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Original Research

Effects of an 8-Week Body-Weight Neuromuscular Training on Dynamic Balance and Vertical Jump Performances in Elite Junior Skiing Athletes: A Randomized Controlled Trial

Vitale, Jacopo A.1; La Torre, Antonio1,2; Banfi, Giuseppe3,4; Bonato, Matteo2

Author Information
Journal of Strength and Conditioning Research: April 2018 - Volume 32 - Issue 4 - p 911-920
doi: 10.1519/JSC.0000000000002478
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Abstract

Introduction

Alpine ski is a popular winter sport, practiced by millions of individuals worldwide (11). It is extremely complex and challenging from physical, technical, and tactical perspectives. The extreme environment of cold and altitude, movement complexity, and the large variety of disciplines make alpine ski racing a difficult sport to study too. Specifically, 4 different disciplines are performed: downhill, slalom, giant slalom, and super giant slalom (50). These disciplines vary by speed, length, number of turns, and gate placement. Downhill and super giant slalom can be considered long and speed events with races lasting between 1 and 3 minutes and skiers reaching up to 130 km·h−1. On the contrary, slalom and giant slalom are shorter, approximately 60–90 seconds, and slower events with a maximum of 60 km·h−1 registered by athletes but with steeper terrain and narrow turns (2,14,48,51). Nonetheless, some common characteristics among all the alpine ski disciplines can be identified. In general, alpine skiing is considered a vigorous physical activity where constant and extremely rapid weight transfers are required to move as fast as possible on the snow (11). This kind of physical activity involves, approximately, a 65% contribution from the anaerobic energetic system (51), and it is commonly stated that force production and neuromuscular coordination should be the focus of training, both for senior, junior athletes, and recreational practitioners (46,51).

Winning a race in top-level competition is often matter of fractions of a second for professional athletes and this small gap underlines the need for a deep study of the factors limiting and influencing alpine skiing performance (22). Previous studies focused the attention mostly on the effect of physiology and strength and conditioning programs on performance (16,28). However, other articles studied the epidemiology, causes, and preventive strategies of injuries to alpine skiers (21,31,40,43). Alpine skiing remains one of the riskiest activities in terms of injury incidence (7,27), and this information should lead strength and conditioning coaches to modify and improve injury prevention methods and strategies. In elite alpine ski racing, injury rate higher than 36% have been reported with the most common injuries occurred at the knee (47). In addition, overuse injuries represent a critical problem among adolescent skiers, and to confirm this, it was underlined that young elite ski athletes obtained similar results compared with the World Cup level with respect to injury incidence (52). Excluding external factors, such as wearing helmets or using appropriate technical material, that can significantly reduce the injury incidence (13,37), one of the possible interventions is to actively work on athletes' intrinsic physical and athletic factors. Nonetheless, no specific exercises or training programs have yet been studied in the scientific literature for the prevention of injuries in alpine skiers.

Neuromuscular control is a key factor of motor skills, and it is defined as the athlete's ability to maintain the body's center of gravity within its base of support (18). Among all the intrinsic risk factors, such as anthropometrics, biological maturity status, and/or ski-racing technique, the lack of neuromuscular control of the lower limbs seems to be the most relevant, and it has been associated with knee and ankle injuries in individuals practicing different disciplines (53). Consequently, it also has been demonstrated that neuromuscular training program is extremely effective in reducing the risk of lower extremity injuries (55), specifically improvements in muscle recruitment patterns, postural stability, and dynamic lower extremity balance have been observed (56). For what concerns young elite athletes and skiers, previous studies identified poor general physical fitness as a risk factor for sports-related injuries (12), and reduced core strength, core strength imbalance, and poor neuromuscular control are considered critical factors for anterior cruciate ligament (ACL) accidents and, in general, for lower limb injuries (15,44). Neuromuscular training requires the use of specific material, such as medicine balls or unstable bases but, however, a more practical and cost-effective solution is to implement this kind of training, into the warm-up routines, with the use of body-weight overload. Therefore, the combination of core stability and plyometric exercises may provide a useful strategy to develop these body-control skills, such as strength, dynamic balance, and coordination, and to reduce injury incidence (20,35,38).

