Soccer is a sport characterized by high demands of sustained, intense, intermittent exercise (26,31,36). During a 90-minute match, players cover up to 10 km while carrying out repeated all-out sprints, jumps, and changes of direction (29,33,36). Previous research has shown a decline in soccer-specific tasks involving passing accuracy and ball touches after repeated bouts of high-intensity exercise (32). Moreover, short burst sprint ability (e.g., ≤15 m), vertical jump, and change of direction performance represent discriminating variables between elite- and lower-level players (34). These findings suggest that soccer players should focus on training tactics to improve the metabolic, muscular, and neuromuscular demands specific to their sport.
In an attempt to improve performance, many adolescent soccer players participate in organized sports performance training programs in winter months when there are fewer weekly soccer practices and games (28). According to the National Strength and Conditioning Association (NSCA), preseason training tactics for soccer players should emphasize a combined training approach involving strength training, plyometric training, and various forms of sprint interval training (SIT) (39). Strength training has been shown to improve acceleration, the ability to change direction, and repeated sprint ability (41), whereas plyometrics enhance jump height and sprint performance by targeting the stretch-shortening cycle (SSC) and increasing the rate of force development (7). Meanwhile, SIT has the capacity to improve measures of aerobic and anaerobic performance compared with traditional endurance training with the added benefit of being more sport-specific and less time consuming (2,4,43).
Specific to SIT, a variety of approaches are routinely used during combined training programs for soccer players, including short burst sprints (e.g., ≤25 m) performed with and without resistance and longer (e.g., 30 seconds) bouts (17,22,28). Incline running represents another form of SIT, with Kavaliauskas et al. (20) reporting that soccer players demonstrated significant improvements in aerobic and anaerobic performance measures following 6 weeks of twice weekly incline training sessions. Of note, SIT performed as part of an incline treadmill training protocol in combination with plyometrics and strength training led to a significantly reduced lower extremity injury rate among female soccer players (U14–U18) compared with a control group (16). To date, however, the performance outcomes of level vs. incline-based SIT remains largely uninvestigated in soccer players.
Therefore, the purpose of this investigation included comparing 2 combined training approaches involving identical strength training and plyometric training programs with different approaches to SIT on indices of athletic performance. One approach consisted of all SIT performed on a level grade, whereas in the other, the predominant focus was incline conditions ranging 5–30% grade. The maximum grade of 30% was chosen based on previous research, indicating that incline treadmill running at 4.47 m·sec−1 and 30% grade resulted in significantly greater measures of lower-body muscle power, muscle activity, and joint range of motion compared with level-grade running at the same speed or same stride frequency (38). All running intensities for both groups and all bout durations were consistent with the definitions associated with SIT and high-intensity interval training (4). It was hypothesized that both groups would demonstrate gains in a variety of performance measures but that gains demonstrated by the incline treadmill training group would be more pronounced because of the reported biomechanical effects associated with incline treadmill running (38).
Experimental Approach to the Problem
This was a parallel, 3-group, longitudinal (pretraining, posttraining) study designed to investigate the effects of 2 different approaches to SIT on indices of soccer performance, with subjects matched according to maximal aerobic running speed (Vmax) and then randomly assigned to a group performing SIT based on incline (INC) or level-grade (LEV) conditions. Subjects were matched by Vmax as previous research has reported on the efficacy of this characteristic for prescribing individualized SIT for both level and incline treadmill grades (2,9,12). A third group served as a control group (CON); these subjects completed all pre- and posttesting while maintaining their normal 75- to 90-minute sport practices 4–5 times·wk−1. Both INC and LEV performed the same plyometric training and strength training exercises with the approach to SIT representing the only difference between training interventions. In addition to the 3 weekly workouts, INC and LEV continued 60-minute soccer training practices 1–2 times·wk−1 during the study. Treadmill group represented the training condition. Dependent variables chosen to examine the demands of incline vs. level-grade SIT and previously shown to be related to optimal soccer performance were as following: countermovement vertical jump (CMJ), standing long jump (SLJ), unilateral triple hop for distance (3HOP_R; 3HOP_L), 9.1-m sprint, 18.3-m sprint, pro agility change of direction (PA), modified Cunningham and Faulkner run to exhaustion (CFMod), Vmax, run to exhaustion at Vmax on a 1.5% treadmill grade (Tmax), unilateral leg press 1 repetition maximum (R leg_1RM; L leg_1RM), bilateral leg press 1RM (2 leg_1RM), dominant side upright hip extension 1RM (HE_1RM), dominant side upright hip flexion 1RM (HF_1RM), and a 30-second “all-out” jump test performed on a horizontal leg press equipped with a force plate to assess a variety of positive (concentric) and negative (eccentric) lower-body power characteristics, termed Plyo Press Power Quotient (3PQ). Subjects were required to attend at least 90% of all training sessions to be included for data analysis. Also, investigators monitored the training status of study subjects through weekly dialogue with each individual.
Subject characteristics are listed in Table 1. Forty-six subjects (24 males and 22 females) between 13 and 18 years of age voluntarily enrolled to participate in this investigation after providing informed consent. Thirty-one subjects (16 males and 15 females) were part of a local youth traveling soccer club; the remaining 15 individuals (8 males and 7 females) served as control group and were equally active and represented competitive sports, such as basketball, softball, baseball, and track and field. Before this investigation, all subjects had participated in various speed, strength, and conditioning programs; however, none was enrolled in any formal training program in the 3 months before the training intervention. For individuals younger than 18 years, a child's consent document was provided by the subject and an informed consent document by the parent or legal guardian. We excluded individuals <13 and >18 years of age and those who had experienced a lower-body injury in the previous 3 months. The Avera McKennan Hospital and University Health Center's Institutional Review Board approved this study, and it conformed to the recommendations of the 1964 Declaration of Helsinki.
