1. INTRODUCTION:

Knee joint is one of the largest joint in the human body, located at the end of the femur and the tibia of both long lever arms. It is where hypofunction and muscle weakness appear a lot¹. In addition, strength and stability are dependent on the ligaments and muscles, and do not depend on the structure of the bone because the joint is connected to one long bone and another long bone, the knee joint causes an anterior cruciate ligament injury and patellofemoral pain syndrome including genu valgus, genu varus when theses stability will not be achieved. Exercises that enhance the stability of the knee joint can be effective for such cases².

The single leg squat (SLS) is a common test used by clinicians for the musculoskeletal assessment of the lower limb³. In addition, SLS is effective in promoting muscle strength than the two leg squat, and have the advantage that it can reduce the load and stretching of the anterior cruciate ligament in accordance with the strength, but the changing movement pattern of the joint during excessive SLS increases the impact on the hip joint. Osteoarthritis in the knee increases the incidence by adding the load on the acetabular labrum and anterior superior chondral. During the SLS, looking at the change of each joint of the lower limb, the pelvis is maintained in a hiked position and backward rotation while knee is moved to be extended to flexion into an external rotation. In the meantime, the ankle undergoes dorsiflexion and eversion.

Many previous studies about SLS, find significant difference in muscle activation⁴. Insistence study, where subjects were asked to do SLS, a single leg standing balance, isometric squats, bipedal squats like give four intervention. During this process, monopolar EMG muscle activation of VL and VM were measured⁵. The results from previous studies show that intermuscular motor unit synchronization was enhanced during contractions during dynamic activities, possibly to facilitate a more accurate control of the joint torque, and to reduce the burden of single leg tasks that require balance control and thus, providing a more independent muscle function. And other previous study claimed when dominant leg doing SLS, non-dominant leg intervention hip abduction and extension then compared to the carrying angle. As a result, hip abduction during SLS insistence change in terms of frontal plane projection angle by 22% ⁴. In others previous study that compared a 60 degree SLS with a maximum angle SLS, the results howed a low correlation with the hip at the maximum angle SLS than 60 degree SLS. Namely, a 60 degree flexion of knee SLS exercise was better than maximum angle flexion of knee SLS exercise⁶.

However, several studies had significantly conflict results. Increased muscular force was generated during a SLS which was different from the existing study where intermuscular motor unit recruitment during dynamic SLS activity decreased⁵. Meanwhile, other studies asserted that correlation was weakness. So, the study was derived. Somewhat different is in conclusions. In addition, in contrast to existing studies, the highest peak muscle activities of the tibialis anterior, the rectus femoris and the biceps femoris were recorded during the squat at 110 degree. However, the soleus muscle record the highest peak muscle activity during half squatting (70 degree knee flexion) with the value⁷.

As demonstrated in previous studies, recent studies had seen SLS done at a knee flexion of 60 degree saw a higher correlation of hip for changing muscle activation of measurement during SLS exercise measure muscle of VL (vastus lateralis) and VM (vastus medialis). Overall, it does not affect the change in lower limb muscles, around knee muscle activation; this study concludes that there is more variance of muscle measured around knee. Additional studies were needed to study the SLS as it was not verified about motion muscular activities other than the two interventions performed. There were needed on hip abduction and hip extension for a comparison of SLS muscle activity during a variety of motions.

Thus, the purpose of this study is to compare the change in intermuscular motor unit recruitment of the lower limb, VL, VM, HAM(hamstring), GL (gastrocnemius lateralis), GM (gastrocnemius medialis) and TA (tibialis anterior) to identify the kinematics of the differences in dynamic and isometric hip SLS and to find better exercises among them.

2. MATERIALS AND METHODS:

2.1. Subjects:

Twenty eight healthy, male (n = 14) and female (n = 14) participants volunteered and gave their written consent to participate in this study. Inclusion criteria were as follows: Subjects had no surgical, neurologic lesions, history of cardiopulmonary disease, osteoporosis, orthopedics disease, instability on knee joint for three months. Exclusion criteria were as follows: Subjects had pain and sprain of knee during SLS exercise. Ethical approval for this research study involving human volunteers was obtained from the University of Sunmoon's Research Ethics Board in Asan, Chungnam (SM-201605-011-2). A total of twenty-eight subjects were chosen using a sample size of output program (G*Power, 3.1.9.2).

