Muscle Coactivation Index In The Ankle Joint On Different Support Surfaces

Research subjects

Our research had 6 subjects (Table 01 ), Junior III and II female athletes, specialized in the high jump and triple jump events. The age of the subjects was between 14 and 17 years and the location of the research was the Athletics Hall in Bacău City. At the time of the evaluation, out of 6 subjects, one athlete had a second degree lesion in the meniscus, while another athlete suffered an injury at the ankle level 3 weeks before the evaluation.

In order to be able to make the recordings, we obtained the written consent of the parents of the six athletes, the research being conducted in accordance with the Helsinki Declaration on Human Subjects Research and with the agreement of the Ethics Committee of “Vasile Alecsandri” University of Bacau.

Evaluation protocol

Regarding the actual assessment of the subjects, we recorded data related to age, height, weight, takeoff leg and results in the sports events in the test competition on April 23th, 2018, and on June 16th, 2018, which took place at the Athletics Hall in Bacau (Table 01 ).

The main assessment method was surface electromyography. This was done by means of the electromyograph (Biopac MP36, Biopac System Inc., Santa Barbara, CA). Before starting the evaluation and putting the electrodes, we cleaned the skin, applied a conductive gel, and calibrated the recording unit. The data recording was done via 2 acquisition channels, which were pre-set for electromyography and whose recording frequency was 1000 Hz. The assessed muscles were the tibialis anterior and triceps surae muscles.

In order to position the electrodes at identical anatomical landmarks at the level of the tibialis anterior muscle, they were positioned 3 cm downwards and 1 cm medial to the tibial tuberosity. For the triceps muscle, the electrodes were applied at 4 cm below the popliteal fossa. The neutral electrodes were positioned at the basin’s level, on the iliac crests, and the subjects performed the event barefoot.

The evaluation test had the following approach: the subjects stayed for 30 seconds on each support surface, while we recorded the spontaneous muscular activity and the maximum contraction value. The order of the evaluation on the four supporting surfaces was: 1 - ground, 2 - elastic trampoline, 3 - balance disk and 4 - air cushion. The data obtained were recorded, processed using the BIOPAC Student Lab 4.0 software. In order to reduce the measurement errors, we used the “smoothing” function of the software, and in order to highlight only the positive values, we used the “root mean square (RMS)” function. The initial evaluation of the subjects took place on April 23th, 2018, and the final evaluation took place 8 weeks later, on the June 16^th, 2018.

Muscle coactivation index

In order to calculate the values ​​of the muscle coactivation index, we used the maximum contraction value obtained by the agonist muscle and divided it to the maximum contraction value obtained by the antagonist muscle + the muscle contraction value obtained by the agonist muscle. The calculation formula was proposed by (Ervilha, Farina, Arendt-Nielsen, & Graven-Nielsen, 2005). CI = 2* $\frac{EMGant}{EMGag+EMGant}*100$. The authors claim that the accuracy of the calculation formula is 92%, ± 6%. We have not found reference values ​​because they differ from one subject to another, but we have found the specification according to which the results obtained through the calculation formula are considered positive when the percentages obtained have a lower value.

6.1. Research results

First we described the results obtained by the tibialis anterior muscle, then by the triceps surae muscle for each subject. The collected data were grouped and are presented in the table no. 02. Following the subjects’ evolution and the analysis of the obtained results, we highlight a series of aspects:

At the ground level, the tibialis anterior muscle of the subject D.I. obtained an initial value of 44% of the muscle coactivation index and a final value of 85%. On the elastic trampoline, the values ​​of the muscle coactivation index decreased compared to the results obtained on the ground, our subject having an initial value of 10% and a final value of 30%. On the balance disk, the values ​​of the muscle coactivation index were 9% at the initial assessment and 54% at the final assessment. At the air cushion level, the initial value of 54% of the muscle coactivation index decreased to a final value of 9%.

Regarding the evolution of the triceps surae muscle, it obtained an initial value of 98% of the muscle coactivation index and a final value of 69% at the ground level. The change of the support surface led to a decrease in the initial values of the muscle coactivation index to 37% for the elastic trampoline, 23% for the balance disk and 67% for the air cushion. At the final evaluation, an 18% muscle coactivation index was recorded on the elastic trampoline, 38% was obtained on the balance disk and 39% on the air cushion. The athlete recorded a performance of 12.02m in the first assessment and 12.36m in the second assessment, therefore, an improvement of 0.34m.

For the subject V.T., the values ​​obtained by the muscle coactivation index on the most stable support surface (ground) were 36% at the initial assessment and 27% at the final assessment for the tibialis anterior muscle. On the elastic trampoline, V., recorded a muscular coactivation index of 8% at the initial assessment and 9% at the final assessment. At the level of the balance disk, the initial value of the index was 18% and the final value of 6%. The air cushion led to an initial value of 27% of the muscle coactivation index and a final value of 18%.

