1. INTRODUCTION:

Soccer is an explosive activity that involves skills such as jumping, kicking, and tackling¹, and injuries such as ankle and knee sprains and muscle fatigue can occur from physical contact between players^2,3. In particular, the ankles receive the most stress due to sudden postural changes and stops, tackling, and jumping^4,5. Soccer players have the highest rate of ankle injuries⁶.

Ankle injury during a match can cause loss of balance ability due to unstable joint movement from proprioceptive sensory impairment, neuromuscular dysfunction, and reduced postural control^7,8.

Visual, vestibular, and somatosensory systems are involved in balance control⁹, and among somatic senses, proprioception represents an immediate external response. Therefore, recovery of proprioception in soccer players is very important in reducing functional instability and joint re-injury¹⁰.Moreover, proprioception helps in accurate execution of various technical movements¹¹, while improving performance by reducing ankle and knee injury^12,13.

Among physical therapy interventions for improving proprioception, sensorimotor training stimulates the brain by maximizing proprioceptive stimulation through exercise performed on unstable ground¹⁴. Sensorimotor training focuses on proprioceptive sensory input in the feet and trains functional tasks under various conditions for normal muscle activation and reflexive stabilization¹⁵.

Transcranial direct current stimulation (tDCS) is simple to simple to apply and can selectively induce and sustain functional changes in the cerebral cortex¹⁶, and reportedly has significant efficacy in various applications, including improvement of physical exercise performance and control of chronic pain¹⁷, fibromyalgia¹⁸, and cancer pain¹⁹. However, pain has been the main topic in most of the studies on sensory function.

The present study applied sensorimotor training for stimulation of the central nervous system and primary somatosensory cortex (S1), using tDCS in soccer players with reduced balance ability after a soccer match. The aim was to determine the effects of changes in proprioception and improvement in balance ability.

2. MATERIALS AND METHODS:

2.1. Study participants:

Ten soccer players who participated in the study provided informed written consent in accordance with the Declaration of Helsinki. The subjects consisted of those who did not have a history of surgery in the hip, knee, or ankle joint; did not have factors that can cause pain such as a burn or wound on the scalp; have no prior experience of tDCS; have no metallic insert in the cranium; have no hearing loss; and have no nervous system or vestibular disorder or prior history of such disorder. The experiment was conducted with subjects randomly assigned to group I (n=5) or group II (n=5) (Table 1).

Table 1. General characteristics of study participants

Characteristics	Group I (n=5)	Group II (n=5)
Age (years)	20.00±1.00	21.00±1.00
Height (cm)	173.00±2.65	174.33±1.15
Weight (kg)	71.33±1.15	64.67±2.89

All data are expressed as means with standard deviation (M±SD)

Group I: Sensorimotor training, Group II: Sensorimotor training and tDCS

2.2. Procedures:

2.2.1. Sensorimotor training:

In group I, sensorimotor training was applied 5 times for 20 min per week. In group II, after sensorimotor training, tDCS was applied to S1, 5 times for 20 min per week. Assessments were performed before, immediately after, and 5 days after a match. For sensorimotor training, the Harmoknee preventive training program by Kiani et al. (2010)²⁰ was used. The training program comprised warm-up, muscle activation, balance, muscle strength, and lumbar stabilization exercises. Each motion was performed for 30-60 s and repeated twice for a total of 25 min.

2.2.2. Transcranial Direct Current Stimulation (tDCS):

For tDCS, a Brain Driver v2.1 device (The Brain Driver Inc., USA) was used. The electrodes were round sponge pads measuring 5×5 cm, and were applied after soaking in 0.9% physiological saline. The electrodes were attached to the S1 area, which is closely associated with proprioception²¹. The anode was fixed on C3′, an area 2 cm behind C3, while the cathode was fixed on the supraorbital area on the opposite side of the anode. Stimulation intensity and duration were set to 1 mA and 20 min, respectively²².

2.2.3. Proprioception Test:

A 4D-MT device (Relive, Gimhae, Korea) was used to assess the reproducibility of ankle joint proprioception. With the subject in a seated position, a sensor was attached to the center of the dorsum of the foot, and 10° ankle dorsiflexion and 30° ankle plantar flexion were performed and maintained for 10 s. After returning to the original position, the joint angles remembered were reproduced. The errors between the target angles of the ankle and the reproduced angles were calculated, from which the mean value was calculated²³.

2.2.4. Balance Test:

BIORescue (RM INGENIERIE, Rodez, France) was used to measure balance ability. For static balance, the legs in standing position were placed apart at a 30° angle while facing forward. Measurements were made with eyes open and closed. After balancing for 1 min, the surface area ellipse was measured. For dynamic balance, the subject stood with legs apart at a 30° angle and moved the body weight within the maximum range possible without losing balance in 8 different directions, prompted by a monitor, during which the limit of stability was measured. The distance of center of gravity movement in forward, backward, and whole directions was measured²⁴.

