Combined sensorimotor and resistance training for young ... - IOS Press

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Isokinetics and Exercise Science 18 (2010) 193–200 DOI 10.3233/IES-2010-0384 IOS Press

Case Study

Combined sensorimotor and resistance training for young short track speed skaters: A case study Martin Behrens∗, Anett Mau-Möller, Henrike Laabs, Sabine Felser and Sven Bruhn Department of Exercise Science, University of Rostock, Rostock, Germany

Abstract. The purpose of the study was to investigate the effects of a combined sensorimotor – resistance training on muscle strength and neuromuscular activation of selective muscles of the right ankle joint. Both treatments were performed successively in each training session. Seven short track speed skaters participated in the study. The training was performed twice a week for 12 weeks. Maximum peak torque during inversion/eversion, the associated neuromuscular activation of soleus and peroneus longus as well as eversion/inversion ratio were measured in a pre- and post-test, respectively. After the training the athletes revealed a significantly increased maximum peak torque during inversion (17.47 ± 4.37N · m vs. 23.57 ± 3.58N · m; p = 0.006) associated with a significantly increased root mean square of the EMG signal in the soleus (12.42 ± 6.09mV vs. 24.00 ± 9.37mV; p = 0.009). The eversion/inversion ratio was significantly decreased after training (0.96 ± 0.12 vs. 0.77 ± 0.11; p = 0.034). Eversion training adaptations were lower than inversion training adaptations probably due to the fact that resistance training did not contain an exercise for peroneus longus. Based on these the combined sensorimotor-resistance training is an attractive option for athletes where maximum strength, active joint stabilization and postural control plays a decisive role. Keywords: Sensorimotor training, resistance training, maximum peak torque, ankle joint

1. Introduction The strength of the lower limb plays a decisive role in many sports as well as in short track speed skating [14]. Therefore, it is essential to train the muscular strength in order to reach an optimal performance in competitions. Resistance training (RT) is the standard method to improve this skill. The results of various studies indicate that RT enhances muscle strength through specific neural [4,12,16,29] and morphological adaptations [31,32, 41]. Data suggest that the neural adaptations include the increased activation of agonists, which is mediated by additional or more consistent recruitment of highestthreshold motor units and increased motor unit firing

∗ Address for correspondence:

Martin Behrens, Department of Exercise Science, University of Rostock, Ulmenstrasse 69, 18057 Rostock, Germany. Tel.: +49 3814982760; E-mail: martin.behrens@ uni-rostock.de.

rates [38]. The primary morphological adaptation is an increase in cross-sectional area of the muscle, due to an enhancement of myofibrillar size and number [17]. A different method with likely positive effects on strength is the sensorimotor training (ST), which is primarily used in the prevention and rehabilitation of joint injuries of the lower leg [9,37,45]. It has been assumed that these effects were related to neural adaptations [43] and different from the adaptations caused by resistance training. However knowledge about the underlying mechanisms is limited. In contrast to RT, ST is characterized by pronounced afferent input [21,22]. In order to maintain balance during ST, the athletes have to control the displacements of the instable platform by using appropriate muscle activation. For an adequate activation the muscles need permanent proprioceptive sensory feedback. How afferent input can affect motoneuron output has been previously shown [19]. This study investigated the recruitment thresholds and firing rates of human motor units and compared the

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M. Behrens et al. / Combined sensorimotor and resistance training for young short track speed skaters