The study took place from January to March and consisted of familiarization procedures (sessions 1–2), pretests (sessions 3–5), an 8-week training intervention (training days 1–24), and posttests (sessions 6–8), with efforts made to complete pre- and posttests at the same time of day.
The week before testing, subjects reported to the training center on 2 separate occasions spaced a minimum 48 hours apart. Session 1 included weighing and measuring each subject using a physician's scale (Detecto; Cardinal Detecto, Webb City, MO), followed by running for 5–7 minutes on a treadmill (Precor 932i, Woodinvale, WA) at a self-selected speed. After running, subjects learned a series of dynamic drills that were performed before every testing and training session. After the warm-up, 4–6 trials of the following tests were performed in the order listed: CMJ, SLJ, 3HOP_L, and 3HOP_R. Next, subjects performed 2–3 trials of 9.1-m sprint, 18.3-m sprint, and PA. Subjects then completed 2 leg_1RM, R leg_1RM, and L leg_1RM on a horizontal leg press (Plyo Press; Athletic Republic, Park City, UT). Session 1 testing concluded by assessing Vmax and discussing the Tmax test. Session 2 began with a HE_1RM strength test using an upright multihip machine (Pro Multi-Hip; Athletic Republic), before using the same machine to assess HF_1RM strength. After HE and HF assessment, a 3PQ test was performed using a load equal to the subject's 40% 2 leg_1RM on a Plyo Press. After a 10-minute rest, the last familiarization procedure was completed, a modified Cunningham and Faulkner test (CFMod). During this procedure, the subject ran to exhaustion on a treadmill set to 80% Vmax and 20% grade.
Five to 7 days after completing familiarization testing, subjects undertook performance testing. These performance tests took place over 3 sessions (sessions 3, 4, and 5) spaced 48–72 hours apart; subjects were encouraged to arrive for testing well rested and having avoided caffeine for 2–3 hours. Pretests were performed in the order listed below. Session 3 began with CMJ testing using a vertical jump measuring device (Vertec; Sports Imports, Hilliard, OH), following standard procedures. Briefly, standing reach height was first determined, followed by maximum jump height. Countermovement depth was self-selected and arm swing was allowed; the highest jump of 3 trials was used for data analysis. Standing long jump was conducted on a portable runway (Plyorobic Runway; Ecore, Inc., Lancaster, PA). Subjects stood on the runway with their feet 20–30 cm apart before completing a maximum effort jump in the forward direction. The maximum distance between a line drawn on the floor and the subject's heel closest to the line was used for data analysis. Unilateral triple hop for distance was performed on the same portable runway, using the same starting line. Subjects balanced on a single leg before attempting a maximum effort jump in a forward direction 3 consecutive times on the same leg and then landing on the same leg. Subjects were instructed to “stick” the landing using only the jumping leg and to hold the landing position for approximately 1–2 seconds (subjects were notified that a failure to hold the landing resulted in a mistrial and reattempt). The best of 3 trials for SLJ and 3HOP was used for data analysis. For all CMJ, SLJ, and 3HOP tests, a 45–60 rest was given between trials. After a 5-minute rest, sprint time for the 9.1- and 18.3-m sprints were measured simultaneously using a 2 gate wireless timing system (Brower Timing Systems, Draper, UT) with timing gates at 9.1 and 18.3 m, respectively. Subjects were instructed to use a 3-point start stance by placing the dominant hand on the ground next to the optical start sensor. The same runway and timing system was used to perform a PA test using standard procedures. Subjects began in a 3-point stance before turning right 90° and sprinting 4.6 m to touch a line on the runway. Subjects then turned left 180° and sprinted 9.1 m in the opposite direction and touched another line on the runway. Finally, subjects turned right 180° and sprinted 4.6 m back across the same starting line. For the sprints and PA tests, a 2- to 3-minute rest was given between trials, and the best of 3 trials was used for data analysis. Following a 5-minute rest, subjects then completed 2 leg_1RM testing on the Plyo Press, followed by R leg_1RM and L leg_1RM. The Plyo Press seat was positioned, so the subjects' feet placed the hip and knee joint angles at approximately 90°. A 3- to 5-minute rest was given between all 1RM efforts (15). Following a 10 minutes of rest, subjects performed a Vmax test on a treadmill (Super Treadmill; Standard Industries, Fargo, ND) set to 1.5% to better simulate overground running (19). Starting speed for the Vmax test ranged 10.5–12.1 km·hr−1 and was based on the self-selected speed chosen by the subject during the familiarization session, with the intention of completing 4 to 6 2-minute stages during the test (12). Briefly, subjects completed 2-minute stages on the Super Treadmill separated by 30-second rests (3). At the completion of each stage, treadmill speed was increased 0.80 km·hr−1. The subject was instructed that at least 90 seconds of the 2-minute stage must be completed for the stage to be considered successful; the speed of the last completed stage was defined as Vmax. If the velocity at exhaustion was maintained for only 60 seconds, then Vmax was calculated to be equal to the velocity of the previous stage plus half the velocity increase between the last 2 stages (23).