2.3. Statistics Analysis:

For statistical analysis, SPSS 22.0 ver. for Windows^® was used to derive mean and standard deviation. One-way repeated measure ANOVA was conducted for the significance test regarding the effects of the sensorimotor training and tDCS depending on the measured time of each group, and Duncan’s multiple range test was conducted for the post test. Statistical significance level was set to α=0.05.

3. RESULTS:

3.1. Changes of Proprioception:

When the ankle dorsiflexion reproduction errors were compared between pre and post match, group I (p<0.01) and group II (p<0.001) showed a significant difference, indicating a severe damage in the proprioception. Upon comparison between pre and 5^th days later from match, group I showed a significant difference (p<0.05), but group II did not. Furthermore, upon comparison between post and 5^th days later from match, group II showed a significant difference. These results suggest that group II had a great recovery to pre match levels (Table 2).

When the ankle plantar flexion reproduction errors was compared between pre and post match, group I (p<0.01) and group II (p<0.001) showed a significant difference, indicating a severe damage in the proprioception. Upon comparison between pre and 5^th days later from match, group I (p<0.05) and group II (p<0.05) showed a significant difference. Furthermore, upon comparison between immediate post and 5^th days later from match, group I (p<0.05) and group II (p<0.001) showed a significant difference. These results suggest that group II had a greater recovered to pre match levels compared to group I (Table 2).

Table 2. Comparison of changes in proprioception (unit: degree)

	Group	Pre match	Post match	5^th days later from match
Dorsi flexion	I	2.42±0.82	2.93±0.57**	2.87±0.44^ߠ
Dorsi flexion	II	2.39±0.37	2.92±0.32***	2.44±0.24^###
Plantar flexion	I	2.46±0.43	2.94±0.43**	2.77±0.45^ߠ^#
Plantar flexion	II	2.48±0.55	2.99±0.53***	2.54±0.54^ߠ^###

All data are expressed as means with standard deviation (M±SD)

Group I: Sensorimotor training, Group II: Sensorimotor training and tDCS

Tested by one-way repeated measures ANOVA and Duncan`s multiple range test according to the time of measurement in each group column

pre and post match (^**p<0.01; ^***p<0.001), pre and 5^th day later from match (^ߠp<0.05), post and 5^th days later from match (^#p<0.05; ^###p<0.001)

3.2. Changes of balance ability:

3.2.1. Static balance:

When surface area ellipse in static balance (with eyes opened) was compared between pre and post match, group I (p<0.01) and group II (p<0.001) showed a significant difference, indicating a severe damage in static balance. Upon comparison between pre and 5^th days later from match, group I (p<0.01) and group II (p<0.001) showed a significant difference. These results suggest that group II had a greater recovery to pre match levels compared to group I (Table 3).

When surface area ellipse in static balance (with eyes closed) was compared between pre and post match, group I (p<0.001) and group II (p<0.001) showed a significant difference, indicating a severe damage in static balance. Upon comparison between pre and 5^th days later from match, group I showed a significant difference (p<0.05), but group II did not. Furthermore, upon comparison between post and 5^th days later from match, group I (p<0.01) and group II (p<0.001) showed a significant difference. These results suggest that group II had a greater recovery to pre match levels compared to group I (Table 3).

3.2.2. Dynamic balance:

When limits of stability in dynamic balance (with forward and backward) were compared between pre and post match, group I (p<0.001) and group II (p<0.001) showed a significant difference, indicating a severe damage in dynamic balance. Upon comparison between pre and 5^th days later from match, group I showed a significant difference (p<0.05), but group II did not. Furthermore, upon comparison between post and 5^th days later from match, group I (p<0.05) and group II (p<0.01) showed a significant difference. These results suggest that group II had a greater recovery to pre match levels (Table 4).

When limits of stability in dynamic balance (with whole directions) was compared between pre and post match, group I (p<0.01) and group II (p<0.001) showed a significant difference, indicating a severe damage in dynamic balance. Upon comparison between pre and 5^th days later from match, group I (p<0.01) and group II (p<0.01) showed a significant difference. Furthermore, upon comparison between post and 5^th days later from match, group I (p<0.01) and group II (p<0.01) showed a significant difference. These results suggest that both group I and group II had modest recovery to pre match levels (Table 4).

4. DISCUSSION:

Vigorous movements in soccer may cause knee or ankle injury, which can become the cause of functional instability due to proprioceptive sensory impairment or ankle muscle weakening^25,26. Consequently, prevention of ankle injury²⁷ and proper treatment are very important for player performance.

