Knowledge

Sport 23 Full Text Article

Electrostimulation Training Effects on the Physical Performance of Ice Hockey Players

By American College of Sports Medicine

Research on the use of electromyostimulation (EMS) as a method of training of healthy skeletal muscle has increased over the past decade. Several studies have indicated that this training modality enables the development of maximal force, albeit with a great diversity in reported strength gains, ranging from 0 to 44%. Differing stimulation modes (frequency, pulse duration), testing procedures, training protocols (number and duration of the sessions), pretraining status, and interindividual differences may account, at least partly, for the observed discrepancies.

Recently, some studies have attempted to investigate the effect of EMS training on the specific performance of athletes from various team sports. For instance, Maffiuletti et al. and Malatesta et al. demonstrated the positive effects of short-term EMS training on the vertical jump performance of basketball and volleyball players. These changes were also associated with isokinetic and isometric strength gains. However, to the best of our knowledge, no study has been published regarding EMS training effects on the specific performance of ice hockey players.

Analysis of physiological profile of elite ice hockey teams reveals the importance of aerobic endurance, anaerobic power and endurance, muscular strength, and skating speed. It was also pointed out that the strength decrement observed during the hockey season can be attributed to the lack of specific strength programs. Our study used EMS training as a complement to standard training practices with the goal of improving both the muscular strength and physical performance of ice hockey athletes. Therefore, the purpose of the present study was to determine the influence of a 3-wk EMS training program on the quadriceps femoris muscle strength and on specific physical abilities of ice hockey players, such as vertical jump and speed skating performance. The quadriceps muscle group was firstly chosen because it develops the largest contractile strength during the push-off of the skating thrust, whereas the hamstrings and gastrocnemius muscles primarily act to stabilize the knee joint. This muscle group was secondly chosen because three of its four component muscles are superficial and can be easily stimulated.

MATERIALS AND METHODS

Subjects

A group of 17 ice hockey players competing in the French Ice Hockey Federation League, division II (age = 22.6 +- 4.5 yr; height = 178.3 +- 4.8 cm; mass = 73.8 +- 7.6 kg) participated in the study. They were randomly divided into two groups with nine assigned to the electrostimulated (ES) and eight to the control players (C). None of them had previously engaged in systematic EMS experience. All the subjects agreed to participate in the study on a voluntary basis and signed an informed consent form. The study was conducted according to the declaration of Helsinki and approval for the project was obtained from the University of Burgundy committee on human research. During the experiment, the ice hockey training was the same for all players and was performed with the same coach with all athletes practicing three times a week in 1.5-h sessions and playing one game per week. No subjects had to stop the experiment due to injuries resulting from EMS training and/or ice hockey practicing.

Training

EMS training. A total of nine EMS sessions were spread over a 3-wk period, with 12 min per session and three sessions per week, as recommended by Sale and MacDougall. EMS sessions, separated from the specific ice hockey training, were always performed at the same time of day and the same days of a week. During EMS, athletes were seated in a leg extension machine with the knee flexed at a 60° angle (0° corresponding to complete leg extension). EMS was delivered to both quadriceps simultaneously with a Compex-2 stimulator (MediCompex SA, Ecublens, Switzerland). Two pairs of self-adhesive positive electrodes (each measuring 25 cm2; 5 x 5 cm), which have the property of depolarizing the membrane, were placed on the vastus medialis and vastus lateralis muscle bellies. Two rectangular negative electrodes, each measuring 50 cm2 (10 x 5 cm) were placed over the femoral triangle of each leg, 1-3 cm below the inguinal ligament. Pulse currents of 85-Hz frequency lasting 250 s were used. The contraction time was 4 s, and the rest time was 20 s. During each training session, 30 EMS contractions were completed. To ensure identical contraction intensity throughout the training session, electrically evoked (isometric) force was consistently measured with a myostatic type dynamometer (Allegro, Sallanches, France). At the beginning of each training session, the subject's maximal voluntary isometric force was measured at 60° (i.e., the angle of stimulation). Then stimulation intensity was individually increased to the maximal tolerated intensity, and to attain at least 60% of each individual pretest maximal voluntary contraction score. This contraction level was reached at the beginning of the stimulation and maintained for 4 s.

