Physical Therapy Korea

Squats are used to increase lower-body muscle strength, and are among the most common exercises performed by the general public and athletes (Escamilla et al, 2001;,Fry et al, 2003;,Mccurdy et al, 2005). Among closed kinetic chain exercises, squats particularly activate the knee extensors and hip extensors, and thus are effective for stabilizing the hip and knee joints (Earl et al, 2001;,Stiene et al, 1996). Squat exercises can vary in depth (partial squats: 40˚ knee angle, half squats: 70 to 100˚, and deep squats: greater than 100˚), as well as in the intensity of load, foot placement and position (conventional squats, single-leg squats and wall squats).

The conventional squats can increase the patellofemoral compression force and stress (Escamilla et al, 2009) as well as lumbar flexion (Noyes et al, 1984;,Paoli et al, 2009;,Schoenfeld, 2010). They may also increase the risk of muscle ligamentous sprains, disc herniation and spondylolysis (Matsumoto et al, 2001;,Vakos et al, 1994). However, wall squats can reduce these risks. Wall squat exercise involves sliding up and down against a wall. Because the wall squat is performed with the bodyweight on the wall, it is easy to perform this exercise as a beginner and to control the patellofemoral compression force and stress. The wall squat also has the advantage of preventing excessive lumbar flexion and extension.

Kim (2012) reported the activity of the quadriceps muscle during wall squats [vastus medialis obliquus: 87.2% of maximal voluntary isometric contraction (MVIC); rectus femoris: 52.3% of MVIC; and vastus lateralis: 89.3% of MVIC]. The average gluteus maximus muscle activity was 17.6% of the MVIC in males and 20.4% of the MVIC in females during wall squats (Bolgla et al, 2014). In the previous study, the majority of the studies on electromyography (EMG) during walls squat were conducted. Schoenfeld (2010) reported the importance of lower extremity strength for squat exercises and Kim et al (2015) reported the relationship between squat depth and lower extremity strength and range of motion. However, the relationship of wall squat performance with knee extensor and hip extensor strength remained unclear. Thus, the purpose of this study was to compare the strengths of the knee extensors and hip extensors between groups with low and high wall squat performance. We hypothesized that the knee extensor and hip extensor strength would be different depending on ability of wall sqaut performace.

Subjects

32 healthy male subjects, who volunteered in this study from among students at Yonsei University, Wonju Campus, were asked to perform wall squat as much as possible. The average wall squat count of 32 subjects was 6.9 times. In order to classify the group according to the performance of wall squat the high and low groups were classified based on completing the total average number. The subjects who were performing less than 30% (less than or equal to 4 times) of the average wall squat count were classified into the low wall squat performance group and the subjects who performed more than 30% (greater than or equal to 8 times) of the average wall squat count were classified into the high wall squat performance group. Therefore, 19 male students (low performance group: 9 subjects, high performance group: 10 subjects) participated in this study. Inclusion criteria included being healthy young male (20∼35 years) for homogeneity and the absence of pain in any part of the body at the time of wall squat. Exclusion criteria included the presence of pain in the back, knee and ankle region and a previous history of back, knee and ankle surgery. The mean height and weight of the subjects was 168.9±7.3 ㎝ and 66.4±14.4 ㎏, respectively. The study was approved by the Institutional Review Board of Yonsei University (approval number: 1041849- 201603-BM-007-02).

Wall squat

The subjects were instructed about wall squat exercise and had a familiarization session of 5 minutes prior to evaluation session. For the wall squat exercise, the subjects stood against a wall with their legs shoulder-width apart, with 135-degree knee flexion and a neutral ankle position. The pelvis and the lumbar spine were in the neutral position and the hands were crossed over the chest (Figure 1A, 1B). The wall squat exercise used a modified Cho (2013) method to accurately control the posture and time. During the wall squat motion, the knees were flexed to an 80-degree angle (Figure 1C, 1D) from the 135-degree starting position, without heel elevation, during a 5-second descent; the full-squat position was held for 5 seconds; and the knees were extended back to a 135-degree angle (Figure 1A) during a 5-second ascent. This was counted as one cycle of a wall squat. We used a mobile phone application (Clinometer level and slope finder, Plaincode Software Solutions, Stephanskirchen, Germany) to measure the start (Figure 1B) and full-squat positions (Figure 1D), and placed a target bar just below the ischial tuberosity at the end position (Figure 1D). The Metronome Beats application (Metronome Beats, Stonekick, London, England) was used to control the descent, full-squat maintenance and ascent times.

Figure 1.

Wall squat (A: initial position in frontal plane, B: initial position in sagittal plane, C: full squat position in frontal plane, D: full squat position in sagittal plane).

