the Sensorimotor Training Using Wireless Sensors: Studying the Effects of Balance Boards with Different Dimensions of Instability

Fig. 1.

Shimmer sensors.

2 Material and Methods

2.1 Measurements

The Shimmer™ measuring instruments are small wireless sensors, Fig. 1. The Bluetooth technology enables to stream the data online and in real-time. The used sensors were a combination of the baseboard and different daughterboards. The used daughterboards were the electromyogram (EMG) as well as the gyroscope sensor. The dimensions are about 53 mm $\times$ 32 mm $\times$ 15 mm [8].

The EMG module allows the one channel measurement of the electrical activity of a muscle. Providing pre-amplification of EMG signal the non-invasive method represents the whole activity of a muscle [9].

The gyroscope daughterboard consists of a single and a dual axis angular rate gyroscope and is able to measure three angular velocity [10].

2.2 Exercisers

Balance Board. The Balance Board is an exerciser with a multidimensional instability, Fig. 2, which offers different fields of application. The height of the exerciser is 9 cm. The Balance Board supports the strengthening of the musculature of the buttocks, the legs, the back as well as the abdomen [5].

Rocker Board. The Rocker Board is characterized by its one-dimensional instability with a height of 7.5 cm, Fig. 2. The exerciser offers either a forward-backward or a left-right instability. The Rocker Board is made to train the coordination, the stamina, the strength as well as the motor skills [11].

The left-right deflection requires movement patterns performed by the extension and the flexion of the knee joints. In contrast, the forward-backward deflection aims for the reaction of the ankle joint.

Fig. 2.

Balance Board and Rocker Board.

2.3 Experimental Setups

During the investigation two different setups were analyzed. The main part of the analyzed data is originated in the first setup. The second setup derived from a previous study [12] and was added for statistical analyzes. The first setup is meant to prove the assumption that the training on the exerciser causes effects on the whole body. The aim of the second setup was the investigation of the participation of both body sides during the sensorimotor training [13].

Supporting the objective to develop a user-oriented experimental setup the design of the study was made in cooperation with experienced physiotherapists of a medical school. The requirement to develop a test procedure which can also be executed with patients causes the drop out of the maximum voluntary contraction (MVC) measurement. Instead of the MVC normalization a reference measurement in front of the exerciser took place.

Setup 1. The first setup comprised of two young (age under 30 years) and healthy students. Both subjects were not familiar with the exercisers. An equal distribution of the sexes was given.

For the investigation two different types of Shimmer™ measurement units were used. A pair of gyroscope sensors were centrally placed on the different exercisers. For the verification of the assumption that the training on the exercisers has effects to the whole body the sensors were placed at five different muscles along the body. The following five muscles were recorded: the M. tibialis anterior, the M. vastus lateralis, the M. gluteus maximus, the M. erector spinae (longissimus) and the M. trapezius. All test points have been measured on the right and on the left body side. Ag/AgCl surface electrodes were applied at the skin. The skin preparation as well as the placement of the electrodes considered the recommendations of the SENIAM project [14].

The test persons had to perform the complete test sequence for each of the three exercisers. The subjects stand on both legs for the whole time. One test sequence comprised of a reference measurement in front of the exerciser with a duration of 15 s as well as of a measurement on the equipment. This part of the procedure was divided into four consecutive phases of changing difficulty, Table 1. All phases were characterized by symmetrical requirements to both body sides. All recordings have been done without shoes. The instructions and the supervision of the correct execution were made by an experienced physiotherapist.

Table 1.

Setup 1 – test procedure.

Phase	Task	Duration
1	Eyes open	s
2	Eyes closed	s
3	Throwing a medicine ball	s
4	Eyes open	s

Investigations have shown that next to handedness, the laterality is important for all paired body parts [15]. Thats why, the subjects had to perform a test to characterize their laterality of the hands and feet. On the whole, six tests for the handedness and ten tests for the laterality of the feet were made. Additionally, a balance test was made, too. The task was to stand on two scales, with each foot on one scale. The distribution of the weight was documented.

Setup 2. The second setup involved $$16$$

healthy subjects of the medical school and the university. One selection criterion was that the subjects have to be a right-hander. Two test persons of the original study were not included because their were left-hander. The test persons ranged from 20 years to 53 years in age. One half of the test persons was familiar with the used exercisers.

For the current investigation only the data of the Balance Board were analyzed. Again, different sensors were used. The skin preparation and the placement of the electrodes followed the recommendations of the SENIAM project [14].

The exercises were characterized by standing the whole time on both legs and symmetrical requirements to the body sides. In this setup one test sequence consists of a reference recording in front of the exerciser and the measurement with five different phases on the Balance Board. Four of the phases were identical to the phases of the first setup. Consequently, only these four phases were considered in the analyzes of the behavior of the left and right body side. Again, all test persons have not worn shoes.

2.4 Data Analyzes

Firstly the EMG data was notch filtered with a blocking frequency of 50 Hz. Secondly a band-pass filter was applied to the data [16]. The next step comprised the normalizations of the EMG data. The calculation of the average muscular activity when staying in front of the exerciser was used as normalization value. Subsequently, the absolute values of the measurement on the exercisers were transformed into relative values by using the normalization value. Consequently, the values were presented as percentage of the stance.

The signal processing also implies the full-wave rectification of the EMG data [16]. The evaluation of the data in the time domain includes the calculation of different statistical parameters. The maximum and mean values were computed for the whole signal over a time window of 512 ms [17]. These values were used for further calculations. On the one hand the course of the maximum values over time was documented. On the other hand the mean value of the maximum voltage values for each phase as well as for the complete procedure was calculated. Next to the mean and the maximum of the EMG the accumulated EMG activity (iEMG) was evaluated. Therefore, the EMG was integrated over time. Consequently, the total accumulated activity was computed by the calculation of the area under the EMG for a chosen time period [18, 19]. This calculation was performed for each phase as well as for the complete test procedure. Furthermore, the course of the iEMG was documented by the summation of the iEMG over the time.

The transformation of the EMG signal from time into frequency domain was achieved by using the Fast Fourier Transformation over signal segments of 512 ms [20, 21]. This transformation allows the computation of parameters in the frequency domain. The total power is described as the accumulation of the power density spectrum (SPD) of the whole frequencies (f), Eq. 1 [20].

$\begin{aligned} E_{totalPower}= \int _{0}^{\infty } S_{PD}(f)df. \end{aligned}$

(1)

The parameter is used as an indicator for muscle fatigue. An increase of the total power indicates that the muscle is fatigued.

In addition to the EMG data the gyroscope data was also analyzed. The motion data was low-pass filtered. Afterwards the direction of motion as well as the current deflection was computed.

3 Results

The test for the handedness of test person one (TP 01) reveals a dominance of the right body side. In five out of six tests the right hand was the preferred one. The analysis of the test for the dominant body side of the feet shows a similar result. In seven out of ten tests the subject used the right foot. An equal distribution of the weight during the scales test was given.

The laterally test of proband two (TP 02) showed that for the verification of the handedness a preference of the right side is given. The subject solved four out of six tasks by using the right hand. Equally, the analyses of the behavior of the feet figured out that there is a dominance of the right body side. In six out of ten tests the right side was preferred. The scales balance test reveals an equal distribution of the body weight.

The accumulated EMG activity was calculated for each muscle and for each exerciser. For the comparison of the participation of the individual muscles the one with the highest activity value was declared as $100\,\%$ . All other activity values of the remaining muscles were set in relation to the $100\,\%$ .