Changes in ankle proprioception in soccer players have a significant impact on mechanoreceptors near the ankle due to fatigue that sets in after 45 min of play as compared to before the match²⁸. Moreover, muscle fatigue causes an increase in fusimotor excitation threshold and interferes with afferent information in muscle receptors, which negatively affects proprioceptive and motor sensory abilities in the joints^28,29. Accordingly, changes in somatosensory input, as well as activation of neuromuscular activities and muscle strength, are very important for posture control and balance deficits after a soccer match³⁰.

The present study applied sensorimotor training, a method that stimulates the central nervous system through activation of muscle strength and neuromuscular activity, and tDCS, a non-invasive stimulation method applied to the S1 to change somatosensory input, in soccer players who showed postural control and balance difficulty after a match. The aim was to investigate the effects of sensorimotor training and tDCS on improvement in proprioception and balance ability.

Sensorimotor training emphasizes functional recovery of the nervous system through relearning of exercise. It also trains functional tasks under various conditions for normal muscle activation and reflexive stabilization, with importance placed on proprioceptive input in the feet^15,31.

Meanwhile, tDCS increases the activation of nerve cells³² and excitability of the motor cortex³³, while also strengthening corticospinal connections and positively affecting functional recovery³⁴. Moreover, tDCS can enhance the exercise effect during brain stimulation. This painless stimulation technique can achieve various functional changes, depending on the intensity, duration, and site of stimulation³⁵. The present study compared ankle motion reproduction errors to determine the improvement in proprioception. After the appearance of a severe damage to the proprioception post match in both ankle dorsiflexion and plantar flexion, group II recovered to significant levels 5^th days later from match. In particular, ankle dorsiflexion showed a great recovery to the pre match levels (Table 2).

It is believed that sensorimotor training directly presents somatosensory information to the ankle or sole³⁶, and that the somatosensory input is increased on inclined or unstable surfaces, with a major impact on changes in mechanoreceptors on the skin of the feet, as well as joint and muscle receptors^37,38. In addition, tDCS increased afferent signal sensitivity by controlling the excitability of the cerebral cortex, in order to increase somatosensory evoked potentials and blood flow to the brain, which result in increased somatic sensation in the soles through improved proprioception^21,39.

Regarding surface area ellipse in static balance (with eyes opened and with eyes closed), a severe damage to static balance ability appeared post match, and 5^th days later from match, group II showed a greater recovery to the pre match levels in comparison to group I (Table 3). In particular, a great recovery was observed in surface area ellipse in static balance with eyes closed.

Furthermore, regarding surface area ellipse in dynamic balance (with forward, backward and whole directions), a severe damage to dynamic balance ability appeared post match, and 5^th days later from match, group II showed a significant recovery to pre match levels. In particular, a great recovery to pre match levels was observed in limits of stability in the forward and backward (Table 4). It is believed that this was due to application of tDCS causing changes in the excitability of the cerebral cortex, which enabled sensorimotor training and input from proprioceptive stimuli under various conditions, with heightened receptor sensitivity to specific afferent signals generated from joints, ligaments, and tendons.

In static and dynamic balance, ankle strategies are used to maintain stability⁴⁰, with posture and balance maintained by providing sensory information to the body through ankle proprioception⁴¹. Therefore, applying sensorimotor training and tDCS facilitated ankle joint use by increasing the sensitivity of ankle proprioception and employing systematic proprioceptive stimulation.

5. CONCLUSION:

The present study demonstrated that application of sensorimotor training and tDCS are safe and effective in soccer players who show difficulty with postural control and balance ability after a match, with significant improvement in balance ability through changes in proprioception. The findings in the present study may be used for clinical application in soccer players to improve performance.

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Received on 08.11.2018 Modified on 21.12.2018

Research J. Pharm. and Tech. 2019; 12(7):3323-3327.

DOI: 10.5958/0974-360X.2019.00560.2

		Group	Pre match	Post match	5^th days later from match
Surface area ellipse	Open eyes	I	17.56±6.15	26.22±12.60**	18.78±8.61^##
	Open eyes	II	18.00±5.98	25.11±7.08***	16.56±5.03^###
	Close eyes	I	14.89±3.66	25.78±5.89***	18.00±4.15^ߠ^##
	Close eyes	II	15.89±2.32	23.78±3.83***	14.00±5.55^###

	Group	Pre match	Post match	5^th days later from match
Forward	I	6943.44±799.90	6036.67±833.88***	6492.22±1022.57^ߠ^#
Forward	II	6875.11±582.20	5793.67±1110.60**	6806.00±762.40^##
Backward	I	5943.44±799.90	5073.11±863.92***	5541.22±1054.66^ߠ^#
Backward	II	5764.00±620.15	4682.56±1217.98**	5824.11±881.96^##
Whole	I	9653.11±863.53	8717.22±414.20**	9215.00±235.22^##
Whole	II	9588.56±1277.71	8436.11±1044.84**	8992.00±942.48^##