Among the many postural-control assessments to predict the risk of injury in athletes, dynamic postural-control tests are preferred by clinicians and coaches because they reproduce movements and tasks expected during sports activities (32). The Y-Balance Test (YBT), a validated derivation of the Star Excursion Balance Test (SEBT), has demonstrated its potential to (a) detect pathologic conditions of the lower limb such as chronic ankle instability (CAI) or ACL deficiency; (b) reveal the effect of induced fatigue on dynamic postural control; (c) demonstrate improvements obtained with exercise intervention in patients with CAI or in healthy individuals; and (d) predict the risk of lower extremity joint injury (19,32). Poor performance on the YBT showed an association with an elevated risk of noncontact lower extremity injury (42); therefore, an increased YBT performance could be indicative of an improved physical fitness and dynamic-postural control in athletes.

To the best of our knowledge, no previous study examined the effect of neuromuscular training in junior alpine skiers. Our rationale for selecting neuromuscular exercises was because they aimed to correct deficits in postural control of lower limb and trunk muscles in athletes (8). Therefore, the aim of the present randomized controlled trial was to evaluate the effects of 8 weeks of neuromuscular training program in elite junior skiers. We hypothesized that this training focused on core stability, plyometric, and body-weight strengthening exercises, included in warm-up routine, could impact on dynamic-postural control and vertical jump performance in elite junior skiers.

Methods

Experimental Approach to the Problem

To assess the effect of neuromuscular warm-up in elite junior male skiers, we devised a 2-armed, parallel group, randomized controlled trial. Participants were recruited from 2 elite junior ski clubs from the north of Italy. Skiers, each year, follow the same training schedule during the season: the resting period takes place between May and June (off-season), they perform athletic preparative training in July and August and go on with precompetitive specific training in September and October (preseason), and they finally compete from November to April (in-season). Specifically, the study ran 8 weeks during the 2017 agonistic season during the months of January and February. Immediately before the intervention period, the skiers regularly conducted 4 training sessions during the week: usually on Monday, Tuesday, Wednesday, and Friday, and in addition, on Saturday or Sunday, they used to compete for an official race. If a race took place between Monday and Friday, athletes always added a training session during the weekend. Each training session lasted between 120 and 150 minutes. Inclusion criteria were age 16–20 years, male sex, at least 6 years practicing skiing, and a practice of 4 times a week for ≥2 hours. Exclusion criteria were tobacco use, use of medications, any medical conditions contraindicating physical exercise as diagnosed by a sports medicine physician, a history of lower extremity injury (e.g., ankle sprain, knee sprain, and ACL lesions), or surgery in the 6 months before testing and previous exposure to neuromuscular training. All participants were randomized into an intervention or control group (CG) in a 1:1 ratio to either experimental group (EG) that performed neuromuscular warm-up exercises or a CG involved in a standard warm-up (Figure 1). A computer-generated list of random numbers was used. M.B. who conducted the randomization did not take part to the intervention. The players in both groups were evaluated with lower quarter YBT, countermovement jump (CMJ), and drop jump (DJ) at baseline (PRE) and at the end (POST) the experimental procedures. Both PRE and POST tests and the training intervention were performed at the same time of day (15:00–15:30) on the same indoor athletic track in dry and windless weather conditions at approximately 20° C.”

F1
Figure 1.:
Flowchart of the study and subjects' screening.

