1. INTRODUCTION:

Spinal stabilization is created through the interaction of gross and fine motor systems.¹ In local muscles, core muscles that provide active support for individual segments of the spine is included. Core muscles such as multifidus (MF), transversus abdominis (TrA), internal oblique (IO) organize spinal segments in stable positions and prevent stress in inactive tissue at the peak range of movement.

In contrast, the gross motor system that include many surface trunk muscles such as rectus abdomin is (RA) and erector spinae (ES) respond to internal and external load of the trunk to regulate changes in body movement and body center.²

Exercises for trunk muscles include bridging, sit-up, leg raise, crunch, and plank exercise. Among them, sit-up exercise is a representative trunk flexion exercise and there are a number of studies that show it has effect on overall abdominal muscle activity. Sit-up exercise is improve abdominal stabilization effect and it has the effect of increased abdominal pressure due to the increase of abdominal tension and muscle strength. On the other hand, bridging exercise, which is a trunk extension exercise and because it increases trunk muscle activity, it is generally used clinically to strengthen lumbopelvic stability.³ Bridging exercise strengthen neuro- muscular control of trunk flexor and extensor muscles. Also, it improves the functional stability of the trunk by strengthening the pelvis and lower body muscles and because it can help strengthen the lumbopelvic area, it is recognized clinically.

However, it was found that an unexpected variable that predicted in trunk loading or position was the risk factor of low back pain(LBP).⁴ Examples of risk factors include excessive and sudden external load that consist specific gravity load, falling, slipping, and internal damage to nervous reactions. Without adequate stability and certain variables, the trunk can have excessive strain that cause tissue damage. Also, in case of sudden neuro- muscular system response, there will be increased trunk muscle activation and mechanical loading on the spine. Here, differences occur between intended movement and actual movement and this increases spine loading. The central nervous system (CNS) attempts to compensate for these differences. It is determined that excessive force applied on the spine is the main cause of spinal damage.⁵

In previous studies about LBP patient and motor control, compared to a person without chronic low back pain (CLBP), a person with CLBP had delayed activation of TrA, MF, and it was reported that there was a slow response and a more severe contraction. Mahmood et al.(2015) researched about microtrauma of the back according to sudden neuromuscular system response in standing position and a different result from previous studies was derived. When sudden load was given to the upper body of a subject with LBP, it was reported that there was small rigidity or preliminary muscle activity in TrA/IO, ES muscles.⁶

On the other hand, Michael (2014) studied about microtrauma according to degree of fatigue of the trunk muscles and it was found that in women rather than men and also in RA muscles rather than other muscles there was a faster onset latency.⁷ In the study by Mahmood et al. (2015), it was shown that high increase of eye open(EO) condition ES muscle short latency and eye close(EC) condition ES muscle latency could cause microtrauma and CLBP. This decrease of electrical activity is a compensatory mechanism which can be considered to be attempts by the CNS to minimize stress on the spinal tissues through muscular inhibition. These changes in trunk muscle activity can lead to pain by back injury and microtrauma.⁶ But, previous studies set conditions to measure microtrauma according to sudden load and currently there are almost no studies on external impact that occur suddenly during trunk exercise. Although there were studies on damages during exercise, it was measured in standing positions.

However, trunk exercises are done lying down. Therefore, to our knowledge, we were trying to explore microtrauma by comparing muscle activation latency and peak according to external impact during the most representative trunk exercises, sit-up and bridging.

The first purpose of this study is to find out which of sit-up and bridging exercise has higher risk of microtrauma. The second purpose is to confirm if visual information reduces the amount of microtrauma. The third purpose is to compare which muscles between deep abdominal muscle and back muscle first response during sudden loading.

2. MATERIALS AND METHODS:

2.1. Subject:

The 24 subjects of the study were 12 healthy men and 12 healthy women who currently attending S University in Asan, Chungnam. And the experiment was conducted after random placement through a draw. Mean age is 22.5±3.3years, height is 175.11±6.3cm, and weight is 70.82±7.2. The subjects received adequate explanation about the purpose and methods of the study before conducting the experiments and participated in the experiments after voluntary agreement. The selection criteria for the subjects included those who did not have back pain and did not have nervous damage of the spine, those who did not have history of surgery in the last three months, those who did not have experience of treatment due to spinal kyphosis or lumbar pain, those who did not have orthopedic problems including recent spinal fracture, and those who did not have a history of cardiovascular diseases. Those who had sensory hypersensitivity, open wounds or inflammatory disease on the abdomen, and back pain during exercise were excluded. This study was conducted with the approval of SunMoon University Institutional Review Board.

