monitored remotely on a weekly basis. All plyometric training had at least 24 hours between sessions and were scheduled at the same time of day (± 2 hours). Prior to each plyometric training session participants underwent a 10-minute dynamic warm up and stretch component (leg swings, ankle bounces, vertical jumps and skips at approximately 50% maximal effort). Both experimental group's plyometric training volume was matched by participants completing the same number of total foot contact jumps per session and per week. Training details relative to the number of foot contacts are given in Table 1. Training intensity for both groups was equalized for box jump exercises only through reducing jump height by 50% for the UPT when compared to the BPT group. All training sessions were conducted on the same training surface (grass and sprung wooden floor). Plyometric training sessions lasted 20-40 minutes and participants were instructed to give their maximal effort for all the exercises each session using fast eccentric/ concentric movements. The level of significance was set at p < 0.05. All data are presented as mean  SD.

RESULTS
All participants (n = 27) completed the 11week training program, i.e. regular running training plus either UPT or BPT for the experimental groups, with the CON group participating only in regular running training. Training compliance was very high in both groups, with UPT participants attending 97% and BPT participants 99% of the once weekly-supervised plyometric training sessions. No significant differences (p > 0.05) in any dependent variables were detected in baseline data between groups (Table 2), or for mean weekly running volume across the 11-week period between groups (Table 3). There was no significant change (p > 0.05) in body mass, VO2max (Table 2), or in mean weekly running volume (Table 3) over the 11-week plyometric training period within the experimental or CON groups.

RE, 3-km TT
After training, the UPT group demonstrated a significant improvement in RES1 (4.9 ± 1.5%, ES = 1.14 [CI = 0.26 -  Table 2). No statistically significant differences in any measure were found between groups (p > 0.05). RER of <1.0 was recorded during all RE trials, indicative that no VO2 slow component was evident and that any oxygen consumption was for locomotion rather than metabolite removal.

Unilateral and Bilateral Maximal Isometric Knee Extensor Torque
No significant differences were observed after the training intervention period for the bilateral maximal isometric knee extensor torque in the UPT (-1.  (Table 2). However, unilateral maximal isometric knee extensor torque for the UPT group significantly improved after the training program  (Table  2). No statistically significant differences in any measure were found between groups (p > 0.05).  Table 2). No statistically significant differences in any measure were found between groups (p > 0.05).