2.2. Measurement Method:

This study used Electromyography (EMG, QUS100, Zero WIRE EMG, Italy, 2009), Electrogoniometer (Digital absolute + axis, 12-1027, USA, 2012), Inbody (Inbody 570, Biospace, Korea, 2013), after removing the material that can effect on the result of study. Volunteers performed six motions in random sequence based on what the usage program drew. Each volunteer performed six different tasks each lasting three seconds: Isometric SLS with contralateral hip flexion (ISLSF), isometric single leg squat with contralateral hip abduction (ISLSA), isometric single leg squat with contralateral hip extension (ISLSE), dynamic single leg squat with contralateral hip flexion (DSLSF), dynamic single leg squat with contralateral hip abduction(DSLSA), dynamic single leg squat with contralateral hip extension (DSLSE) ISLSF kept posture that dominant leg sustain the weight and non-dominant leg maintain the motion of hip and knee: Angle of hip flexion was 90 degree, angle of knee flexion was 90 degree, and midline of trunk maintained an ideal alignment. Knee flexion was maintained at 60 degree during three second for SLS. ISLSA kept a posture where the dominant leg sustained the weight while the non-dominant leg maintained the motion of hip and knee: The angle of hip abduction was 90 degree, angle of knee flexion was 90 degree, and midline of trunk maintained an ideal alignment. ISLSE kept the posture where the dominant leg sustained the weight while the non-dominant leg maintained the motion of hip and knee: Angle of hip extension was 90 degree, angle of knee flexion was 90 degree, and midline of trunk maintained an ideal alignment.

Dynamic SLS intervention was performed like that exercise. DSLSF was performed the sit down and up at 60 degree of knee flexion angle during three second that motion was like ISLSF. DSLSA was performed the sit down and up at 60 degree of knee flexion angle during three second that motion was like ISLSA. And DSLSE was performed the sit down and up at 60 degree of knee flexion angle during three second that motion was like ISLSE.

Tester requested right posture of volunteers for accuracy result data before the task starting. Meanwhile, verbal feedback was used to encourage subjects to hold their trunks in a vertical position.

Tester pre-educated the subjects on the method of performance for the task. Volunteers performed a series of squat down to a knee flexion angle of 60 degree with contralateral knee flexion degree of 90 degree. In order to ensure consistent knee flexion degree, volunteers were given verbal real-time feedback from a Electro goniometer (Digital absolute + axis, 12-1027, USA, 2012) on the lateral aspect of the volunteers’ knee joint. Volunteers were asked to balance on their dominant leg while trying to keep an upright upper body posture by tester. For each time, a metronome and a stopwatch of a cell phone was used for visual feedback and accuracy.

2.3. Experiment Procedures:

In order to obtain Bipolar EMG currents from lower limb, the skin surface above the muscles were shaved, slightly abraded with sand paper and cleaned with alcohol wipes to ensure high signal conductivity. Bipolar Ag-AgCl electrodes were placed over the muscle of VM, VL, HAM, GM, GL and TA according to electrode locations recommended in previous studies. Six electrode pairs were placed on each muscle, in the direction of the muscle fibers, to get information of six muscles at the same time. EMG was placed on the location of muscle palpation. The tester executed palpation giving resistance to moving direction of volunteers' muscles. A part of the palpation muscle was used the EMG after confirming of muscle activation during resistance at muscle. Also, other muscles were used the EMG in the same way. However, it could not be used in the abrasion. The signals were electronically band-pass filtered between 10 and 500Hz. The sampled signal was applied to RMS(root mean square). RMS values were computed for each collection period.

2.4. Statistical analysis:

In this study, SPSS version 22.0 software (SPSS Inc, Chicago, IL, USA) was used for all statistical analysis. In order to calculate the subject characteristics, the mean value and the standard deviation of all the variables of the subjects, descriptive statistics were applied to our study. One-way repeated ANOVA was used to compare muscle activities of VL, VM, HAM, GL, GM and TA depending on the six interventions. Bonferroni's correction was used as post-hoc comparison and statistical significance level was set at α=.05.

3. RESULTS AND DISCUSSION:

As a result of the paired t-test, there was a significant difference that was about intermuscular motor unit recruitment of lower limb between isometric and dynamic. Concretely, when subjects took SLS exercise during hip flexion with knee flexion of non-dominant lower limb by isometric and dynamic, muscle of HAM(t=6.186, p<0.01), VL(t=9.671, p<0.01), VM(t=8.87, p<0.01), GL(t=3.906, p=0.001), GM(t=4.36, p<0.01), TA(t=2.462, p=0.02) was a significant difference. Secondly, during hip abduction with knee flexion, muscle of HAM (t=7.646, p<0.01), VL (t=8.635, p<0.01), VM (t=13.263, p<0.01), GL (t=5.206, p<0.01), GM(t=6.327, p<0.01), TA(t=3.079, p=0.05) was a significant difference. Finally, during hip extension with knee flexion, muscle of HAM (t=6.186, p<0.01), VL (t=9.671, p<0.01), VM(t=8.87, p<0.01), GL(t=3.906, p<0.01), GM(t=4.36, p<0.01), TA(t=2.462, p<0.01) was a significant difference[Figure 1].