By tracking the evolution of the the muscle coactivation index for the triceps surae muscle, we can notice the following: at the ground level, the initial value of the muscle coactivation index was 27% and the final value 47%. At the trampoline level, subject V recorded an initial value of 10% of the muscle coactivation index and a final value of 50%. The values ​​obtained on the balance disk were 13% at the initial evaluation, respectively 15% at the final evaluation. The air cushion recorded a 2% increase from an initial value of 31% to a final value of 33%. The athlete recorded a performance at the first assessment of 11.49m and a second one of 11.64m, therefore, an improvement by 0.15m (Table 02 ).

For subject B.B., the most important changes in the coactivation index were recorded in the evolution of the tibialis anterior muscle. The coactivation index values ​​had a decrease of the initial values directly proportional with the support surface becoming harder (60% - ground, 43% elastic trampoline, 25% balance disk and 27% air cushion). This trend was also maintained at the final assessment, with the athletes recording 15% on the ground, 42% on the elastic trampoline, 27% on the balance disk and 11% on the air cushion.

At the triceps surae muscle level, the values in the initial evaluation ​​showed a muscle coactivation index of 51% on the ground support, 57% on the elastic trampoline, 43% on the balance disk, and a value of 2.5% on the air cushion. In the final evaluation, subject B. recorded a muscle coactivation index of 33% on the ground, 31% on the trampoline, 19% on the balance disk, and 11% on the air cushion. The results obtained by the subject B.B. were very good because both muscle groups obtained better values ​​of the muscle coactivation index on all the support surfaces in the initial assessment and in the final assessment, as well. The athlete recorded a performance at the first evaluation of 10.80 m, and a second one of 10.96 m, thus, an improvement of 0.16 m.

The results obtained by the subject R.S., highlight the perfect evolution of the muscular coactivation index at the initial evaluation of the tibialis anterior muscle. The values ​​obtained for the muscle coactivation index (60%) at the ground level decreased to 23% on the elastic trampoline, 21% on the balance disk and 24% on the air cushion. The positive evolution was also maintained in the final assessment, where we can notice a decrease in the values ​​of the muscular coactivation index, the subject R., recording a value of 34% on the ground, 13% on the elastic trampoline, 47% on the balance disk, respectively 7% on the air cushion.

In the initial evaluation of the coactivation index for the triceps surae muscle, the initial values ​​obtained on the ground had a percentage of 29%, with the change of the support surface (trampoline - 90%, balance disk - 51% and air cushion - 55 %), the values ​​show a significant increase. In the final evaluation, the results of the muscle coactivation index had an improvement, the subject R., registering a value of 37% on the ground, 26% on the elastic trampoline, 21% on the balance disk and 55% on the air cushion. The increase in the initial values ​​of the muscular coactivation index for the R.S. subject is justified by the existence of a second degree lesion in the meniscus. The athlete reached a performance of 11.46 m in the first evaluation of 11.62 m, therefore, an improvement of 0.16 m.

Concerning the values ​​obtained by the subject M.C. at the ground level, the muscle coactivation index of the tibialis anterior muscle had an initial value of 17% and a final value of 31%. The change of the support surface led to an increase in the initial values ​​for the elastic trampoline to 35%, while the balance disk values ​​dropped to 16% and those for the air cushion to 8%. At the final evaluation, the coactivation index values ​​on the elastic trampoline were 21%, 7% on the balance disk and 17% on the air cushion.

In the triceps surae muscle, the subject M.C had a perfect evolution. On the ground, the initial value of the muscle coactivation index was 15%. The value obtained by the muscle coactivation index on the elastic trampoline was 1%, 4% on the balance disk and 15% on the air cushion. At the final evaluation, the values ​​obtained had a muscle coactivation index of 36% at the ground level, 22% at the elastic trampoline, 51% on the balance disk and 17% on the air cushion. The athlete recorded a performance at the first assessment of 1.60 m and a second evaluation of 1.63 m, thus, an improvement of 0.34 m.

The analysis of the results obtained by the subject V.R., at the initial evaluation, reveals a poor evolution of the athlete, probably due to the ankle sprain suffered 3 weeks before.

In the initial assessment of the tibialis anterior muscle, the subject had a value of 30% for the muscle coactivation index at the ground level. The change in the support surface increased the muscle coactivation index values ​​up to 63% on the elastic trampoline, 39% on the balance disk and 38% on the air cushion. After 3 weeks, at the final assessment, the values ​​obtained by the subject V. were much better. At the ground level, a muscle coactivation index of 38% was recorded, while on the elastic trampoline it was 4%, on the balance disk 39%, and on the air cushion 12%.

At the triceps surae muscle level, the muscle coactivation index achieved an initial value of 13% at the ground level. The change of the support surface resulted in an increase of 71% in the muscle coactivation index on the elastic trampoline and 65% on the balance disk. At the same time, the values ​​dropped to 6% when assessing it on the air cushion. The final evaluation revealed a change in the muscle coactivation index to 97% on the ground, 28% on the elastic trampoline, 54% on the balance disk and 18% on the air cushion.