Subjects

A total of 24 Italian elite junior male skiers (age range: from 16.9 to 19.1 years [age range: 17-19 years old]) were recruited and then randomized to either EG (± SDn = 12, age 18 ± 1 years; body mass 66 ± 21 kg; height 1.70 ± 0.1 m; weekly training volume 10 ± 2 hours) or CG (n = 12, age 18 ± 1 years; body mass 62 ± 14 kg; height 1.73 ± 0.1 m; weekly training volume 10 ± 2 hours). Comparisons of the demographic characteristics of the 2 groups showed no differences in age, body height, or mass. The participants were asked not to engage in any form of physical activity other than their normal routines, to avoid tobacco and caffeine consumption, to maintain their regular lifestyle, their usual diets, and to keep themselves always sufficiently hydrated for the duration of the study. Data on medical history, age, height, body mass, training characteristics, injury history, and performance level were collected at baseline. Height was measured to the nearest 1 cm and body mass to the nearest 0.5 kg. The study protocol was approved by the Institutional Ethics Review Committee of the University of Milan (approved on December 10, 2015, Prot. N. 54/15) and conducted in accordance with current national and international laws and regulations governing the use of human subjects (Declaration of Helsinki II). A signed informed consent form, outlining the study protocol benefits and risks for participating in the study, was obtained from each participant and, for subjects younger than 18 years old, parental or guardian consent was also obtained.

Procedures

Lower Quarter Y-Balance Test

The Y-Balance Test was performed using a standardized testing protocol that has been shown to be reliable (41,42). None of the subjects were known to have had previous exposure to YBT, which might have interfered with the validity of the testing protocol. The subjects were fully familiarized with the testing procedures. After watching a standard video demonstration, they were provided initial YBT instructions and allowed 6 practice trials with 6 reaches in each direction (42). Assessment with the YBT was performed using a YBT kit comprising a stance platform to which 3 pieces of polyvinyl chloride pipe are attached in the anterior (A), posteromedial (PM), and posterolateral (PL) reach directions. The posterior pipes were positioned 135° from the anterior pipe with 45° between them. Each pipe was marked in 5-mm increments for measurement. The YBT was performed with the distal aspect of the great toe centered at the junction of the Y. The subjects had to reach with the opposite leg in the anterior, PM, and PL directions and push a target (reach indicator) along the pipe that standardized the reach distance. The target remained over the tape measure after completion of the test. The order of the tests was performed according to the guidelines published in Plisky et al. (42): 3 trials were performed with the subjects standing on the right foot while reaching with the left foot in the anterior direction, followed by 3 trials with the subject standing on the left foot and reaching with the right foot in the A direction. The procedure was repeated for the PM and then the PL reach directions. During the trials, the reach foot was not allowed to touch the floor or gain balance from the reach indicator or support pipe. If the subject was unable to perform the test according to the above criteria in 6 attempts, she failed that direction, no data were collected, and another trial was attempted. Reach distance was measured from the most distal aspect of the toes of the stance foot to the most distal aspect of the reach foot in the anterior, PM, and PL directions. The YBT scores were analyzed using the average of the last 3 trials for each reach direction for each lower extremity and the average of the total of reach directions (composite score). The YBT composite score was calculated according to Plisky et al. (42) by dividing the sum of the maximum reach distance in the anterior, PM, and PL directions by 3 times the limb length (LL) of the subject, then multiplied by 100: ([A + PM + PL]/[LL × 3]) × 100. The maximum value measured for each excursion direction was also analyzed, as well as the summed composite score of the maximums for each lower extremity. To control for the effect of LL between subjects, the YBT values were normalized to percent of average anatomical LL (average of right and left side).

Vertical Jump Tests

The procedures were performed as described by Maulder and Cronin (34) in which 3 CMJs and DJs were performed on an Optojump Next (Microgate, Bolzano, Italy). In this setup, the Optojump photoelectric cells consisted of 2 parallel bars (1 receiver and 1 transmitter unit) that were placed approximately 1 m apart and parallel to each other. The transmitter contains 32 light-emitting diodes positioned 3 mm above ground level at 31.25-mm intervals. The Optojump bars were connected to a personal computer. Jump height was measured using proprietary software (Optojump software; version 3.01.0001). The Optojump system measured the flight time of CMJ with an accuracy of 1 ms (1 kHz). Jump height was estimated using the following equation (9):where h is jump height, g is gravitational acceleration (9.81 m·s−2), and tf is flight time. Before testing, subjects performed 15 minutes of standardized warm-up consisting of 5 minutes of submaximal running followed by a dynamic stretch routine comprising functional exercises: front-to-back leg swing, side-to-side leg swing, lateral lunge (squat to flow), and sumo squat to stand. Stretching was not allowed because it might have introduced confounding factors because of stretching one side more vigorously than the other. After taking several submaximal jumps to familiarize themselves with the jump movement, the subjects performed 3 trials of CMJ and DJ. Regarding CMJ, from the standing position, the subjects dropped into a squat position to a knee angle of ∼90°, cued by a countdown of “1, 2, 3, and jump” given on reaching the correct squat position and then jumped vertically. Take off was monitored with no preliminary steps of movement during the eccentric phase. Regarding DJ subjects were instructed to step off the box, rather than to jump of the box, cued by a countdown of “1, 2, 3, and go” touch down on both feet, and jump off as quickly and as high as possible. The hands were kept on the hips during the CMJ and DJ. Both legs were used during the landing phase. Subjects were allowed 20 seconds' recovery time between each trial. Countermovement jump and DJ were executed starting from a standing position with feet aligned parallel. Countermovement jump and DJ not meeting these criteria were repeated. The best CMJ and DJ were recorded for analysis.