When subjects take SLS exercise during hip flexion, abduction and extension with knee flexion of non-dominant lower limb by isometric and dynamic, Only one muscle, VM(F=3.364, p=0.04), was a significant difference. However, other isometric muscles, HAM(F=1.598, p=0.209), VL(F=1.285, p=0.282), GL(F=0.375, p=0.689), GM(F=0.578, p=0.563), TA(F=0.014, p=0.986), and dynamic muscles, HAM(F=0.703, p=0.498), VL(F=0.555, p=0.576), VM(F=0.699, p=0.5), GL(F=0.97, p=0.384), GM(F=0.488, p=0.616), TA(F=0.187, p=0.829), wasn't a significant difference. Flexion of non-dominant lower limb by isometric and dynamic, muscle of HAM (t=6.186, p<0.01), VL (t=9.671, p<0.01), VM(t=8.87, p<0.01), GL(t=3.906, p=0.001), GM(t=4.36, p<0.01), TA(t=2.462, p=0.02) was a significant difference. Secondly, during hip abduction with knee flexion, muscle of HAM (t=7.646, p<0.01), VL (t=8.635, p<0.01), VM (t=13.263, p<0.01), GL (t=5.206, p<0.01), GM (t=6.327, p<0.01), TA(t=3.079, p=0.05) was a significant difference. Finally, during hip extension with knee flexion, muscle of HAM (t=6.186, p<0.01), VL (t=9.671, p<0.01), VM (t=8.87, p<0.01), GL (t=3.906 p<0.01), GM(t=4.36, p<0.01), TA(t=2.462, p<0.01) was a significant difference[Figure 2].

This study used EMG during six SLS exercises and compared the results to measurement of the change in lower limb muscles activation. The results showed that each exercise appeared a significant difference in intermuscular motor unit recruitment of lower limb between isometric and dynamic values.

When doing SLS exercise, existing study measurement around knee usually was limited by VL and VM⁸. However, results from this study showed that many muscle activations measure until VL, VM, HAM, GL, GM, and TA where there was many muscle activation while doing SLS. Moreover, there were many significant results.

According to another preceding study, the angle at which there was most muscle activation is 60 degree during SLS exercise. This study also raised the reliability with 60 degree by progressing⁹. The biggest difference between this study and the previous study is that it gave a variety of motion in the non-dominant leg when dominant leg is squatting. In this study, SLS exercise provided efficient SLS exercise guide that apply hip flexion, abduction and extension in non-dominant leg by intervention and measurement for the many effective muscles, efficient at the same time.

Finally, this study is a intervention study across different preceding studies that used isometric and dynamic variables at SLS, but intervened studies of two studies are not sufficient. In addition, existing studies that use SLS almost always measure by dynamic value¹⁰.

Therefore, through this study using the motion of hip of isometric and dynamic differences in muscle activity, the difference in isometric and dynamic muscle activity were compared. This allowed to identify the advantage of an effective way to muscle activity of SLS exercise. The following was a comparison based on isometric and dynamic values by six motions.

The first description of this study compared the difference that was flexion, extension, abduction of non-dominant leg during SLS. Results of this experiment were a significant difference that was flexion, extension, abduction of between isometric and dynamic.

According to the previous study that compared VM and VL when non-dominant leg do hip flexion and dominant leg do SLS, muscle activation of VM and VL more increased during SLS ¹¹. However, previous studies measured two muscles, around the knee, not able to know the value of other muscles. In addition, only two muscles could not represent all muscles involved in the knee muscles located above the knee. In addition, this previous study could not generalize because the number of volunteers is few people also don't fit the ratio of men and women. However, this study measured both extensor muscles and flexor muscles base on the knee, also to make generalizations about sample size of output program(G*power program3.1.9.2) confidence of study increase by appropriating the proportion of male and female volunteers. A number of previous studies compared muscle activation isometric and dynamic SLS. Among them, the studies compared dynamic squat with quasi-static squat. Those results were a different in rapid dynamic squat group and quasi-static squat group, but that was a bit different. There was another study that compared isometric and dynamic squats. But, intervention of knee flexion angle was different. The study claimed that there was not much difference between isometric and dynamic ¹².