Testing

Isokinetic test. Maximal voluntary torque of the right knee extensor muscles (N-m) was measured before and after the 3-wk period, using a Biodex isokinetic dynamometer (Biodex Corporation, Shirley, NY) validated by Taylor et al.. A 7-min period of standardized warm-up and familiarization with the measurement apparatus was conducted with submaximal repetitions at each experimental angular velocity. Then subjects performed three maximal voluntary knee extensions at five concentric angular velocities (60, 120, 180, 240, and 300°·s-1) and at two eccentric velocities (-60 and -120°·s-1) with a 90° range of motion (starting position = 10° knee flexion). In each case, only the best performance was retained. A 4-min rest period was allowed between each trial. To minimize hip and thigh motion during all contractions, a series of straps were applied across the chest, pelvis, mid-thigh, and lower leg. The latter strap secured the leg to the dynamometer lever arm. The alignment between the center of rotation of the dynamometer shaft and the axis of the knee joint (lateral femoral condyle) was checked at the beginning of each trial. The subject's arms were positioned across the chest with each hand clasping the opposite shoulder. Torques were gravity corrected at each joint angle, using the torque produced by the weight of the limb at a joint angle corresponding to the maximal gravity effect. For each angular velocity, the 60° knee flexion maximal voluntary torque (constant angular torque technique) was directly computed by the Biodex software and included in the analyses.

Vertical jump test. Jumping ability was evaluated with a contact mat (Globus, Codogne, Italy). The squat jump (SJ), countermovement jump (CMJ), and drop jump (DJ) from a height of 30 cm were randomly performed according to Asmussen and Bonde-Petersen's recommendations (1). Three tests were carried out for each type of jump, and the best result was retained. Fifteen consecutive CMJ (15J) were also performed to evaluate the resistive capacities of the knee extensors. During this 15J test, jump height and power were measured for each jump and then averaged together.

Sprint test. Times, determined at the hip level for 10- and 30-m sprints on ice, were measured with infrared photoelectric cells (TEL.SI s.r.l., Vignola, Italy) positioned 10 and 30 m from the start line and controlled by commercially available software. The players set off upon a visual signal and skated as fast as possible the 30-m distance. This sprint allowed us to directly measure both times with the 10-m time as intermediate. Only the best performance of three trials was retained.

Statistical Analysis

Mean values and standard deviations (SD) were calculated for all variables. A repeated measures analysis of variances (ANOVA) followed by a Newman - Keuls post hoc procedure was used to test differences between both groups and the effects of the EMS program on dependent variables (strength, jump, and sprint performances) in each group before and after the 3-wk period. Relationships between isokinetic strength, vertical jump, and skating performance were also examined using Pearson product correlations. In all statistical procedures, a 0.05 level of significance was adopted.

RESULTS

Before training, no significant difference was observed between ES and C groups in physical characteristics, knee extensor strength, and skating performance. C group had, however, significantly higher values for 15J height (P < 0.01) and power (P < 0.05) compared with ES group (Table 1). When considering both groups (N = 17) before the 3-wk period, a significant negative relationship was observed between the 10- and 30-m skating performance and the 240°·s-1 concentric torque (r = -0.61, P < 0.01 and r = -0.76, P < 0.01, respectively, for 10 and 30 m; Fig. 1).

Muscular strength. After 3 wk of EMS training, the isokinetic torque increased significantly (Fig. 2) for ES in eccentric (37.1 +- 21.9% at -120°·s-1 and 24.2 +- 17.9% at -60°·s-1; P < 0.01), and concentric conditions (41.3 +- 37.6% at 60°·s-1 and 49.2 +- 48.9% at 300°·s-1; P < 0.05). Except for the -60°·s-1 eccentric condition, the C group did not exhibit any significant torque increase. When comparing torque changes after the 3-wk period, it appears that the ES group had significantly higher torque increases than the C group. The -60°·s-1 eccentric torque increase was, however, not significantly different between the ES and C groups.

Vertical jump performance. Vertical jump results, obtained before and after the 3-wk period, are shown on Table 1 for both ES and C groups. After EMS training, the ES group vertical jump height decreased significantly (P < 0.05) for the SJ (-8.4 +- 6.9%), CMJ (-6.1 +- 6.0%), and DJ (-5.2 +- 4.6%). No significant difference was found before and after the 3-wk period for members of the C group. For the ES group, the 15J power increased after training (14.3 +- 17.2%; P < 0.05), whereas gain in 15J height was not significant. No significant difference was obtained for the C group.

Skating performances. For the ES group, the 10-m skating time significantly declined (-4.8 +- 5.8%, P < 0.05), whereas no change was observed for 30-m sprints (Fig. 3). C-group skating performances were comparable before and after the 3-wk period.