Neuromuscular Training Protocol

The neuromuscular warm-up protocol for EG was developed from theory and findings from previous injury-prevention research (5). Training took place twice a week (Mondays and Wednesdays) for 8 weeks (16 sessions) during the warm-up session immediately before regular training. A certified strength and conditioning coach conducted the sessions and gave verbal and visual feedback on exercise technique. In addition, the strength coach encouraged to focus on the quality of the movement and put emphasis on core stability, hip control, and proper knee alignment. When introducing the program to the participants, the main focus was to improve awareness and neuromuscular control during standing, running, planting, cutting, jumping, and landing. Each 30-minute session consisted of circuit training with 10 exercises and 3-minute rests between circuits. The exercises were progressed through 3 phases (Table 1) using periodization methods. Initially, low-volume, high-intensity exercises were performed until the technique was mastered. The volume was increased when the exercise was executed correctly according to the coach's judgment. The exercises were progressed from a stable to an unstable position to increase demands on lower extremity strength and core stability. However, the training program did not include exercises that emulated the YBT.

T1
Table 1.:
Phases of the neuromuscular training program.

However, the conventional warm-up of CG consisted of light aerobic exercises and dynamic stretching of the major muscle groups before the regular practice sessions. Participants of both EG and CG were blinded about the aim of the study, to avoid the risk of contamination between the 2 groups. The strength and conditioning coaches documented the execution of the warm-up program on the players' attendance form for each training session. Compliance to the warm-up program for both EG and CG was defined as the proportion of sessions attended during the experimental protocol. Participants needed to complete 75% of the warm-up to be considered compliant. No acute injuries occurred during the training sessions.

Statistical Analyses

Descriptive statistics (mean ± SD) for the outcome measures were calculated. Ninety-five percent (95%) confidence intervals for each test parameter were derived from the mean ± SD of EG and CG group. Test-retest reliability and measurement error of the YBT, CMJ, and DJ were analyzed by repeating the test at 2 sessions 1 week apart and then comparing the scores using interclass correlation coefficients (ICCs). The ICCs was used to establish intersession repeatability of all measures, where r < 0.50 was classified as weak, from 0.50 to 0.79 as moderate, and ≥0.80 as strong. The normality of the distribution of the subjects' characteristics at baseline was checked using the D'Agostino Pearson test. Because all variables were normally distributed, differences between EG and CG were checked using an unpaired Student's t-test. Intragroup and intergroup differences between EG and CG for YBT, CMJ, and DJ were checked using 2-way analysis of variance with Bonferroni's multiple comparisons test. The level of significance was set at p ≤ 0.05. Statistical analysis was performed using GraphPad Prism version 6.00 for Mac OSX (GraphPad Software, San Diego, CA, USA). Standardized changes in the mean values were used to assess magnitude of effects (effect size [ES]). Values <0.2, <0.6, <1.2, and >2.0 were interpreted as trivial, small, moderate, large, and very large, respectively (4).

