During a soccer match, professional players cover total distances of 9–12 km (7,26). In addition, the intensity of running varies during the match, and the distance that is covered with a high-intensity speed is 10–15% of the total covered distance (13,30). During the match, the soccer player performs a variety of movement skills such as jumping, kicking, overhead play for header and short sprint in a direct line, or changing directions.
Vertical jumping ability is necessary for the soccer player during all phases of overhead play (31). Various training methods have been proposed for improving jumping ability. It was suggested that the high resistance training (RT) and power resistance (30–60% of repetition maximum [RM]) improve jumping performance (3,5). Other methods that should be mentioned are the plyometric training (36), a combination of high-intensity RT, either with plyometric (29,34) or running sprint training (22,35), and a combination of strength training and electrostimulation (23,24).
Recently, it has been suggested that sensorimotor training contributes to power performance enhancement. More specifically, balance training may increase the rate of force development (RFD) and jumping ability in untrained (11,15,17,32) recreationally active individuals (27) and trained subjects (20). In addition, some studies report that sensorimotor training increased strength in persons with motor impairments and unfit young individuals (10,11,19), although there are also other studies that included young healthy and recreationally active subjects and report no effect of sensorimotor training on strength performance (16,32). Interestingly, there is a scientific study that used combined sensorimotor and RT programs (SM-RT). In this 8-week training program, Bruhn et al. (11) compared 2 conditions. In the first one, the 4-week sensorimotor training program preceded the 4-week RT program, and the results showed a continuous increase in muscle strength and the RFD during the entire training period. In the second condition, the 4-week RT program was performed before the 4-week sensorimotor training, and the results showed an increase in muscle strength during the first 4 weeks. However, after the implementation of sensorimotor training during the last 4 weeks, a decrease in force was observed while RFD remained unaltered. Therefore, the 2 interventions had different effects on performance depending on the sequence.
However, the effect of combined SM-RT within the same training session has not been determined yet. This is of particular interest because although these 2 training components enhance muscular performance activating different neural tracks (i.e., sensory and motor neural tracks for sensorimotor training and RT, respectively), they cause similar adaptations (33). Thus, a cumulative effect could be expected when combining these 2 training modules. However, this issue has not been thoroughly examined. In a recent study, strength and balance improved in elderly people after combined SM-RT performed during the same day but on different times of the day (15). Additionally, a combined training program with plyometrics and sensorimotor components was more effective than single plyometrics one when it was applied in prepubertal children (12). However, there are no data available regarding combined SM-RT programs within the same training session in trained adults. Therefore, this study was designed to give information regarding the extent of effectiveness of a combined SM-RT program compared with an intervention including RT only. More specifically, the purpose of this study was to examine the effect of an SM-RT program within the same training session on muscle strength, balance, and jumping ability of active soccer players.
Experimental Approach to the Problem
This study was designed to test the effects of a combined SM-RT and a pure RT program on various performance attributes of soccer players. Twenty recreationally active male soccer players were randomly assigned into 2 groups consisted of 10 players each (SM-RT and RT group). A 6-week training program with 2 training units per week was applied to both groups. The SM-RT group followed a balance training program on a balance board coupled with maximal strengthening with the leg press. The other training protocol for the RT group included only maximal strengthening with the leg press. The following dependent variables were evaluated before and after training: (a) the range of displacement of the center of pressure (COP) in mediolateral and anteroposterior axes, (b) the vertical jump during squat jump (SJ), (c) the maximum isometric contraction at leg press, (d) the maximum RFD, (e) the RFD at the intervals 0–30, 0–50, 0–100, 0–150, 0–200 milliseconds, and (f) the leg press 1RM. All pretraining and posttraining tests were performed at the same time of the day.