Ia afferent-induced activation of the motoneuron pool with the voluntary-induced one during force matched isometric knee extensions. The authors have reported that motor units of vastus medialis recruited via Ia afferent input had lower recruitment thresholds and firing rates than those recruited voluntarily. That indicates that the amount of afferent input probably correlates with the recruitment threshold and firing rates of motor units. Findings suggest that both, RT and ST, are capable of increasing the mechanical output. Bruhn et al. [11] have investigated the combined effect of RT and ST on muscle strength. The study design included two groups; one which completed four weeks of ST followed by four weeks of RT (ST-RT) while the other was trained in the opposite design (RT-ST). Both training regimen had positive effect on muscle strength, however, ST-RT led to a higher improvement of maximum strength during maximal voluntary contraction. The authors have suggested that ST can have preconditioning effects on the RT. Based on the above-mentioned research it is of interest to elucidate the effect on maximal strength of ST and RT, which are performed successively in each training session. The ability to perform forceful movements plays a decisive role in short track speed skating [14]. Furthermore, the movement on skids requires a high active ankle joint stabilisation as well as postural control. In addition to strength improvements, ST has probably the ability to enhance joint and postural stabilisation as well [15,24,37]. A combination of both methods could be useful to reach an optimal performance in competition. The purpose of the present study was to investigate the effects of a combined ST and RT, performed successively in each training session, on maximal strength and neuromuscular activation in young short track speed skaters. The variables considered included maximum peak torque during inversion/eversion of the right ankle joint, the associated activations of selected muscles and the eversion/inversion ratio.

Table 1 Physical and training characteristics of the subjects (mean ± SD) Age (yrs) Height (cm) Weight (kg) Training (h/week) Range of motion (◦ ) Eversion Inversion

Experimental group (n = 7) 17.1 ± 1.3 176.4 ± 7.6 67.9 ± 6.3 25.9 ± 6.1 35.6 ± 6.7 46.3 ± 2.6

experiment. The physical and training characteristics of the experimental group are presented in Table 1. The initial condition of all subjects was measured in a pre-test. Afterwards, the short track speed skaters had to go through the combined training for 12 weeks. Following the training period the post-test took place. The athletes had not engaged in such a specific training treatment before and they continued their systematic training during the experiment. 2.2. Procedures

2.1. Subjects and study design

2.2.1. Training The combined ST-RT was performed successively in each training session twice a week for 12 weeks. Every training session consisted of 30-min of ST followed by 45-min of RT. All sessions were surveyed, supervised and documented by the authors of the study. Sensorimotor training consisted of different balancing tasks on wobbly and unsteady surfaces with high demands on postural stabilization (soft mat, ankle disc, air cushion, posturomed). Therefore this training intervention was characterized by complex activation patterns in the muscles surrounding the ankle joint [22]. Each type of exercise was performed unilaterally with the knee slightly bent (approximately 30◦ ), barefoot and with hands akimbo. Each stabilization task was carried out four times for 45-s with 30-s rest between the sets. The RT consisted of 1–3 repetition high-intensity trials (> 90% maximal voluntary contraction) which could result in optimal recruitment and activation frequency of motor units [28,39]. The trials were performed 3–4 times on four different strength devices: leg press, leg curl, leg extensor machine and calf raise machine. To ensure the high-intensity of RT the load was adapted to the subjects’ performance.

Seven healthy short track speed skaters with no history of neurological disorders or injuries of the lower leg took part in this study. All subjects gave their signed informed consent before they participated in the

2.2.2. Strength assessment Isokinetic concentric strength was assessed before and after the 12 week training period. The CYBEX NORM Testing and Rehabilitation System (Computer

2. Methods


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Fig. 1. (A) Changes in maximum peak torque (PTmax ) during inversion and eversion. Data are expressed as means ± standard error of the mean. Maximum peak torque during inversion was significantly increased (p = 0.006). (B) Maximum peak torque (PTmax ) during inversion and eversion of the individuals before and after the combined training.

Sports Medicine , Inc.) was used to assess maximum peak torque (PT) during inversion/eversion of the right ankle joint. The system recorded the torque with a sampling frequency of 100 Hz. Each subject underwent 10 minutes warm-up on a bicycle ergometer at 100 W. The subjects’ position and the dynamometer were adjusted according to the manufacturer’s guideline for inversion/eversion testing. Subjects were tested in the supine position with a knee angle between 100◦ and 110◦. They were stabilized in the chair with Velcro straps securing the trunk and thigh to eliminate compensatory movements. Each subject‘s foot was attached to the adapter and secured by two Velcro straps across the foot’s dorsum. After positioning the subjects the individual range of motion was determined (see Table 1) and security stops were set. Before the test was started the subjects were asked to cross their arms in front of the chest. The positions of all test persons were documented and saved for postmeasurement. The subjects performed five submaximal concentric inversion/eversion test trials with an angular velocity of 240◦ /s. After familiarization and a rest period of 1-min