Results

Unpaired t-test showed that groups were equally matched, showing no significant differences in age, body mass, height, and weekly training volume. The ICCs (1,1) of the A, PM, PL, and composite YBT scores were 0.90 (SEM = 0.08), 0.87 (SEM = 0.1), 0.84 (SEM = 0.2), and 0.90 (SEM = 0.07), respectively, for the right limb and 0.89 (SEM = 0.12), 0.91 (SEM = 0.08), 0.83 (SEM = 0.07), and 0.90 (SEM = 0.03), for the left limb. Moreover, the ICCs (1,1) of CMJ and DJ were 0.92 (SEM = 0.01) and 0.89 (SEM = 0.02), respectively. The changes in YBT composite score and in the A, PL, and PM reaching directions, respectively, for the right and left limb, before (PRE) and after (POST) the 8-week training protocol are shown in Figures 2 and 3.

F2
Figure 2.:
Changes in YBT composite score and in the anterior-, PM-, and PL-reaching direction for the right limb before (PRE) and after (POST) the experimental training protocol. Data are reported as mean ± SD of normalized reached distances. Ninety-five percent (95%) confidence intervals are reported in brackets. EG = experimental group; CG = control group; YBT = Y-Balance Test; PM = posteromedial; PL = posterolateral. *Intra group difference with p ≤ 0.05; **Intra group difference with p < 0.01; §Inter group difference with p ≤ 0.05.
F3
Figure 3.:
Changes in YBT composite score and in the anterior-, PM-, and PL-reaching direction for the left limb before (PRE) and after (POST) the experimental training protocol. Data are reported as mean ± SD of normalized reached distances. Ninety-five percent (95%) confidence intervals are reported in brackets. EG = experimental group; CG = control group; YBT = Y-Balance Test; PM = posteromedial; PL = posterolateral. *Intra group difference with p ≤ 0.05; **Intra group difference with p < 0.01; §Inter group difference with p ≤ 0.05; §§§Inter group difference with p < 0.001.

No significant differences between EG and CG in PRE training conditions were found in A, PL, PM directions, and composite score for both lower limbs. Two-way analysis of variance with Bonferroni's multiple comparisons test showed for the EG an improvement of YBT composite score for the reaching right (91.9 ± 4.3% vs. 98.0 ± 5.2%, +6.6%, p = 0.03, ES = 1.4) and left (91.8 ± 3.2% vs. 97.8 ± 3.2%, +6.5%, p = 0.005, ES = 1.9) limb, respectively, whereas no significant differences for the CG were detected. For what concerns post hoc intergroup differences, it was observed that EG and CG differ in POST YBT composite score for the reaching right (98.0 ± 5.2%, vs. 89.3 ± 4.5%, +8.9%, p = 0.003, ES = 1.9, interaction p < 0.0001) and left (91.8 ± 3.2% vs. 97.8 ± 3.2%, +6.5%, p = 0.0002, ES = 1.9, interaction p < 0.0001) limb, respectively. Significant improvements over the pretraining in A direction were observed in the EG for the reaching right (71.0 ± 5.7% vs. 77.1 ± 5.3%, +8.7%, p = 0.016, ES = 1.1) and left (70.9 ± 4.6% vs. 76.9 ± 4.6%, +8.5%, p = 0.012, ES = 1.3) limb, respectively, whereas no significant differences for the CG were detected. For what concerns post hoc intergroup differences, it was detected that EG and CG differ in POST A direction for both right (77.1 ± 5.3%, vs. 69.1 ± 5.6%, +10.4%, p = 0.004, ES = 1.4, interaction p < 0.0001) and left (76.9 ± 4.5% vs. 69.4 ± 5.8%, +9.8%, p = 0.007, ES = 1.2, interaction p < 0.0001) limbs. Significant improvements over the pretraining PL direction were observed in the EG for the reaching right (102.1 ± 7.7% vs. 108.1 ± 8.1%, +5.8%, p = 0.037, ES = 0.8) and left (102.5 ± 5.1% vs. 108.5 ± 4.4%, +5.9%, p = 0.006, ES = 1.2) limb, respectively, whereas no significant differences for the CG were detected. The post hoc intergroup analysis highlighted that EG and CG differ in POST PL direction for the reaching right (108.1 ± 8.1% vs. 100.1 ± 4.3%, +7.5%, p = 0.002, ES = 1.8, interaction p < 0.0001) and left (108.5 ± 4.4% vs. 99.8 ± 5.8%, +8.1%, p = 0.004, ES = 1.5, interaction p < 0.0001) limb, respectively. Significant improvements over the pretraining PM direction were observed in the EG for the reaching right (102.5 ± 3.8% vs. 108.6 ± 5.1%, +6.0%, p = 0.038, ES = 1.8) and left (102.1 ± 5.4% vs. 108.1 ± 4.2%, +5.8%, p = 0.004, ES = 1.1) limb, respectively, whereas no significant differences for the CG were detected. Again, the post hoc intergroup analysis detected that EG and CG differ in POST PM direction for the reaching right (108.6 ± 5.1% vs. 98.6 ± 7.5%, +9.3%, p = 0.001, ES = 1.7, interaction p < 0.0001) and left (108.5 ± 4.4% vs. 98.3 ± 4.2%, +8.1%, p = 0.0002, ES > 2.0, interaction p < 0.0001) limb, respectively.