On the same question, our study tested hip flexion, abduction and extension of non-dominant leg with between isometric and dynamic squat of dominant leg. As a result, it was performed with isometric squat and dynamic squats, VL, VM, HAM, GL, GM and TA were significantly different. And muscle activities of all motion were high during isometric squat than dynamic squat. That is, when the squat was performed and the isometric with dynamic values were compared, muscle activity was high.

For the second experiment of this study, the non-dominant leg flexion, abduction, extension three motions in the experiment to measure the comparison to have the activity of the muscle, respectively isometric and dynamic, each operation of the only non-dominant leg in the VM of isometric a result that was a significant difference in came out. This study similarly test doing Earl et al.(2001) when doing squat to dynamic, compared to muscle activity of VMO and VL, no significant difference of two muscle don't assert. When a dynamic SLS was done for this study and a similar result was acquired, there being no significant difference in the lower limb muscles was the basis of the result ¹³. Also, Jefferey (2006) study performance static squat, a support surface on the bottom surface to give unstable and stable surface experiment comparing the VL and VM. As a result, asserted of a significant difference at all, that was a little different conclusion in the present study. This is because the results of this study saw a significant difference only when the VM static squat ¹⁴. That study was intervened by angle of knee flexion 100 degree while this study was intervened by the angle of knee flexion 60 degree. It came out differently significantly different muscles.

However, result of this study was that contralateral leg movement was hip flexion, center of gravity(COG) moved to forward and activate posterior muscle like HAM, and contralateral leg movement was hip extension, COG was moved to backward and activate anterior muscle like VL, VM of quadriceps muscle. Also, when contralateral leg movement was abduction, there was change of COG what moved lateral side as compared with supporting leg. And muscles located medial side was activated, one of these muscle was VM. However, according to claims from this study it should have also been activated VM, HAM, but the reason is because we did not mediate a 60 degree angle of the SLS, VM that is most active. Another similar study claimed that quadriceps muscle was most activated during 80~90 degree SLS. This study suggested that a best knee angle for 60 degree when performed SLS, because 60 degree was most activated muscle of VM. Thus, other similar study claimed that quadriceps muscle is most activation at 80~90 degree. And also Escamilla and Isear et al.(1998) claim that gastrocnemius was most activation at knee flexion 60 degree and 90 degree ¹⁵.

As such, there was a motion that brings about a most activated angle in each muscle. But this study was not significantly different because this study proceeded with the experience with only a knee flexion of 60 degree. Because the ankle dorsiflexion during the descent and plantarflexion during the ascent, it is a common belief that the gastrocnemius contracts eccentrically during the descent to help control the rate of ankle dorsiflexion, and concentrically during the ascent to aid in ankle plantarflexion. And performed with dynamic SLS, In this study was not significant difference. But Rafael (2001) claimed that each other compared to VM and VL produced approximately the same amount of activity, which was in agreement with data from several other studies. But, that study arranged about tibiofemoral shear and compressive forces, patellofemoral compressive force, knee muscle activity, and knee stability, and also variety angle apply differently from each muscle and all procedure the experience using bipedal squat ¹⁶. In conclusion, the result of within isometric and dynamic in this study was not significant difference all result of isometric and dynamic except VM of isometric SLS. And this study obtained the result that 60 degree SLS was most activation muscle, VM, during isometric SLS.

4. CONCLUSION:

The purpose of this study is to compare the change in intermuscular motor unit recruitment of lower limb, VL, VM, HAM, GL, GM and TA, kinematics of the differences in a dynamic and isometric hip SLS and finding better exercises. As a result, compared to isometric SLS with dynamic SLS, isometric SLS was more high muscle activation than dynamic SLS. And when SLS was maintained to 60 degree of knee flexion, extension and abduction were more high muscle activation than flexion of VM. Therefore, the isometric SLS exercise was more effective of lower limb. Also, doing SLS exercise, extension and abduction were more effective of lower limb.

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Received on 21.06.2017 Modified on 28.06.2017

Research J. Pharm. and Tech. 2017; 10(7): 2289-2294.

DOI: 10.5958/0974-360X.2017.00406.1

Intermuscular Motor Unit recruitment of Lower limb according to contralateral hip motion during dynamic and isometric single leg squat