Session 4 began with maximum isoinertial strength testing for HE_1RM and HF_1RM. Dominant side was determined by asking the subject with which leg he or she kicked a ball (40). Briefly, the platform of the Pro Multi-Hip was set to a height that aligned the subject's hip joint with the axis of the machine's lever arm. The HE_1RM test began with the subject flexing the hip to approximately 120°, placing the (flexed) knee over the top of the cam lever pad and bracing themselves with outstretched arms using the machine's hand railing. The subject extended the hip and knee to an end point position of approximately 30 and 0°, respectively, whereas the pad of the lever arm remained behind the knee. Subjects next completed a HF_1RM test, which began with the hip at approximately 30° of hip extension and the pad of the lever arm slightly superior to the patella before then flexing the hip to 90°. Maximum strength for HE and HF was defined as the maximum load moved through the full range of motion using good form and held for 1–2 seconds. Following a 10-minute rest, subjects concluded session 4 testing by completing a Tmax test. Briefly, the Super Treadmill was set to 1.5% grade, and subjects warm up with two 15-second bouts at Vmax separated by a 60-second rest. On the next attempt, subjects ran to exhaustion at Vmax.
Session 5 testing began with 3PQ testing. Subjects warmed up by performing 2 sets of 3–5 repetitions at the prescribed load, 40% of 2 leg_1RM. The warm-up sets were separated by 60–90 seconds of rest. Following a 2–3 minutes of rest, subjects performed the 30-second 3PQ test. The measures assessed with 3PQ testing include a variety of positive (concentric) and negative (eccentric) neuromuscular characteristics, such as peak force, peak positive and negative power, average positive and negative power, average rate of power development, and percent positive and negative fatigue, respectively. After a 10-minute rest, subjects completed performance testing by undertaking a CFMod test on the Super Treadmill. Subjects completed 2 warm-ups at 80% of Vmax and 10 and 15% grades for 8 seconds each, respectively. Following a 1–2 minutes of rest, the Super Treadmill was raised to 20% grade and the subject ran to exhaustion. All posttests were performed in the same order and used the same settings and starting speeds as pretesting.
Both groups completed the same number of running treadmill and plyometric training workouts (12 each; 24 total) in an alternating fashion over 8 weeks. Subjects were not allowed to perform workouts on 3 consecutive days. On treadmill training days, INC performed 15–26 bouts ranging 6–30 seconds on grades ranging 5–30% while running at Vmax. In contrast, LEV performed 10–14 bouts for either 6 or 30 seconds on a 1.5% grade while running at intensities ranging 110–138% Vmax. The top intensity of 138% Vmax was based on a previous investigation of well-trained distance runners that reported that level grade 140% Vmax could be maintained for approximately 30 seconds (12). The only 6-second bouts for either group were the last 2 sets of every treadmill session and consisted of short-burst sprints on a 1.5% grade at intensities ranging 140–165% Vmax. Work-to-rest durations for both groups were 1:4, but subjects were given additional rest if physically unable to undertake the next effort. The treadmill training protocol for each group is listed in Table 2. Both groups performed the same plyometric training program, which included a variety of unilateral and bilateral footwork drills in anterior-posterior, mediolateral, and multidirectional movement patterns, bilateral anterior-posterior and mediolateral foam block hops, unilateral and bilateral box jumps in the anterior-posterior direction, and unilateral and bilateral weighted squat jumps on the Plyo Press at various percentages of each condition's 1RM (1RM), respectively. The unilateral and bilateral footwork drills involved a 4-square pattern and have been described as low-intensity by Chu (6). Briefly, the 4-square pattern consists of 2 122 cm lines (one vertical, one horizontal) that bisect to form a cross, leaving 4 quadrants each 61 × 61 cm. The lower left, upper left, upper right, and lower right squares were numbered 1, 2, 3, and 4, respectively. A typical footwork drill (either bilateral or unilateral) labeled 1–2, for example, involved an anterior-posterior movement for the prescribed duration. Similarly, a footwork drill labeled 1-2-3 involved starting on 1, jumping forward to 2, laterally to the right to 3, and finishing by jumping diagonally back to 1. During 4-square footwork drills, subjects were instructed to the following: (a) keep the feet, knees, hips, and chest facing forward at all times regardless of pattern; (b) maintain a slight knee bend; (c) stay on the balls of the feet; (d) seek accuracy and precision; and (e) move as intensely as possible. The plyometric training protocol is listed in Table 3. Finally, both groups performed the same strength training program: on treadmill days upright HE and HF exercises (for each side) and bilateral leg press were performed; on plyometric days, unilateral upright hip abduction, hip adduction, and leg presses were performed. All strength exercises involved 3 sets, repetitions ranging 8–12, and intensities ranging 60–90% 1RM, respectively. The strength training program is listed in Table 4.
The statistical program JMP (v.14.0; SAS Institute, Cary, NC) was used for all data analysis. Descriptive statistics of each outcome variable, including means, standard deviations, and tests of normality were determined. A 2-way repeated-measures analysis of variance (ANOVA) was used to test for the effect of training (time: pre, post) and training program (group: INC, LEV and CON) on CMJ, SLJ, 3HOP_L, 3HOP_R, 9.1 m, 18.3 m, PA, L leg_1RM, R leg_1RM, 2 leg_1RM, HE_1RM, HF_1RM, Vmax, Tmax, CFMod, and 3PQ. Absolute and relative percentage change from pre- to posttraining was calculated for all variables, and 1-way ANOVA was used to determine differences between the groups. A significance level of P ≤ 0.05 was set for all statistical analyses. Where significance was found, a Tukey post hoc test was performed.