Furthermore, referring to the vertical jumps, the changes in CMJ and DJ tests, and relative ES, before (PRE) and after (POST) the 8 weeks of experimental training protocol are reported in Table 2. Two-way analysis of variance with Bonferroni's multiple comparisons test did not reveal any significant difference between PRE and POST training both for EG and CG. Nonetheless, CG showed a clear worsening trend at POST in DJ (ES = 0.7), whereas, on the contrary, EG managed to maintain vertical jump performance.

T2
Table 2.:
Changes in CMJ and DJ results, expressed in centimeters (cm), before (PRE) and after (POST) the experimental training protocol.*†

Discussion

To our knowledge, this is the first randomized controlled study that showed the effects of an 8-week body-weight neuromuscular training on dynamic balance and vertical jump performances in elite junior skiing athletes. The main findings of this study were the following: (a) comparisons of the preintervention and postintervention YBT showed significant improvement in the EG compared with the CG and (b) comparisons of the preintervention and postintervention CMJ and DJ tests highlighted no significant differences between EG and CG.

As a primary result of our study, we observed that 8-week neuromuscular training protocol had positive effects in improving postural stability in our group of young skiers. In fact, we observed significant improvements in YBT in A, PM, PL directions, and composite score of YBT, for both lower limbs, for the skiers that performed neuromuscular training, whereas, on the contrary, the CG significantly worsened his performance. These results were almost expected and in line with previous studies showing that neuromuscular training improved postural control in athletes (1,3,5,10,23,26,29). We selected the exercises for our neuromuscular training based on previous findings from injury-prevention research on plyometrics and core stability (5,24,36,39) because it was noted that both these training techniques had positive effects on several athletic components. Core stability is the dynamic control of the trunk allowing for production, transfer, and control of force and motion to distal segments of the kinetic chain (30), whereas plyometrics is a high-impact training technique, based on the use of the stretch-shortening cycle, that enables a muscle to reach maximal force in the shortest amount of time (54). It was previously reported that 15 healthy subjects improved their maximal reached distances in SEBT, compared with a CG, after 6 weeks of core-stability training (17), and similarly, Benis et al. (5) reported that an 8-week neuromuscular training intervention, focused on core stability, plyometric, and body-weight strengthening exercises, improved dynamic balance ability, evaluated by the YBT, in elite basketball players. In addition, it was also showed that 23 junior netball players had significant improvements in 20-m sprint time, 5-0-5 agility time, CMJ, and SEBT performance after a 6-week of neuromuscular training intervention incorporating plyometrics (29). An advantage of this study was that the body-weight neuromuscular training was performed without the use of special equipment and integrated into the warm-up routine to ensure higher compliance. Subjects' compliance increases with short duration programs involving sessions of up to 25 minutes (26). The participants of this study completed at least the 85% of the warm-up intervention (lower limit was set at 75%), and therefore, they were all considered compliant. Furthermore, we focused the participants' attention on the execution of technique and biomechanical movements to better address neuromuscular imbalances and to reduce the risk of common lower limb injuries. Professional supervision is essential when proposing injury-prevention programs, and therefore, during each training sessions, an expert and certified strength and conditioning coach gave continuous verbal and visual feedback to the skiers on the accuracy and precision of technique (49). We examined our athletes with the YBT because it is a validated and reliable tool to screen individuals for limitations in dynamic balance (19,20,41,42). Poor YBT performance has been associated with an elevated risk of noncontact lower extremity injury in athletes (41), and therefore, even if we did not directly investigate the effects of neuromuscular training on lower limb injury, the improvements in postural stability for EG suggest that YBT result could serve as an important outcome measure for assessing athletes at risk for lower limb injury in these athletes.