2.2. Procedure:

The author recorded a maximum muscle strength of the subjects for the lower erector spinae (ES), biceps femoris (BF), rectus abdominis (RA), rectus femoris (RF), gluteus maximus(GMAX), gluteus medius (GMED) using the electromyography(EMG) before the kettlebell exercise. For the ES, the instructor gave a maximum resistance during the back extension in the prone position. For the BF, the person gave a maximum resistance during the knee flexion in the prone position. For the RA, the subjects fulfilled a isometric contraction during the sit-up movement. For the RF, the tester gave the maximum resistance to the subjects while the subjects conducted the knee extension in the sit-up posture. For the GMAX, the educator gave the maximum resistance when the subjects are conducted the hip extension in the standing posture and knee flexion 90°. For the GMED, the subjects are received maximum resistance during hip abduction in the standing position holding grab a pole. Test leader were instructed to active the maximum strength to the subjects. Maximum strength test was performed for 2 seconds.

This study was conducted with 1-cycle during 2 seconds passing by the lower phase and upper phase during the two handed kettlebell exercise (THKE) consist of Russian kettlebell swing(RKBS), American kettlebell swing (AKBS) and kettlebell goblet squat (KBGS), or the single handed kettlebell exercise (SHKE) consist of one-arm kettlebell swing (OKBS), kettlebell rack(KBR) and kettlebell snatch (KBS). The subjects repeated until they can do the correct action. The author measured the six muscles during the kettlebell exercises. Before the start of the study, test-leader was obtained consentof subjects about taking off all clothes except for the underwear to them. Kettlebell (Gymmaster Daeho Ba2 style kettlebell; Gymmaster, Korea) weight was determined by the 3kg. All subjects coached to take the correct posture of kettlebell exercise for accurate measurement. Kettlebell exercises were carried out at random.

In the SHKE, the arm was used with dominant side. The distance between the feet was located to wider than shoulder width of each subject. Start point of the kettlebell exercises was conducted to do by holding the kettlebell in the upper phase, with the help of a test-leader. It was emphasized that the hip extensors should generate the motion in their hips, with knees slightly bent during the whole movement.

Method of kettlebell exercise referred to the previous study.⁷ The participant was cued to initiate the swing through the sagittal plane by simultaneously reducing their hips, knees, ankles and to use the momentum to swing the kettlebell pass by the legs, and return to their initial starting position. The both elbows and both wrists was to be kept straight during the entire swing. Ending position of lower extremity is knee flexion 40°, hip flexion 90°.AKBS was performed in the similar way, but starting point and ending point of swing was shoulder flexion 130°. KBGS was initiated with the subject in a squat position with a holding kettlebell in chest level with both hands, hip flexion 0° and knee flexion 0°. The movement performed in motion like a general squat by holding initiate posture. Ending position is knee flexion 90° and hip flexion 130°. OKBS was performed in the same way as the RKBS, but the movement used only dominant one-arm. KBR was started with the subject in a squat position by holding the kettlebell with the dominant one-arm in the shoulder height. The movement was performed as to lift the burden to shoulder height. Ending position of lower limb is knee flexion 60°, hip flexion 90°. KBS was initiated with the participant in a squat position with a hip flexion 0°, knee flexion 0°, shoulder flexion 180°, elbow flexion 0° and the kettlebell in the dominant one-arm. The subjects were cued to initiated the movement by simultaneously reducing their hips, knees and ankles by anterior snatching the kettlebell, and to use the movement the kettlebell into lower phase with the dominant one-arm. Next, they returned into initiate position by posterior snatching the kettlebell. Ending position of lower limb is hip flexion 90°, knee flexion 60°.