Twenty amateur soccer players (age range = 19–23 years) participated in the study (Table 1). They were randomly assigned to 2 groups (n = 10 for each group), that is, the SM-RT and RT group. The participants were informed about the tests and the study procedure by the researcher in the Laboratory of Coaching in the Department of Physical Education and Sport Science, and they signed informed consents for participating in the study. All participants reported no medical contraindications, such as chronic disease or lower limb injuries, which could preclude them from participation in this study. They had the right to stop participating in the training program depending on their own decision. This study was approved by the Committee of Ethics of Aristotle University of Thessaloniki, Greece.
Before testing, participants performed a warm-up session, consisted of 10-minute cycling (100 W at self-paced speed), on a cycle ergometer (MONARK Ergomedic 814 E, class Α, din 32932, Vansbro, Sweden) and warm-up exercises for upper limbs, lower limbs, and trunk. Then, they performed the balance test, the vertical jump test, and the 1RM test, and after 10-minute rest, the maximal isometric voluntary contraction (MIVC) was performed to avoid fatigue and postactivation potentiation. Tests were performed before and after the intervention training program.
Static balance test was used for testing balance. The participants performed “one-leg stance” on a force plate (Kistler 9281CΑ, Winterthur, Switzerland, with sampling rate at 1,000 Hz). The support leg was the leg that the participants used to perform an instep kick. The contralateral leg was flexed so that the foot was stabilized to the knee of the support leg (“stork stance”). According to the researcher's command, the participant should raise the heel off the ground and keep his balance at this position for at least 5 seconds. The balance variables analyzed were the range of displacement of the COP in mediolateral and anteroposterior axes.
Vertical Jump Test
The participants stood on the force plate and performed SJs with knee angle at 90° and their hands on their waist. They were instructed to bend their knees slowly reaching 90° knee angle and then stay on this position for 2 seconds to minimize the prestretching effect during the concentric phase. According to the researcher's command, the participant performed the jumping task to achieve the maximum SJ height. For familiarization, each participant performed a series of submaximal SJ and 3 maximal SJs before the final trial with the procedure (9). Finally, they performed 3 maximal SJs, and the best in height SJ was chosen for final assessment. The interval among trials was 2 minutes. For SJ, the outcome variables were maximal jump height, peak vertical ground reaction force normalized to body weight and duration of the push-off phase, and take-off knee angular velocity. Videos were captured with a video camera set on a stable tripod (Panasonic AG-188 NTSCS; Tokyo, Japan) with a frame rate of 60 Hz. All video files were digitized and stored in AVI form in a personal computer. Digitizing of the joint and fixed markers and further analysis of the kinematic variables were assessed by APAS (Ariel Dynamics Inc., San Diego, CA, USA).
Repetition Maximum Test
Before testing, each participant performed a warming up session that consisted of submaximal repetitions on the leg press machine with an intensity of 40–80%, of perceived assessment for 1RM. Thereafter, repeated contractions starting 5% below the perceived assessment for 1RM were performed. After a successful trial, the load was increased by 2%. This procedure was continued until the participant was not able to perform the task in the maximal range of motion. The interval between the repetitions was 3 minutes.
Maximal Isometric Voluntary Contraction Test
The participants were sitting with the upper body fixed with straps on a hip angle at 100° and knee angle at 120° (4). During testing, their arms were crossed on their chest. Before determination of MIVC, each participant completed a series of submaximal isometric contractions with gradually increasing intensity until maximum. Then, they performed 3 maximal isometric contractions for the final assessment, with an interval of 5 minutes between the trials. If the range of the trials was more than 5%, they performed 1 more trial. The participants were pressing against a load cell (LC 4204-K600; A&D, Co., Ltd, Abingdon, UK) as “forcefully and fast” as they could, according to the researcher's command. They received continuous motivation and had visual feedback about their force level, which were stored at a sampling rate of 1 kHz. A low-pass digital filter with a cutoff frequency of 6 Hz was chosen for all force signals (37). Maximum RFD and RFD at 0–30, 0–50, 0–100, 0–150, and 0–200 milliseconds were evaluated during MIVC. For the calculation of maximum, RFD was derived from the first derivative of force using a time window of 1 millisecond. The threshold for force onset was set at 2% of the MIVC (25,28).