three maximal concentric efforts were performed with the same velocity. The subjects were instructed to work as hard as possible in both directions of the movement. They had no visual feedback of the produced torque but verbal motivation was given in a standardized manner. The best of the three efforts was used to determine maximum PT for inversion and eversion. The individual eversion/inversion ratio was calculated from the eversion and inversion torque values. This ratio acts as an indicator for ipsilateral muscular balance or imbalance around a joint [8,33]. Value 1 means a force balance between inversion and eversion muscles. A ratio below 1 presents stronger foot invertors than evertors. 2.2.3. Electromyography Surface electromyography (EMG) recordings were used to detect adaptive changes in neural function [1] after the combined training. Bipolar surface electrodes (Ambu Blue Sensor N, diameter 2 cm, center-to-center distance 2 cm) were firmly attached to the shaved, abraded and cleaned skin over soleus and peroneus longus of the right leg. The electrodes were in line with the presumed direction of

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Fig. 2. (A) Changes in root mean square (RMS) in soleus and peroneus longus during inversion and eversion. Data are expressed as means ± standard error of the mean. Root mean square in soleus was significantly enhanced (p = 0.009). (B) Root mean square (RMS) in soleus and peroneus longus during inversion and eversion of the individuals before and after the combined training.

the underlying muscle fibres. All electrode positions were carefully determined and photographed to ensure high quality of results and to reconstruct electrode positions. Impedance level was measured by a digitalmultimeter (McVoice MY-68) and kept below 2 kΩ. Signals were sampled at 1.5 kHz, amplified and bandpass filtered (10–1300 Hz). The determined EMG parameters included root mean square (mV) and median frequency (Hz). Concerning the determination of root mean square EMGs were rectified and afterwards time normalized for about 1-s because of the different individual motion loop. For the median frequency determination the EMGs were only time normalized. Both parameters were defined over the complete motion loop. 2.3. Statistical analysis Differences between the values before and after the combined ST-RT were tested for significance by One Way Repeated Measures ANOVA. The level of significance was established at p 6 0.05. SPSS 15.0 (SPSS

Inc., Chicago, IL, USA) was used for statistical analysis. Data are presented as group mean values ± standard error of the mean (Figs 1–3). In addition the alteration of maximum PT and root mean square for every single subject is displayed (Figs 1, 2).

3. Results Maximum PT during inversion was significantly enhanced (17.47 ± 4.37 N · m vs. 23.57 ± 3.58 N · m; p = 0.006) (Fig. 1A) after the combined ST-RT associated with a significantly increased root mean square in soleus (12.42 ± 6.09 mV vs. 24.00 ± 9.37 mV; p = 0.009) (Fig. 2A) while median frequency remained unmodified (36.89 ± 5.13 Hz vs. 35.92 ± 3.79 Hz; p = 0.701) (Fig. 3). Maximum PT during eversion revealed a nonsignificant improvement after the training (16.35 ± 3.05 N · m vs. 17.82 ± 2.43 N · m; p = 0.092) (Fig. 1A). This trend was accompanied by non-significant enhancement of root mean square in peroneus longus


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Fig. 3. Changes in median frequency (MF) in soleus and peroneus longus during inversion and eversion. Data are expressed as means ± standard error of the mean. Median frequency did not change after the combined training.

(37.51 ± 16.60 mV vs. 50.79 ± 36.88 mV; p = 0.152) (Fig. 2A) while median frequency did not change (96.77 ± 18.91 Hz vs. 95.03 ± 13.74 Hz; p = 0.946) (Fig. 3). The single observations revealed a consistent improvement of nearly all subjects regarding maximum PT in both movement tasks and root mean square in the two tested muscles (Figs 1B and 2B). The significant enhancement of maximum PT during inversion resulted in a significantly decreased eversion/inversion ratio (0.96 ± 0.12 vs. 0.77 ± 0.11; p = 0.034).