Two subjects required an additional week to finish the investigation because of experiencing brief illnesses unrelated to the training. However, all subjects completed all training sessions. Additionally, there was no significant difference in total time spent running between INC and LEV during treadmill sessions. Table 5 highlights the performance responses of INC, LEV, and CON. INC experienced significant time effect improvements in all performance outcomes, whereas LEV experienced significant time effect improvements in CMJ, 9.1-m sprint, PA, Vmax, Tmax, 2 leg_1RM, L leg_1RM, R leg_1RM, and HE_1RM. CON did not experience significant improvements in any outcome measure. Furthermore, there were significant group by time interactions for improvements in 9.1-m sprint, 18.3-m sprint, 3HOP_L, 3HOP_R, PA, CFMod, and HF_1RM with INC improving to a greater extent than LEV and CON.
Power and Fatigue Outcomes
Table 6 highlights the power and fatigue responses of INC, LEV, and CON during 3PQ testing. Significant time effect improvements in all power and fatigue outcome measures were demonstrated by INC and LEV; CON did not experience significant improvements in any outcome measure. There were no significant group by time improvements between INC and LEV.
Soccer players routinely engage in sports performance training programs during winter months that consist of plyometric training, strength training, and both incline and level-grade SIT. Specific to incline-based SIT, to date, a paucity of research exists, highlighting the efficacy of using this approach to improve measures related to the athletic performance of soccer players, including speed, strength, power, change of direction, and anaerobic capacity. Therefore, the purpose of this investigation included comparing 2 combined training approaches involving identical strength training and plyometric training programs with different approaches to SIT by examining measures that are both unique to the effects of incline and level-grade SIT, as well as applicable to soccer performance. The findings of the present investigation show that level and incline-based SIT in combination with plyometric training and strength training significantly improved measures of speed, strength, change of direction, and anaerobic capacity in a group of adolescent soccer players. Additionally, incline compared with level-based SIT led to significantly greater improvements in 9.1-m sprint, 18.3-m sprint, 3HOP_L, 3HOP_R, PA, CFMod, and HF_1RM. Therefore, based on these findings, we accept our hypothesis that incline vs. level-based SIT would lead to greater performance gains. In our estimation, the current investigation is the first to compare the training effects between time-matched SIT performed using INC and LEV conditions.
Combined or concurrent training programs have been used extensively in soccer for years, and a number of previous investigations have examined the effects of these methods, including, for example, strength training and power training, various sequences of strength and plyometric training, plyometric training and speed training, strength training and speed training, strength training and SIT, and optimum power training (OPL) combined with either resisted sprints (RS) or plyometrics (21,22,24,28,35,42,43). Moreover, the incorporation of SIT has been primarily field based. In contrast, the use of treadmill intervals, with a specific focus on performing those intervals on an incline, has not been investigated.
Previous studies have examined training programs combining traditional strength (back squat) and power-oriented (power clean) exercises in young soccer players (42). For example, Wong et al. (42) investigated combined strength and power training in 14-year-old, male, soccer players performing 2 training sessions·wk−1 for 12 weeks (42). Pre- and posttest absolute differences in means between the experimental and control groups revealed significant improvements in a variety of performance measures including 3.2 cm and 0.07 seconds, respectively, for CMJ and 10-m sprint performance. In the present investigation, INC demonstrated greater and similar significant improvements compared with CON in absolute differences of the means in the same (CMJ) and similar (9.1-m sprint) tests, whereas LEV did not. Total training sessions (24 sessions) and time committed to individual training sessions (approximately 60 minutes) was the same in both Wong et al. (42) and the present investigation. However, a greater emphasis was placed on lower-body resistance training in Wong et al. (42) and specifically on bilateral exercises. In contrast, in the present investigation, one exercise (bilateral leg press) was performed bilaterally. Previous research indicates that unilateral limb training may be more effective at improving muscle strength for applying force unilaterally in sprinting and change of direction movements (13). Importantly, during incline treadmill running, hip range of motion and joint angular velocity during push off at the ankle, knee, and hip have been shown to be significantly greater compared with level-grade sprinting at the same stride frequency (38). This increased range of joint motion during intense triple extension muscle contractions, together with increased joint angular velocity, leads to significantly greater hip muscle power during incline running and indicates that incline treadmill running may represent a sport-specific training tactic (38).
Others have investigated different sequences of combined strength and plyometric training in elite 19-year-old soccer players (21). Training twice weekly for 8 weeks, Kobal et al. (21) had players complete traditional training (TD, strength followed by plyometric training), complex training (CP, plyometric followed by strength training), or contrast training (CT, alternating strength and plyometric training set by set). The strength and plyometric exercises were back half-squat and drop jumps (from boxes of 30 and 45 cm), respectively. Although all groups significantly improved back half-squat (TD, 46.3%; CP, 48.6%; CT, 53.0%) and CMJ (TD, 14.2%, CP; 13.0%; CT, 14.7%), only TD demonstrated significant mid to posttest improvements in 10- and 20-m sprint performance. As to the lack of significant pre- to posttest improvement in sprint performance by any of the groups, the authors postulated that it may have been the result of the “interference phenomenon,” which has been shown previously to affect maximum sprint performance in high-level soccer players while not affecting 1RM strength or CMJ performance during combined training (8). In the present investigation, in addition to significant group effects demonstrated by INC and LEV in a similar strength exercise (2_leg 1RM), both INC and LEV also demonstrated significant group effects in similar short-burst sprint performance tests (9.1 and 18.3 m, respectively). One possible explanation why INC and LEV demonstrated significant improvements in short-burst sprint performance is that all plyometric exercises in the present investigation were repeat jump exercises vs. single drop jumps performed in Kobal et al. (21). Previous research of repeat plyometric jump exercises indicate that they are more effective at improving jump height (both drop jump and repeated CMJ) than single jump plyometric exercises, which may have implications for high-velocity movements involving repeated SSCs, such as sprinting (25).