The intervention training program was designed for 6 weeks, 2 training sessions (units) per week (Table 2). Both groups performed in the beginning of each training session warm-up of 10 minutes. The training session ended with a 10-minute cool-down. Before the start of the 6-week intervention program, a preliminary 2-week exercise program consisting of endurance, flexibility, and speed-force training was assessed for injury prevention. During the second week, participants were accustomed with the testing procedures. After group allocation, the main training program was applied. The content of the training program for each group is shown in Table 2. Every week, there was an adjustment of the resistance to maintain the intensity constant. For the SM-RT group, the balance training program was according to the literature (16) and preceded the strength training program to avoid fatigue effects that affect stability negatively. Both training programs were performed during the transitional preparation period, and the soccer players did not participate in another training program.
Two-way repeated-measures analysis of variance was used to examine the effect of group (SM-RT vs. RT) and time (pretraining vs. posttraining, repeated measures) on the outcome variables. Effect sizes (ES) were calculated, and pairwise comparisons were assessed with the Tukey's post hoc test. Level of significance was set at α < 0.05, and Bonferroni's correction was used.
The intraclass correlation coefficients (ICCs) and their 95% confidence interval (CI95%) for maximal SJ height and peak vertical ground reaction force normalized to body weight and duration of the push-off phase were 0.97 (CI95%: 0.87–1.00) and 0.98 (CI95%: 0.88–1.00), respectively, and the ICC for the take-off knee angular velocity was 0.95 (CI95%: 0.83–1.00). The ICCs for maximum RFD and RFD at 0–30, 0–50, 0–100, 0–150, and 0–200 milliseconds were 0.97 (CI95%: 0.86–1.00), 0.98 (CI95%: 0.90–1.00), 0.94 (CI95%: 0.82–1.00), 0.93 (CI95%: 0.81–1.00), 0.94 (CI95%: 0.83–1.00), and 0.94 (CI95%: 0.86–1.00), respectively. The ICCs for the range of displacement of the COP in mediolateral and anteroposterior axes were 0.98 (CI95%: 0.98–0.99) and 0.97 (CI95%: 0.97–0.98), respectively.
As shown in Figure 1, COP (in centimeters) in anterior-posterior and mediolateral axes decreased significantly after training (F(1,18) = 97.1, p < 0.01 and F(1,18) = 28.5, p < 0.01, respectively). More specifically, for the anterior-posterior axis, the ES of these differences were −1.94 and −1.68 for the SM-RT and RT group, respectively. The respective values for the mediolateral axis were −0.68 and −0.69. No group main effect was detected for the anterior-posterior (ES, −0.71 and −0.49 for pretraining and posttraining measurements, respectively) and mediolateral (ES, −0.21 and −0.15 for pretraining and posttraining measurements, respectively) axes. Regarding the interaction between the factors, there was no statistically significance as well (p > 0.05).
Vertical Jump Test
The SJ height increased statistically significantly between premeasurement and postmeasurement, for both training groups (F(1,18) = 36.5, p < 0.01). Similar increase was demonstrated for peak vertical ground reaction force normalized to body weight (F(1,18) = 17.4, p < 0.01), push-off duration (F(1,18) = 6.7, p ≤ 0.05), and knee angular velocity at take-off phase (F(1,18) = 20.6, p < 0.01). None of the above parameters demonstrated statistically significant interaction between groups and the 2 examinations (p > 0.05) (Table 3).
Repetition Maximum Test
There was a statistical significant increase in 1RM between the premeasurement and postmeasurement for both training groups, the combined SM-RT program group, and the RT program group (F(1,18) = 83.0, p < 0.01). No statistical significant intergroup differences and no significant interaction effects could be determined (p > 0.05) (Table 3).