Jump tests are valid and reliable tools to measure lower limb strength in athletes. In this study, we did not observe any significant difference in jump performance for both groups. Nonetheless, we noted that CG showed a worsening trend in DJ and CMJ, whereas EG managed to maintain the performance. At a first analysis, it seems that our subjects did not reach a sufficient high-intensity training stimulus to determine higher vertical jump performances. Generally, it has been reported that neuromuscular training improved vertical jump performances allowing for an improvement in leg muscle strength (17,25,33,45). However, it is crucial to underline that these training sessions lasted at least 60–90 minutes, and neuromuscular training was not designed to serve a warm-up routine. Our results are in accordance with a meta-analysis performed by Bien (6) that highlighted that plyometric training determined improvements in athletic performance only in prepubertal and midpubertal male youth, whereas postpubertal men benefited from a combined training method including also resistance training (6). Our sample was composed by male adolescent athletes and no specific resistance training was performed. Furthermore, young athletes exhibit the greatest improvements in strength and athletic performance in response to higher training intensities when movement quality and technical competency are upheld (45). Drop jump and CMJ results were expected since we performed the study during the competitive period of the season, where athletes were already conditioned and when improving lower limb strength was not the main focus of the training. In this period of the year, young skiers have one competition a weekend, and consequently, during the week, athletic trainers and strength coaches have to properly prepare athletes to recovery, to prevent injuries, and to be ready for the next competition. Nonetheless, it seems that integrating our neuromuscular training program in the warm-up routine before skiing practice allowed EG to maintain lower limb strength, whereas CG had a trend to reduce vertical jump performance.

This trial has some limitations. First, this study encompassed only 8 weeks, so it is unknown whether coaches can implement the warm-up consistently over the entire season and it is interesting to evaluate its effects during preseason and off-season training periods too. Second, we are aware that we speculated about the possible role of the improvement of postural stability for the reduction of lower limb strength. Third, probably the vertical test that we choose for our study were not the most suitable to mimic specific technical requirements during alpine skiing.

Practical Applications

Alpine skiing is a complex sport from a physical perspective, and unfortunately, high rates of lower limb injuries are registered at all ages and at all competitive levels. This sport is high intensity in nature and requires athletes to have lower body strength and postural control without ignoring high technical competency. The current randomized controlled study suggests that completion of an 8-week body-weight neuromuscular training program, performed into the warm-up routine and including core stability and plyometric exercises, may be an effective intervention to specifically increase lower limb joint awareness and improve postural control, whereas it seems to not positively affect athletes' lower limb strength. Nonetheless, the inclusion of neuromuscular exercise in the warm-up routine of alpine skiers allowed to not worsen their vertical jump performances during the competitive season. Because poor YBT performances has been associated with an elevated risk of noncontact lower extremity injuries, this study could serve to inform athletic trainers and sports medicine physicians on the need to develop sport-specific neuromuscular training programs that target (a) the proprioceptive ability, (b) lower limb joint biomechanics, and (c) dynamic balance ability to decrease the overall lower limb injury risk in elite junior skiers.

Acknowledgments

The authors have no conflicts of interest to disclose. No sources of funding were used and the results of this study do not constitute endorsement of any product by the authors or the NSCA. The authors would thank the skiers for participating in this study and the authors extend their gratitude to Luca Marchina for his valuable technical assistance during data acquisition.

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Keywords:

Y-balance test; core stability; plyometrics; ski; injury

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