Examining combined plyometric training and speed training, Saez de Villarreal et al. (35) had 14- to 15-year-old soccer players train 2 sessions·wk−1 (approximately 40 min·session−1) for 9 weeks using jumping, skipping, and hopping exercises in combination with 10-m all-out sprints. The experimental vs. control group demonstrated significant improvements in CMJ and 10-m sprint performance by 9.4 and 4.8%, respectively. In the present investigation, improvements in CMJ posted by INC and LEV were greater and less, respectively, whereas in the similar short-burst sprint performance test of 9.1 m, INC and LEV recorded significant improvements that were similar and less, respectively. In the study by Saez de Villarreal et al. (35), all plyometric training involved bilateral body weight exercises (half-squats with jumps, hurdle jump, and vertical jumps). In the present investigation, all plyometric sessions involved a combination of unilateral and bilateral exercises. In those sessions emphasizing footwork drills, total time spent performing exercises ranged 4.5–6.5 minutes, with unilateral footwork drills constituting 1–2 minutes of the total. Furthermore, 4 of the 12 plyometric sessions (sessions 3, 5, 7, and 9) emphasized more traditional plyometric exercises, such as box jumps and weighted squat jumps; in these sessions, the number of high-intensity box jumps and weighted squat jump contacts ranged 96–128 per session (for a total of 448 in the 4 sessions). By comparison, total contacts·session−1 in Saez de Villarreal et al. (35) ranged from 60 (week 1) to 200 (week 9). Based on the present investigation, despite performing a majority of low-intensity plyometric footwork drills aimed at improving kinesthetic awareness, the relatively modest number of unilateral and bilateral high-intensity box jumps and weighted squat jumps seems to have provided an adequate training stimulus. Additionally, this may be a prudent approach for younger athletes who may be engaging in plyometric training for the first time (1,6). Finally, although the present investigation did not record the number of ground contacts during the low-intensity footwork drills, based on Chu's recommendation for quantifying the 4-square exercises, investigators and coaches could manually count foot strikes or use force plates to determine the degree of improvement with training (6).
In a 9-week study of 17-year-old, male, soccer players involving 2 sessions·wk−1 of combined strength and speed training (COM) compared with strength training (STR) alone, Kotzamanidis et al. (22) reported that both groups demonstrated a number of significant time effect improvements, but COM demonstrated significant time effect improvements in several additional measures (half squat, CMJ, unilateral step-up, and 30-m sprint performance). For this discussion, comparisons between COM, INC, and LEV are most appropriate becaue several performance measures were similar. For example, COM demonstrated significant improvements in half squat, CMJ, unilateral step-up, and 30-m sprint performance of 8.6, 6.8, 14.0, and 3.7%, respectively. In the present investigation, INC recorded significant gains in the same (CMJ) or similar (2 leg_1RM, 1 leg_1RM, and 18.3-m sprint) performance measures that were greater than COM, whereas LEV demonstrated greater gains compared with COM in 2 leg_1RM and similar gains in CMJ, 1 leg_1RM, and 18.3-m sprint performance. Interestingly, the strength training program performed by COM was similar to INC and LEV in that both investigations involved 3 lower-body exercises. More specifically, COM used a strength training routine involving 2 exercises·session−1 that targeted the powerful knee extensor muscle group and to a lesser extent the hip extensors. In contrast, INC and LEV performed one exercise·session−1 targeting the knee extensor muscle group and another that specifically targeted the hip extensor and hip flexor musculature. Specific to the hip flexor, previous research has indicated the importance of this muscle group in achieving and maintaining maximum sprint speed (5) and that the biarticular rectus femoris (assisting in hip flexion) transfers energy generated by the monoarticular hip extensors to assist in the powerful knee extensions that occur in high-velocity triple extension movements, such as sprinting and jumping (18,30).
A different investigation by Wong et al. (43) examined combined strength training and SIT during 2 training sessions·wk−1 for 8 weeks. Resistance exercises included jump squats and back half squats, whereas SIT consisted of sixteen 15-second bouts (with 15 seconds rest) at 120% of maximal aerobic speed (MAS) for a total of 4 minutes of running. Performance measures that were similar to the present investigation in which the experimental group recorded significant improvements included back squat 1RM (20.3%), CMJ (3.9%), 10-m sprint (5.8%), 30-m sprint (2.7%), and distance covered while running at MAS (9.2%). In the present investigation, not only did INC and LEV demonstrate significant improvements in 2 leg_1RM, CMJ, 9.1, and 18.3-m sprint performance that were similar but also recorded significant improvements in Tmax (INC: 44.7%; LEV: 43.6%), giving insights into distance covered at a high intensity. Specifically, in Wong et al. (43), the average improvement in distance covered running at MAS represented 298 m; in the present investigation, the improvement in Tmax while running at Vmax reflected an average increase in distance run of 308 m (INC) and 301 m (LEV), respectively. The similarities in running intensities and total time spent running between the experimental group in Wong et al. (43) and INC and LEV in the present investigation add further evidence to the efficacy of using low-volume SIT performed at maximal or supramaximal intensities to improve anaerobic endurance (2,9).