Maximal Isometric Voluntary Contraction Test
As shown in Figure 2, all variables showed no between-group statistically significant difference during the baseline measurements (ES range from −0.05 to 0.54 and −0.18 to 0.19 for pretraining and posttraining measurements, respectively). However, comparison between premeasurements and postmeasurements showed significant changes for all parameters for both training groups, with ES ranging from 1.10 to 0.64. More specifically, significant increase was observed in force output (F(1,18) = 38.0, p < 0.01) and RFD during 0–30 milliseconds (F(1,18) = 4.1, p < 0.01), 0–50 milliseconds (F(1,18) = 18.7, p < 0.01), 0–100 milliseconds (F(1,18) = 28.8, p < 0.01), 0–150 milliseconds (F(1,18) = 40.0, p < 0.01), 0–200 milliseconds (F(1,18) = 27.6, p < 0.01), and maximum RFD (F(1,18) = 34.3, p < 0.01). The interaction between the 2 factors did not reach the level of statistical significance in any of the parameters (p > 0.05).
In this study, the effects of an SM-RT program and an RT program on lower limb strength, explosive ability, and balance were examined. Both intervention programs improved maximum leg press strength. Because of the program duration of 6 weeks, the mechanisms that underlie this improvement could be attributed to neural and peripheral (hypertrophy) factors (14).
In line with previous observations (8), both training programs improved maximal RFD and RFD in all selected time intervals. This could be explained by the fact that the selected training exercise was similar with the MIVC testing and the given command to the subjects, which was to contract as forcefully and fast as possible (8). The RFD improvement in all selected time phases is in agreement with previous research (1,2) that followed similar methodology. More specifically, we used high resistance intensity, low number of repetitions, and the instruction to make the contraction fast. Other studies showed that balance training improved RFD only in initial phases of contraction up to 60 milliseconds (16). In this study, it seems that in an SM-RT program, the effect of the RT component is probably strong enough to increase RFD in time phases longer than 60 milliseconds.
Both training programs improved balance ability as indicated by the decrease in the displacement of the COP in both axes. Previous research showed that sensorimotor training on unstable surfaces improves balance ability (18), and these results are in agreement with this study. Furthermore, a link between muscle strength and balance has been reported. Specifically, balance improves significantly after muscle strength training (6,10,11,21), and this improvement is attributed to neuromuscular mechanisms that led to faster and more accurate mechanical reaction of the muscle to maintain balance. Moreover, the improvement of balance has been attributed to the sensory mechanisms, as it was found by the decrease in H-reflex (33).
Both training programs improved the vertical jumping ability during SJ. Previous studies showed that both RT (3,5) and sensorimotor training (32) improved vertical jumping by enhanced neuromuscular activation. In this study, the improvement of the jumping ability is further supported by the increase in angular velocity of the knee. The basic hypothesis of this study was that the SM-RT would be superior to RT because it combines 2 programs, which cause an enhancement in muscle performance using different pathways of neuromuscular system. More specifically, muscle performance improves after sensorimotor training or RT through sensory or motor pathways, respectively. It is known that a sensorimotor training program causes a decrease in H-reflex, whereas an RT program may keep H-reflex constant or increase it (33). However, the results of the combined SM-RT program did not verify training-specific differences. Until now, it seems that this program is beneficial in children (12). This could indicate that in trained individuals, the presence of RT stimulus masks the effect of the sensorimotor training when assessed combined in midterm training periods as assessed in this study. However, it cannot be excluded that trained subjects could require a longer training period to advance performance. This still needs further examination.
The present research showed that a combined SM-RT and a pure RT program results in similar improvements in balance, SJ, and force output. Therefore, the strength and conditioning professional should consider that RT may have the potential to achieve benefits that occur with sensorimotor training and possibly play an important role on injury prevention.
The authors would like to thank Dr. Christos Papadopoulos, Director of the Sport Biomechanics Laboratory, Department of Physical Education and Sport Science at Serres, Greece, and the soccer players who participated in this study.
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