6.2. Result interpretation

In the acute phase of an injury, there is an increase in the muscle coactivation index (seen at the injured athletes, R.S. and V.R.). Eight weeks after the initial assessment, we assume that the pain relief due to injuries improved the muscle coactivation index since it has been shown that the related signals sent by the pain receptors can interfere with the corresponding signals sent by the proprioceptors, thus having a negative impact on the entire motor control system (Abd-Efattah, Abdelzeim, & Elshennawy, 2015). A single athlete (MC) recorded higher (ununjustified) values ​​of the muscle coactivation index at the final assessment. The final values ​​of the muscle coactivation index were better when compared to the initial values ​​of the muscle coactivation index. We also note an improvement in the values of the muscle coactivation index ​​when the support surface became more unstable. Taking into account that the final evaluation was made one week before the most important competition of the season, it is possible that the improvement of the values ​​of the muscular coactivation index is also justified by the competition period, the moment of the final evaluation being actually the peak of the athletes. The muscle coactivation index may be an effective tool for measuring the quality of the motor acts, because the lower the muscle coactivation index is (i.e., there is an agonist-antagonist muscle balance), the higher the sports performance will be, with a lower risk of injury occurrence.

6.3. Discussions

Starting from Sherrington in (1906 -1897) by justifying and raising the consideration for the mutual inhibition as law, the antagonist muscles have been considered inactive throughout the duration of the agonist muscle contraction for a long time. The remarkable advances made in physiology have demonstrated the existence of the inhibitory neurons, which, by means of the signals received from the neuromuscular spheres, regulate the intensity of the antagonist muscle contraction. The hereby research, having an observational purpose, was to highlight the changes occurring at the level of the muscle coactivation index while changing the support surface. At the same time, we have been trying to find correlations between the changes in the muscle coactivation index and the existence of injuries in the lower limbs. For all the subjects evaluated on the stable support surface, the muscle coactivation index was higher in the tibialis anterior muscle. This suggests the important role of the tibialis anterior muscle statically, which, along with the peroneal and tibialis posterior muscles, provides stability to the ankle joint (Louwerens, van Linge, de Klerk, Mulder, & Snijders, 1995). When the support surface changed, the values ​​obtained were reversed, the triceps muscle becoming dominant. The results highlight the importance of the triceps muscle in dynamics, muscles playing a particularly important role in long jump. The ability of the ankle joint muscles to synchronize the muscle contraction with the ongoing movement is a functional adjustment achieved in training through the muscle coactivation. The same functional modification was found by (Bazzucchi, Riccio, & Felici, 2008) in tennismen, but at the elbow joint level. The changes in the muscle coactivation index were also reported by Linens, Ross, Arnold, Gayle, & Pidoce (2014) in subjects with chronic ankle joint instability, and by Kitatani et al., (2016) in patients with cerebrovascular accident (CVA)/strokes. At the same time, Rossi et al., (2014) reported changes in the muscle coactivation index during exercise or pilates. The ultimate goal of all these exercises is to decrease the values ​​of the muscle coactivation index so that the antagonist resistance is minimal.

In athletes, the changes in the muscle coactivation index were reported by Kellis, Zafeiridis and Amiridis (2011) in the ankle muscles, especially during their contact with the ground. The same authors claim that an increased muscle coactivation may play an important role in the injury occurrences, especially when the running biomechanics is deficient. DeMers, Hicks and Depl (2017), demonstrated that through the feed-forward mechanism (i.e. by the occurrence of the anticipated muscle contraction), the muscle coactivation plays an important role in reducing the risk of ankle joint injuries, especially when the coactivation index values ​​are below 60%.

Regarding the results obtained according to the support surface, the air cushion and the balance disk were the most demanding support surfaces. Authors such as Ridder, Willems, Vanrenterghem and Roosen (2015) or Nepocatych, Ketcham, Vallabhajosula and Balilionis (2018) state that the bosu ball (similar to the air cushion) offers the greatest postural deviations and Harput, Aouylu, Ertan and Ergun, (2013) states that it is not the support surface which determines the amount of muscle contraction, but the type of the exercise practiced.

In injured subjects, modifying the coactivation index is a normal phenomenon, as the onset of pain leads to a change in the muscle contraction pattern (Tam, Santos-Concejero, Coetzee, Noakes, & Tucker, 2017). In these situations, athletes are advised to rest, as a continuation of their sports activity leads to chronic pain and structural reorganization of the motor engramme of the somatosensitive cortex (Borghuis, Hof & Lemmink, 2008).

Since the ankle is the first joint to contact the ground, it plays a particularly important role in balancing the postural deviations coming from the trunk. Taking this into consideration, by optimizing the biomechanics of the lower limb, the muscle coactivation can ensure the optimal operation of the entire motor control system.