Finally, Loturco et al. (24) investigated 12 sessions of OPL combined with either RS or plyometrics (PL), respectively, in 22-year-old, professional, soccer players over 5 weeks. Optimum power load was individually determined from an upright weighted jump squat test, sled pull sprints consisted of loads ranging 5–20% of body mass, and plyometrics were either horizontal or vertical jumps (or a combination thereof). Regarding CMJ, OPL with PL demonstrated a small effect size (0.20), whereas OPL with RS was small (−0.39). Similarly, small and trivial effect sizes for SLJ were recorded by OPL with PL (0.40) and OPL with RS (−0.03), respectively. Interestingly, mean maximum power output of all subjects in the weighted jump squat test was a load equal to 60% of body mass or an average load of 118 kg (body mass plus 60% of body mass load). In the present investigation, 40% of 2 leg_1RM was used for 3PQ testing, a load equal to 122 and 129 kg for LEV and INC, respectively. Although the experimental groups in Loturco et al. (24) reported trivial effect sizes for mean maximum power output, in the present investigation, both INC and LEV demonstrated significant time effect improvements in maximum positive power during 3PQ testing of 16.7 and 11.2%, respectively. Furthermore, velocity of movement during the weighted jump squat test in Loturco et al. (24) was reported as small and trivial for OPL with RS and OPL with PL, respectively, whereas in the present investigation, both INC and LEV demonstrated significant time effect improvements in this measure during 3PQ testing of 8.3 and 6.2%, respectively. However, the training status of the subjects (Brazilian first division professionals vs. American youth club players) most likely had an impact on the demonstrated results because relatively novice athletes may respond to a greater degree when introduced to new training tactics (37).
Regarding incline treadmill running, Swanson and Caldwell (38) reported that incline sprinting on a treadmill using intensities and grades similar to those used in present investigation compared with level-grade running at the same stride frequency led to significantly greater: (a) extensor range of motion at the ankle, knee, and hip; (b) average electromyographical amplitude of key lower-body musculature during stance; and (c) average power and energy generated during hip flexion and hip extension in the swing phase of running, respectively. Additionally, horizontal braking and propulsive forces associated with foot strike, stance, and toe-off during incline treadmill running at grades of >15% are decreased and increased, respectively, by 50 and 75% compared with level-grade running (14). Combined with the alterations in the kinematics and electromyographical of key propulsive muscles during incline sprinting, this suggests that incline running represents a particularly functional training tactic for replicating powerful triple extension movements of the ankle, knee, and hip while running. Moreover, this tactic may also contribute to an enhanced ability to orient the resultant force vector in a manner described by Morin et al. (27) that maximizes the propulsive forces involved in acceleration and sprinting ability. Despite relatively little research into the physiological and performance effects associated with incline treadmill training, several investigations have reported on this training tactic in distance runners (9–11). In these investigations, runners performed 10 to 14 thirty-second bouts twice weekly for 6 weeks on a 10% grade while running at Vmax. In well-trained distance runners, this training led to significant improvements in Tmax (10); however, in this same investigation, level-grade interval training performed using Vmax and 60% Tmax (an average interval of 136 seconds) as training prescriptors led to even greater group by time results in Tmax. In another investigation, runners using Vmax, 30 seconds and 10% grade as training prescriptors compared with 68% Vmax, 60% Tmax (average interval of 173 seconds) and 10% grade, respectively, demonstrated significant group by time improvements in key physiological and performance measures, including VLT and CFMod, leading to the conclusion that shorter, faster SIT performed on an incline was more effective than longer, slower incline-based interval training (9). Interestingly, total time spent running during the shorter, faster incline training sessions (5–7 min·session−1) of each of these investigations (9–11) was very similar to that performed by INC in the present investigation (4–7 min·session−1).
In summary, our findings show that strength and plyometric training combined with level or incline-based SIT produced significant improvements in a number of measures associated with sprinting, jumping, change of direction, strength, and anaerobic capacity in adolescent soccer players. Moreover, inclined-based SIT led to even greater improvements in sprinting ability (9.1 and 18.3 m), powerful triple extension muscle actions (3HOP_L and 3HOP_R), change of direction (PA), glycolytic bioenergetics (CFMod), and hip flexor strength (HF_1RM). However, whether the improvements in these measures enhance on-field performance remains to be determined. Furthermore, we acknowledge limitations to this investigation, including the subject age range, a lack of reporting on biological ages of the subjects, and having used both male and female players. Moreover, although the control group in the present investigation was included to account for the effects of physical maturation during the study, we acknowledge that by not ensuring a matched weekly load of sports practices between the control and experimental groups, the strength of comparisons to the control group are lessened. Also, although treadmill running allows greater training precision and control over environmental factors, it may be that overground SIT would produce different results. In addition to these limitations, we recommend future investigations to examine SIT using other combinations of training prescriptors. For example, shorter, more intense, level-grade bouts or alternating between a level-grade and incline training session each week may be a more effective approach to SIT.