2.3. Electromyographic Measurements:

Before attaching the pad were checked for skin lesions. EMG pad was used a disposable common electrode, distance between the two pad was defined to 2cm. On the basis of previous articles, EMG pad electrode placement was attached in a estimated direction along the muscle fiber of ES, BF, RA, RF, GMAX, GMED. All the electrode arrangement was placed in the dominant arm muscles. In order to minimize the disturbance factors when measuring the activation, the raw EMG signal band pass amplified and filtered using preamplifier located close to the sampling point. The preamplifier had a common mode rejection ratio of 100dB, high cut frequency of 600Hz, and low cut frequency of 8Hz. The EMG signals were converted to root mean squre(RMS) signal. And it analyzed the data saved. After that, the author was recorded in Excel the mean RMS values that it saved, was calculated the percent of MVIC of each muscle according to each kettlebell exercise.

2.4. Statistical Analyses:

A one-way analysis of valance (ANOVA) with repeated measures was used to assess differences in the EMG activity for each of the muscles measured. Analyses of variance with repeated measures and post-hoc analysis for descriptive statistics with LSD corrections were used to assess the hypotheses dealing with the effects of and differences between the different types of kettlebell exercises (RKBS, AKBS, KBGS, OKBS, KBR, KBS) on muscle activation. Statistical analyses were performed with SPSS version 18.0(SPSS Inc., Chicago, IL, USA). All results are presented as mean ± SDs. Statistical significance was accepted at p<0.05.

3. RESULTS AND DISCUSSION:

Comparison for percent of MVIC of muscles according to kettlebell exercises is recorded in the figure 1. First, ES was significant difference statistically during all kettlebell exercises(p<0.05). Post-hoc test result showed significant difference statistically in RKBS-KBR(p<0.01), RKBS-KBS, KBGS-KBR(p<0.01), KBGS-KBS, AKBS-KBR, OKBS-KBR(p<0.01), OKBS-KBS. In BF, there was significant value statistically during all movements(p<0.01). Post-hoc test showed significant value in RKBS-KBGS, RKBS-KBR, RKBS-KBS, OKBS-KBR, OKBS-KBS and AKBS-KBGS(p<0.05), AKBS-KBR(p<0.05), AKBS-KBS. In RA, there was no significant difference statistically when subjects do all exercises. RF showed significant value statistically during all exercises (p<0.01). Post-hoc test result showed significant difference statistically on RKBS-KBGS(p<0.05), AKBS-KBGS(p<0.05), KBGS-OKBS, OKBS-KBR(p<0.05), OKBS-KBS. The GMAX and GMED did not showed significant difference statistically during all performance.

The main purpose of this study was to compare and analyze the muscle activity of the core and lower limb during the THKE and SHKE. This article sought to refer with other previous studies reported in the rank using percent of MVIC about a variety of kettlebell exercise. The second aim was to prescribe appropriate exercise, as necessary to analyze the activity of the muscles according to the kettlebell exercise.

As a result, when do the six kettlebell movements, muscle activity of ES showed significant difference statistically (p<0.05). In the previous study, Anderson et al.(2016) reported that upper erector spinae was more active on contralateral side than ipsilateral side during OKBS with dominant arm, but lower erector spinae has been reported that there is no significant difference]. In other words, there was significant difference between OKBS and upper ES, but lower ES did not significant value, so author considered the need for additional research, this study was to compare and analyze add to the SHKEs and THKEs. Similar to the preceding article,⁸ the correlation between THKEs and OKBS in our study was no significant difference. As the author expected, THKEs was greater than the muscle activity SHKEs. Interestingly, the activity was highest in KBGS like a regular squat.

Second, it was analyzed for the BF muscle. BF activity has increased significantly in the swing movement than the other kettlebell exercises. Because, while knee joint flexes in the swing, lower limb muscles work to perform the squatting motion in the swing specifically through rapid eccentric control of the hamstrings. BF muscle is activated greater than the normal squat only lifting gravity because the action take place by concentric contraction of the hamstrings when you swing the kettlebell upward load on the knee extension movement.⁹ In previous study, Van et al. (2015) conducted both left-armed and right-armed according to 2-armed and 1-armed kettlebell swing, mean time to peak was significant difference.¹⁰ Comparisons of the mean time to peak activation for each muscle showed that the BF was the first muscle to activate from among BF, GMAX, GMED during the swing. As shown in the preceding article, it is not an exaggeration to say that the core muscle that hold the center of the body during the kettlebell swing. During one-armed than swing with two-armed kettlebell swing, faster time to peak was reported to have. But, our study result, BF muscle had the meaning in the comparison between the swing and rest of the movement than the comparison of the movement between one-handed and two-handed. When the hamstring had weakness, the author is recommended to swing the kettlebell by increasing kettlebell tool's load.