4. Discussion The purpose of the present study was to investigate the effects of a combined ST and RT performed successively in each training session, on maximal strength and neuromuscular activation in young short track speed skaters. After the training the short track speed skaters revealed a significantly increased maximum PT during inversion and a non-significant enhancement of maximum PT during eversion. The changes in PT were associated with improvements of root mean square in soleus and peroneus longus. The eversion/inversion ratio was significantly decreased after the combined training. 4.1. Muscle strength The gain in inversion strength was higher in comparison to eversion strength. The training adaptation concerning inversion was probably caused by RT as well as by the ST due to the fact that soleus was involved in

sensorimotor training and resistance training (calf raise machine). To our knowledge no study so far has been conducted that evaluated the effects of a combined ST-RT on maximum strength in athletes. Several studies have demonstrated positive effects on maximum strength after ST [7,10,11] or RT [4,5,10,11]. The measured gain in maximum PT during inversion was likely a result of neural adaptations, induced by RT and ST, as well as morphological adaptations, just caused by RT. Morphological changes have not been measured in this study but numerous studies have reported alterations of muscle morphology and architecture as a consequence of RT with short training periods [2,6,26]. The influence of ST on morphological adaptations is unknown. No study has evaluated the effect of ST on morphological changes. These changes are improbable because the tension of the muscle is only submaximal during ST. The adaptation concerning eversion strength was likely caused only by ST because RT did not contain an exercise for peroneus longus. Therefore, if RT and ST activate the same muscles the combined training could probably lead to greater improvement in maximum strength. 4.2. Neuromuscular activation Improvements in maximum inversion and eversion strength were associated with increased root mean square while median frequency remained unmodified in the corresponding muscles. However, the gain in root mean square was higher in soleus than in peroneus longus. It may well be that the significantly increased root mean square in soleus can be ascribed to neural

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adaptations caused by RT as well as ST. However, both treatments induce different neural adaptations at different sites of the central nervous system. The increased motoneuronal output after RT was probably the consequence of the enhanced recruitment of additional motor units and/or the increased discharge rate of the active motor units [25]. Besides, the alteration of motor unit synchronization could be taken into account [18]. Furthermore, RT is able to elevate motoneuron excitability and to reduce presynaptic inhibition of Ia muscle spindle afferents [1,3]. As already mentioned above, ST induces neural adaptations as well but the knowledge about the alterations is limited. It has been assumed that ST increases afferent feedback [21]. This afferent feedback is likely altered by changes in physiological processes such as sensory threshold of specific peripheral mechanoreceptors, nerve conduction velocity and sensorimotor integration at the spinal and/or supraspinal level [27]. The central mechanism for increased maximum strength after ST is probably an alteration of presynaptic inhibition. In perturbation experiments it has been proved that presynaptic inhibition was enhanced after ST, assessed by the Hoffmann reflex [43,44]. It has been assumed that the increased presynaptic inhibition influences the regulation of afferent input and leads to advantageous conditions for central activation information [42]. This assumption remains speculative because up to the present no study evaluated the alteration of presynaptic inhibition during maximum contraction after ST. Gruber and Gollhofer [21] have discussed an opposite approach and regarded a heightened afferent drive, induced by a decreased presynaptic inhibition, as the primary mechanism for increased neuromuscular activation at the onset of a maximum contraction following four weeks of ST. The non-significant enhancement of the root mean square of the EMG in peroneus longus can probably be ascribed to neural adaptations only caused by ST because RT did not contain an exercise for peroneus longus. Therefore, if RT and ST activate the same muscles the combined training could possibly lead to a greater improvement in neuromuscular activation. In contrast to the root mean square, median frequency did not change in soleus as well as peroneus longus after the combined training. The interpretation of median frequency during dynamic contractions might be difficult because the movement per se introduces additional factors (e.g. changes in force throughout the range of motion, changes in fiber and muscles length) that might affect its characteristic [34]. However, a study of Lars-