DISCUSSION
The aim of this study was to compare the effect of an 11-week UPT versus BPT program on RE and TT endurance running performance in recreationally trained distance runners. The main finding of the present study indicates that very high volume UPT and BPT significantly improved RE, 3-km TT run performance and explosive strength (jump height) to a similar degree in recreational endurance runners. These results suggest that to optimize endurance running performance UPT or BPT should be added concurrently to a regular run training program.
Both plyometric groups showed a significant improvement in 3-km TT performance (2.4-2.5%, ES = 0. 17-0.26) with no significant difference between groups. These results are consistent with previous research albeit in shorter time frames (6-9 weeks), in competitive distance runners that noted similar improvements in TT run performance after bilateral or unilateral and bilateral plyometric training (5,22,23,25,28). A recent study (23) examined the effects of plyometric training on TT run performance. Twenty-five trained runners included 2-3 sessions of high volume, lowmoderate intensity unilateral and bilateral plyometric exercises per week over 6 weeks to a regular endurance training program. The authors found a significant improvement in 3-km TT performance (2.6%). Other authors (5,25) reported marginally greater magnitudes of improvement in TT runs (5.0% & 3.9% respectively) after a 6-8 week low-volume, high intensity bilateral plyometric training period in moderately to well-trained distance runners (ES = 0.4). Participants in the current study performed a very high volume, low intensity unilateral or bilateral plyometric training program. Whilst foot contact volume in the present study was unusually high for both UPT and BPT groups when compared to that identified in a recent systematic review (7), no participants incurred any injuries. These results support existing research reporting improvements in distance run performance through a structured plyometric training program, although exercise intensity may be of greater importance than either mode (UPT versus BPT) or training volume. The present study shows that either mode of plyometric training represents a time-efficient and cost effective (minimal equipment requirements) way to train without increasing weekly running volume.
In the present study, both plyometric groups showed a significant improvement in RE across the two running speeds (4.9-5.6%, ES = 1.1-1.3) with no significant difference between groups. In a recent study (5), eleven moderately to welltrained endurance runners added one additional session of low-volume, high intensity bilateral plyometric training per week over 8-weeks to their usual endurance run training. They reported a large improvement in RE (7%, ES = 1.01) whereas peak VO2 remained unchanged. Previous plyometric training programs used to enhance RE have demonstrated lower magnitudes of improvement (3.0-6.0%) after 6-9 weeks of 2-3 moderate to high-volume, low to moderate intensity combine unilateral and bilateral plyometric training sessions per week in moderately to well-trained distance runners (27,28,31) with no change in VO2max. Various factors may have contributed to the magnitudes of change differences in RE between low-volume, high intensity versus high volume, low intensity based plyometric training programs. Morphological (muscle-tendon unit properties) and neural (motor unit recruitment, intermuscular coordination) changes to the musculotendinous system (4) may have been influenced by the type of plyometric exercise (18). It has been proposed that improvements in explosive strength can be attributed to an increase in motor unit recruitment allowing lower limb muscles to resist eccentric loading during stance of the running gait cycle thereby facilitating a more energy efficient isometric contraction (7,11,26). It is possible that such fast stretch shortening cycle plyometric exercises used by Berryman et al. (5) enhanced RE to a greater degree than that seen in the present and previous studies as a result of improved motor unit recruitment and the development of a more optimal mechanical stiffness of the musculotendinous system. Further research is warranted to determine the exact mechanisms from different types of muscle contractions as a result of explosive resistance and plyometric training on RE.
In the present study, only UPT significantly improved unilateral maximal isometric knee extensor torque (10.8%, ES = 0.41) after training. Previous research (9,18,22,30) demonstrates equivocal findings of the change in maximal isometric strength of the leg extensors (0-10%) where the impact of neuromuscular characteristics on RE and endurance performance has been investigated over short durations (6-9 weeks). From a neuromuscular perspective, an increase in absolute motor unit recruitment as a result of explosive strength training would lead to a lower relative intensity per motor unit thereby delaying the activation of metabolically inefficient type II fibers during distance running (4,7,11). Lower relative force outputs that are commonly observed during bilateral strength and explosive strength training (6,13,17) may explain the lack of significant change in maximal isometric knee extensor torque in the BPT group.
Despite both plyometric groups significantly improving all CMJ measures after 11 weeks with no significant difference between groups the magnitude of improvement in bilateral CMJ from BPT (12.6%, ES = 0.56) was greater than that following UPT (7.7%, ES = 0.27). Theoretically, an enhanced stretch shortening cycle function as evidenced from the bilateral CMJ measure would result from higher movement velocities and greater absolute force outputs generated through BPT (17,18). The most important morphological adaptation from plyometric training cited in the literature (3,11,28) as previously mentioned seems to be an optimal stiffness and elasticity of the muscle-tendon unit that is achieved through a reduction in the magnitude and velocity of muscle shortening thereby allowing muscle fibers to better resist eccentric loading and remain in an energy efficient isometric state. Conceivably, this would explain the improved RE and 3-km TT run performance observed from the BPT group.
It must be acknowledged that the present study is not without limitations. Only one of the weekly plyometric training sessions was supervised and endurance training for all participants was self-reported over the 11-week period, although weekly contact was continued to ensure training was maintained throughout the programme. A second limitation was that the phase of the menstrual cycle when testing took place was not controlled for in the female participants who did not use a hormonal contraceptive. Similarly, the use of the treadmill during the 3-km TT may be problematic because over ground running would likely offer greater validity and reliability as a participant approaches exhaustion where speed and rhythm are easier to change. Finally, treadmill speeds for RE were not selected relative to each participant's physiological capabilities, possibly compromising the external validity of the findings.

PRACTICAL APPLICATIONS
The ultimate goal of a distance runner's training program is to enhance performance and minimize injury risk. The purpose of the present study was to compare the effects of UPT versus BPT, which were matched for volume and intensity on RE and TT run performance in recreationally trained male and females distance runners. The results indicate that BPT may be safer due to a reduced injury risk associated with the bilateral force deficit (13,17,21), and is equally effective as UPT at enhancing distance running performance. A reduction in relative lower limb impact to joint and tissue structures from bilateral jumping could be of greater benefit to runners undertaking high volume weekly endurance training that have a greater exposure to injury risk associated with high mileage running (21). Therefore, coaches should plan a periodized BPT program that emphasizes high volume and low intensity exercises to support the optimization of performance in recreationally trained male and female distance runners.