Soccer at all levels is characterized by speed, acceleration, jumping, change of direction, and aerobic and anaerobic capacity, and ever-growing numbers of players routinely participate in off-field training programs throughout the year. Hence, training tactics shown to significantly enhance these measures are of great interest to players and coaches. One particular training tactic performed by many soccer players in off-season training is SIT, with players using both incline and level-grade conditions. Previous investigations of incline running indicate that there may be unique metabolic, muscular, or neuromuscular gains compared with level-grade running, which may be of interest to players, coaches, and strength and conditioning professionals. The results of the present investigation provide further insights into sports performance training tactics for adolescent soccer players and indicate that combined training tactics involving SIT, plyometrics, and strength training can be used safely and effectively in this age group. Specific to the SIT component of the present investigation, with increased access to commercial grade treadmills, replicating either of the SIT protocols performed by INC or LEV is within the capacity of many players, coaches, and soccer organizations. Additionally, based on several key speed and power measures that showed additional improvements, we recommend, if logistically possible, performing SIT using the INC approach in conjunction with strength and plyometric training. Based on the findings of the present investigation, if performing incline-based SIT, we recommend a training session to involve a training intensity equal to an individually determined MAS (100% Vmax) and bout durations and treadmill grades ranging 10–30 seconds and 5–30%, respectively, for a total of 4–6 minutes of running. Alternatively, if performing level-grade SIT, we recommend a training session be consist of 30-second bouts in combination with supramaximal aerobic speeds ranging 105–138% Vmax for a similar amount of total time spent running. Furthermore, with either approach, we recommend incorporating 2–3six-second sprint efforts using intensities of 140–165% Vmax at the end of the training session to further enhance the neuromuscular adaptations. Finally, we encourage training such as this to be completed during times of the year considered the “off-season,” traditionally, during the winter or summer months leading up to the fall or spring competitive seasons, respectively. However, special attention must be given to athletes who perform SIT on treadmills, including “spotting” them at all times and providing instruction on proper running form and technique.
The authors thank the subjects for their effort, willingness, and enthusiasm for participating in this study. They also greatly appreciate the assistance of Augustana University exercise science students Clark Vargo, Alaina Klapperich, Brooke Bleeker, and Hunter Haman; the staff of Avera Sports; and the support of Avera McKennan Hospital and University Health Center. The authors of this study received no funding for this investigation and have no conflicts of interest.
This investigation was self-funded and the authors have no conflict of interest.
1. Bedoya AA, Miltenberger MR, Lopez RM. Plyometric training effects on athletic performance in youth soccer athletes: A systematic review. J Strength Cond Res 29: 2351–2360, 2015.
2. Billat LV. Interval training for performance: A scientific and empirical practice. Special recommendations for middle- and long-distance running. Part II: Anaerobic interval training. Sports Med 31: 75–90, 2001.
3. Billat VL, Slawinski J, Bocquet V, et al. Intermittent runs at the velocity associated with maximal oxygen uptake enables subjects to remain at maximal oxygen uptake for a longer time than intense but submaximal runs. Eur J Appl Physiol 81: 188–196, 2000.
4. Buchheit M, Laursen PB. High-intensity interval training, solutions to the programming puzzle. Part II: Anaerobic energy, neuromuscular load and practical applications. Sports Med 43: 927–954, 2013.
5. Chapman AE, Caldwell GE. Kinetic limitations of maximal sprinting speed. J Biomech 16: 79–83, 1983.
6. Chu D. Jumping
into Plyometrics. Champaign, IL: Human Kinetics, 1998. pp. 69–74.
7. Faude O, Roth R, Di Giovine D, Zahner L, Donath L. Combined strength and power training in high-level amateur football during the competitive season: A randomised-controlled trial. J Sports Sci 31: 1460–1467, 2013.
8. Faude O, Schnittker R, Schulte-Zurhausen R, Muller F, Meyer T. High intensity interval training vs. high-volume running training during pre-season conditioning in high-level youth football: A cross-over trial. J Sports Sci 31: 1441–1450, 2013.
9. Ferley DD, Hopper DT, Vukovich MD. Incline treadmill interval training: Short vs. Long bouts and the effects on distance running performance. Int J Sports Med 37: 958–965, 2016.
10. Ferley DD, Osborn RW, Vukovich MD. The effects of uphill vs. level-grade high-intensity interval training on VO2max, Vmax, V(LT), and Tmax in well-trained distance runners. J Strength Cond Res 27: 1549–1559, 2013.
11. Ferley DD, Osborn RW, Vukovich MD. The effects of incline and level-grade high-intensity interval treadmill training on running economy and muscle power in well-trained distance runners. J Strength Cond Res 28: 1298–1309, 2014.
12. Ferley DD, Vukovich MD. Predicting the intensity for performing supramaximal incline treadmill interval training in distance runners. J Strength Cond Res 33: 1354–1361, 2019.
13. Gonzalo-Skok O, Tous-Fajardo J, Suarez-Arrones L, et al. Single-leg power output and between-limbs imbalances in team-sport players: Unilateral versus bilateral combined resistance training. Int J Sports Physiol Perform 12: 106–114, 2017.
14. Gottschall JS, Kram R. Ground reaction forces during downhill and uphill running. J Biomech 38: 445–452, 2005.
15. Haff GG, Triplett NT. National Strength Conditioning Association: Essentials of Strength Training and Conditioning. Champaign, IL: Human Kinetics, 2016. p. 396.