son et al. [30] has demonstrated a good reproducibility of mean frequency during maximum dynamic isokinetic contractions. Solomonow et al. [40] have suggested that recruitment control strategies, which are responsible for an increase in contraction force, could be assessed by determination of median frequency. The unmodified median frequency in this study indicates that there were probably no changes in recruitment control strategies after training. It is difficult to compare the EMG results of this study with others because the root mean square and median frequency were determined over the complete concentric motion loop and not at the point in time of PT. Normally, both parameters are determined at the onset of maximal voluntary contraction and around the maximum force produced during maximal voluntary contraction [22,44]. Consequently, the time of contraction as amplitude increased could not be exactly defined. There is a different pattern in the literature concerning the improvement in amplitude of surface EMG during maximal voluntary contraction after RT and ST. There are training studies which have reported enhancements of amplitude (RT: [23,29,36]; ST: [11,21]), while other studies did not (RT: [4,25,32], ST: [22,43]). These varying results can be explained by methodological and/or training specific differences. Inasmuch as we are aware, in variance with the amplitude of surface EMG no study has been conducted in which the effect of RT on frequency has been evaluated. Only one investigation was found that determined the consequence of ST on frequency. Gruber et al. [22] have indicated an enhanced rate of force development after ST, which was associated with an increased median frequency in gastrocnemius and soleus during the first 200 ms. 4.3. Eversion/inversion ratio The force ratio, an indicator for muscular balance [8, 33], was calculated by dividing the eversion value by the inversion value. In general, a correct relationship between agonist and antagonist is necessary for the athletes to be effective and without injury [35]. The question arises which value stands for an accurate relationship between eversion and inversion muscles. In contrast to the knee joint, there are only few studies in literature which have evaluated the strength relationship of the ankle musculature in healthy subjects. From a functional point of view, foot invertor and evertor force must be equal to stabilize the foot during daily activities (e.g. going, running). However, Wong et al. [46] have revealed that peak torques of invertors are consis-


tently higher than that of the evertors. That was proved for men and women at test speeds of 30◦ /s, 60◦ /s and 120◦ /s. The same relation between invertor and evertor peak torque could be verified by Gross and Brugnolotti [20] for test speeds of 60◦ /s and 120◦ /s. Cawthorn et al. [13] have tested foot invertor and evertor force in three positions (10◦ dorsiflexion, neutral dorsi- and plantarflexion, 10◦ plantarflexion). In contrast to our study, the authors have provided the force ratio by dividing the inversion value by the eversion value. The ratio values were marginal over 1 in all three positions, i.e. the eversion muscles were slightly weaker than the inversion muscles. This result could be verified by our study. The short track speed skaters showed a force balance (0.96) around the ankle joint before the combined training. However, after the training the ratio decreased significantly because the enhancement in inversion strength was considerably higher compared to eversion strength. This strength difference was probably the consequence of the lack of a specific exercise for peroneus longus in RT. Therefore, in order to avoid different strength adaptations agonist and antagonist should be trained with the same training treatment. 5. Conclusions The present study demonstrated positive effects on muscle strength and neuromuscular activation in young short track speed skaters as a result of 12 weeks combined ST-RT. The training adaptations concerning inversion were higher in comparison to eversion. This was probably the consequence of a lack of a specific exercise for peroneus longus in RT. Therefore, if RT and ST activate the same muscles, the combined training could probably lead to a greater improvement in maximum strength and neuromuscular activation. Both training methods might have induced different neural adaptations which were also responsible for the effects. We cannot quantify how many of the adaptations in muscle strength and neuromuscular activation were definitely caused by the combined training or by the specific short track speed skating training. Based on the obtained results, it may well be that the combined ST-RT may useful for sports in which maximum strength, active joint stabilization and postural control play a decisive role.

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