Next, the author was analyzed the activity of RA. The muscle had no significant difference statistically (p>0.05). However, RA muscle showed a higher activity in all exercise. In the preceding article, Andersen et al.(2016) showed low activity the muscle in contralateral side compared to ipsilateral side of RA muscle during the OKBS, showed low activity the muscle compared to two-armed kettlebell swing.⁸ The other leading article, Aspe et al.(2014) reported that the overhead squat and back squat, the activity of RA muscle was a significant difference statistically, was reported the high activity of the muscle in anterior trunk muscle(rectus abdominis and external oblique) during the eccentric phase of the overhead squat.⁵ Based on the above article, the author of this study was expected to demonstrate the greatest activation in the KBS that potential energy of hand moving from the highest point during eccentric phase, second, expected to show the greatest activity in the AKBS. As a result, the author as expected, KBS is higher the activity of RA than the RKBS. Generally, compared to the KBS and other kettlebell exercises, it was no significant difference. And, KBGS showed an activity greater than KBS. As a result, rest kettlebell exercise was somewhat more effective compared to kettlebell swing movement about the RA. The author was not recommended as a therapeutic exercise to strengthen only abdominal muscle, but was recommended the kettlebell exercise as movement improving overall balance ability, strength and cooperation with the RA and the surrounding muscles.

Analyzed comparison of activation of the RF during the kettlebell exercises, there was a significant difference statistically (p<0.05). In this study, it showed the low activity of the RF in the swing movement compared with the KBGS. While origin of quadriceps move to the bottom of the front by pelvic anterior tilting during the knee flexes in the squat, RF occurred concentric contraction. According to the preceding article, Marchetti et al.(2016) reported that RF activity was greatest in the knee flexion 90°.¹¹ Similarly, because our study was knee flexion 90° in the ending position during the KBGS and knee flexion 40° in the swing movement, RF activity is the biggest in the KBGS. But overall, RF muscle showed relatively low activity compared to other muscles. The author recommends the KBGS when you try to improve the quadriceps muscle strength. Core and lower limb activations of the two-handed and single-handed kettlebell exercises Finally, the author analyzed about the gluteus muscles. The muscles have been suggested during the kettlebell swing, as the beneficial technology for neuromuscular training. So before this experiment, education about kettlebell movement was included verbal instruction to activate particular in the gluteus muscles. And about gradual proper kettlebell technique performing, provided feedback to remind the proprioceptive sense. According to recent evidence of McGill and Marshall (2012), two-armed kettlebell swing was reported that the mean peak activity of GMAX, GMED is producted higher significantly for healthy people.⁷ In another previous article, Caterisano et al.(2002) reported that GMAX was a significant difference in the concentric phase according to squat depth during the back squat.¹² In our study, activation of GMAX and GMED was no significant difference(p>0.05). But, commonality of the preceding articles and this study showed significantly greater activity. The author recommend all kettlebell exercises in order to promote the gluteal muscle strengthening with enough kettlebell weight and oral instruction for stimulation.

4. CONCLUSION:

The author compared and analyzed activation of ES, BF, RA, RF, GMAX and GMED using the EMG according to two-handed and single-handed kettlebell exercises. As a result, the author recommended the RKBS for patients with the LBP, recommended the KBGS for patients with ES weakness, recommended the kettlebell swing movement for patients with the BF weakness, recommended the KBGS for patients with the quadriceps weakness. In conclusion, the author recommended all kettlebell exercise as movement improving overall balance ability, strength and cooperation with the RA and the surrounding muscles. This study is considered to be affected to increase the use and value of kettlebell exercise in clinical to deal with strength, endurance and LBP.

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Received on 12.12.2017 Modified on 24.12.2017

Research J. Pharm. and Tech. 2018; 11(3): 1101-1106.

DOI: 10.5958/0974-360X.2018.00207.X

Core and Lower limb Activations of the two-handed and Single-Handed Kettlebell Exercises