16. Heidt RS Jr, Sweeterman LM, Carlonas RL, Traub JA, Tekulve FX. Avoidance of soccer injuries with preseason conditioning. Am J Sports Med 28: 659–662, 2000.
17. Howard N, Stavrianeas S. In-season high-intensity interval training improves conditioning in high school soccer players. Int J Exerc Sci 10: 713–720, 2017.
18. Jacobs R, Bobbert MF, van Ingen Schenau GJ. Mechanical output from individual muscles during explosive leg extensions: The role of biarticular muscles. J Biomech 29: 513–523, 1996.
19. Jones AM, Doust JH. A 1% treadmill grade most accurately reflects the energetic cost of outdoor running. J Sports Sci 14: 321–327, 1996.
20. Kavaliauskas M, Kilvington R, Babraj J. Effects of in-season uphill sprinting on physical characteristics in semi-professional soccer players. J Sports Med Phys Fitness 57: 165–170, 2017.
21. Kobal R, Loturco I, Barroso R, et al. Effects of different combinations of strength, power, and plyometric training on the physical performance of elite young soccer players. J Strength Cond Res 31: 1468–1476, 2017.
22. Kotzamanidis C, Chatzopoulos D, Michailidis C, Papaiakovou G, Patikas D. The effect of a combined high-intensity strength and speed training program on the running and jumping
ability of soccer players. J Strength Cond Res 19: 369–375, 2005.
23. Kuipers H, Verstappen FT, Keizer HA, Geurten P, van Kranenburg G. Variability of aerobic performance in the laboratory and its physiologic correlates. Int J Sports Med 6: 197–201, 1985.
24. Loturco I, Kobal R, Kitamura K, et al. Mixed training methods: Effects of combining resisted sprints or plyometrics with optimum power loads on sprint and agility performance in professional soccer players. Front Physiol 8: 1034, 2017.
25. Makaruk H, Czaplicki A, Sacewicz T, Sadowski J. The effects of single versus repeated plyometrics on landing biomechanics and jumping
performance in men. Biol Sport 31: 9–14, 2014.
26. Mohr M, Krustrup P, Bangsbo J. Match performance of high-standard soccer players with special reference to development of fatigue. J Sports Sci 21: 519–528, 2003.
27. Morin JB, Bourdin M, Edouard P, Peyrot N, Samozino P, Lacour JR. Mechanical determinants of 100-m sprint running performance. Eur J Appl Physiol 112: 3921–3930, 2012.
28. Mujika I, Santisteban J, Castagna C. In-season effect of short-term sprint and power training programs on elite junior soccer players. J Strength Cond Res 23: 2581–2587, 2009.
29. Pettersen SA, Brenn T. Activity profiles by position in youth elite soccer players in official matches. Sports Med Int Open 3: E19–e24, 2019.
30. Prilutsky BI, Zatsiorsky VM. Tendon action of two-joint muscles: Transfer of mechanical energy between joints during jumping
, landing, and running. J Biomech 27: 25–34, 1994.
31. Rampinini E, Sassi A, Azzalin A, et al. Physiological determinants of Yo-Yo intermittent recovery tests in male soccer players. Eur J Appl Physiol 108: 401–409, 2010.
32. Rampinini E, Sassi A, Morelli A, et al. Repeated-sprint ability in professional and amateur soccer players. Appl Physiol Nutr Metab 34: 1048–1054, 2009.
33. Reilly T, Reilly N, Secher P, Snell P, Williams C. Physiology of sports. London, United Kingdom: E. & F.N. Spon, 1990. pp. 328–331.
34. Reilly T, Williams AM, Nevill A, Franks A. A multidisciplinary approach to talent identification in soccer. J Sports Sci 18: 695–702, 2000.
35. Saez de Villarreal E, Suarez-Arrones L, Requena B, Haff GG, Ferrete C. Effects of plyometric and sprint training on physical and technical skill performance in adolescent soccer players. J Strength Cond Res 29: 1894–1903, 2015.
36. Stolen T, Chamari K, Castagna C, Wisloff U. Physiology of soccer: An update. Sports Med 35: 501–536, 2005.
37. Suchomel TJ, Nimphius S, Bellon CR, Stone MH. The importance of muscular strength: Training considerations. Sports Med 48: 765–785, 2018.
38. Swanson SC, Caldwell GE. An integrated biomechanical analysis of high speed incline and level treadmill running. Med Sci Sports Exerc 32: 1146–1155, 2000.
39. Turner AN, Stewart PF. Strength and conditioning for soccer players. Strength Cond J 36: 1–13, 2014.
40. van Melick N, Meddeler BM, Hoogeboom TJ, Nijhuis-van der Sanden MWG, van Cingel REH. How to determine leg dominance: The agreement between self-reported and observed performance in healthy adults. PLoS One 12: e0189876, 2017.
41. Wisloff U, Castagna C, Helgerud J, Jones R, Hoff J. Strong correlation of maximal squat strength with sprint performance and vertical jump height in elite soccer players. Br J Sports Med 38: 285–288, 2004.
42. Wong PL, Chamari K, Wisloff U. Effects of 12-week on-field combined strength and power training on physical performance among U-14 young soccer players. J Strength Cond Res 24: 644–652, 2010.
43. Wong PL, Chaouachi A, Chamari K, Dellal A, Wisloff U. Effect of preseason concurrent muscular strength and high-intensity interval training in professional soccer players. J Strength Cond Res 24: 653–660, 2010.