Effects of various plank exercises on activation of hamstring muscle

Ⅰ. Introduction

Plank exercise (PE) resist gravity through activity of the forearms and feet while the subject is in a prone position (Kang et al., 2016; Lee et al., 2016). These exercises stabilize the spinal column and provide stability in the pelvic and trunk regions when the trunk is aligned (Snarr and Esco, 2014). Compared to other body stability exercises such as sit-ups, PEs are performed without excessive lumbar flexion and do not cause posterior disc herniation. For these reasons, PEs are commonly considered useful and safe exercises to promote body stabilization (McGill, 2007; Peterson, 2013).

A typical PE is performed in supine position on the floor, keeping the legs and spine straight while supporting the body with the elbows and toes. This posture strengthens the core, internal oblique (IO), and external oblique (EO) muscles (Snarr and Esco, 2014), reduces the incidence of lumbar injuries, and enhances performance activity (Cho, 2010; Nadler et al., 2002). Variations on the typical PE have attempted to enhance its core muscle strengthening effects; for example, PE on an unstable supporting surface further increases core muscle activity, and the incorporation of slings promotes greater core muscle activity than typical PE (Kang et al., 2016; Lee et al., 2016, Han and Son, 2019). However, because PE requires that the hip joint be kept straight, they can activate the gluteus maximus and hamstring (HAM) (An and Kim, 2017). Although strengthening the HAM can enhance the stability of the hip and knee joints, its excessive activation may induce tightness, which can cause low back pain through posterior pelvic tilt and excessive lumbar flexion (Neumann, 2013, Cho et al., 2019). Moreover, PE on unstable supporting surfaces can increase lower limb muscle activity, leading to excessive HAM activation. Therefore, it is necessary to study PE postures that can minimize HAM activity while maximizing core muscle activity.

Ⅱ. Method

1. Subjects

The subjects of this study were 30 healthy adults. All subjects voluntarily agreed to participate in the experiments after listening to an explanation of the purpose and procedure of this study, provided written consent. Subjects who had received no lumbar treatment within 6 months prior to the study, had no musculoskeletal or neurological disorders, and had no low back pain were included in this study, whereas those who did not consent to the experiment were excluded <Table 1>.

Table 1.

Characteristics of subjects(N=30)

Ⅲ. Results

The RA, EO, and ES muscles showed the highest muscular activity when PEB was performed and the lowest activity when PES were used; however, there were no significant differences. The HAM showed lower activity during PES than during PEB or PE; however, no significant differences were detected <Table 2>.

Table 2.

Comparisons of trunk muscles and hamstring muscle activation between normal length of hamstring group and shortened length of hamstring group

Ⅳ. Discussion

In a previous study, a PE performed using a Swiss ball showed a significant increase in the thicknesses of the RA, EO, and ES muscles (Shin, 2014). PE performed in prone position significantly increased RA, EO, IO, and ES muscle activity as the degree of difficulty increased. Core muscle activity further increased to maintain trunk stability against an unstable supporting surface. In the current study, RA, EO, and ES muscle activity increased more during PEB than during PE; however, no significant differences were detected.

Although previous studies have reported that PES significantly increased EO, RA, and ES muscle activity compared with PE (Lee et al., 2016), the current study detected decreases in the activity of all three muscles. HAM activation was greatest during PE and lowest during PES. In previous studies, exercises were performed with the subject extending the hip and knee joint(Kang et al., 2016; Kim et al., 2016). Knee flexion during exercise may have contributed to the different results obtained in the current study, reducing the external moment arm from the center of the body, and providing less load to the core muscles during PES than during PE. To decrease HAM activity, we asked subjects to place slings under their femurs during knee flexion at insufficient activity; however, this PE modification did not significantly reduce HAM activity.

This study was limited by the small number of participants, making it difficult to compare our results with those of other studies. Future studies should increase the number of subjects and establish multiple treatment groups to allow more meaningful comparisons. In addition, HAM activity should be determined while subjects perform PEs with knee joints extended and a sling placed at the ankles to examine differences when measurements are conducted with the knee joint flexed.

Ⅴ. Conclusion

In this study, we asked subjects to perform PEs using slings and with their knees flexed to identify a PE that could minimize HAM activity while maximizing core muscle activity; however, the exercise did not significantly affect either activity. Therefore, it is necessary to conduct further research on changes in HAM activity during various types of PE under different conditions.

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