Strength measure

Knee extensor and hip extensor strength were measured on a SMARTKEMA strength system with a load cell sensor (Relive Inc., Gimhae, Korea). For the measurement of knee extensor strength, the subject was seated at the edge of the therapeutic table, with 90-degree knee flexion and 90-degree hip flexion. The subject was told to stabilize himself by holding the side of table with his hands and maintaining an upright sitting posture. A glass suction cup on the floor was attached to a restraining belt, which was tied to the subject’s ankle to limit the vertical direction of knee extension. The length of the belt was adjusted so that the subject could extend the knee 45 degrees (Figure 2A). The subject performed isometric movements by extending the ankle against the strap anchored by the glass suction cup and maintaining the maximum voluntary isometric contraction for 5 seconds.

Figure 2.

Strength measure method (A: knee extensor strength measure method, B: hip extensor strength measure method).

For the measurement of hip extensor strength, the subject lay on the table in a prone position with knee flexion while his leg was slightly off the side of the table. A thigh strap was tied to the femur, 2 ㎝ above the popliteal fossa. The length of the belt was adjusted so that subject’s leg would be in the position of 5-degree hip extension (Figure 2B). The examiner stabilized the rotation of the lumbar vertebra during hip extension. The subject exerted a 5-second maximum voluntary isometric contraction twice against the strap. The mean values of the middle three seconds were averaged to generate the mean MVIC values.

Because of strength of lower extremity is influenced by bodyweight, the measured strength was normalized to bodyweight according to previous study (Claiborne et al, 2006) to yield the strengthto- bodyweight ratio (㎏f/㎏). And composite values that is strength of the knee extensor added strength of hip extensor were also analyzed because two muscles are working as synergists to descending for eccentric contraction and ascending for concentric contraction during the wall squat. The intra-rater reliability of the strength measurements was calculated with data from two trials per subject. An intra- class correlation coefficient (ICC) [3,1] model and 95% confidence intervals (CI) were used to evaluate the intra-rater reliability of each strength measurement and strength-to-bodyweight ratio. The strength measurement during knee extension demonstrated excellent intra-rater reliability [ICC (3,1)=.97, 95% CI: .931, .987], and the intra-rater reliability was also good for the hip extension measurement [ICC (3,1)=.94, 95% CI: .872, .976].

Statistical analysis

Statistical analyses were conducted with SPSS ver. 22.0 (SPSS Inc., Chicago, IL, USA). The Kolmogorov-Smirnov test was used to confirm that the data were normally distributed. An independent t-test was used to compare the strengths of the knee extensors and hip extensors between the low and high wall squat performance groups. A value of p<.05 was taken to indicate statistical significance.

Results of all outcome measures are summarized in Table 2. The ratios of knee extensor and hip extensor strength to bodyweight differed significantly between the low and high wall squat performance groups (knee extensors: p<.001; hip extensors: p=.03) (Figure 3). And the ratios of composite value to bodyweight differed significantly between the low and high wall squat performance groups (p<.01). However, no significant difference was observed in the strengths of the knee extensors and hip extensors between the low and high wall squat performance groups (knee extensors: p=.39; hip extensors: p=.06).

Figure 3.

Knee extensor and hip extensor strengths and strength-to-bodyweight ratios (KE: knee extensor strength, KE/w: knee extensor strength-to-bodyweight ratio, HE: hip extensor strength, HE/w: hip extensor strength-to-bodyweight ratio, Com: composite values; KE+HE, Com/w: composite valueto- bodyweight ratio; (KE+HE)/㎏, ^*p<.05, ^**p<.001).

We investigated the differences in knee extensor and hip extensor muscle strengths between low and high wall squat performance groups. Previous studies have investigated the association between the squat 1 repetition maximum (RM) and squat performance (Bazyler et al, 2014) or studied muscle activity during wall squats (Ayotte et al, 2007;,Boren et al, 2011;,Escamilla et al, 2009;,Kim, 2012), but there has been insufficient research regarding the differences in muscle strengths between low and high wall squat performance groups. This study demonstrated that there was no significant difference in knee extensor or hip extensor strength between the low and high wall squat performance groups; however, the high wall squat performance group had higher bodyweight- normalized knee extensor and hip extensor strengths than the low wall squat performance group.

The wall squat exercise involves descending to a full-squat position (80-degree knee flexion) and ascending to the initial position (135-degree knee flexion). Previous studies (Cheron et al, 1997;,Dionisio et al, 2008;,Escamilla et al., 1998;,Flanagan et al, 2003) have shown that quadriceps activation is high in the descending section of the squat. Kim (2012) reported the activation of the vastus medialis obliquus (87.21% of MVIC), rectus femoris (52.34% of MVIC) and vastus lateralis (89.37% of MVIC) during wall squats. In the ascending section of the squat, activation of the GM was 56-86% of the MVIC (Ayotte et al, 2007). Therefore, for successful squat performance, knee extensor and a hip extensor strength will be required. In this study, the differences in the knee extensor and hip extensor strength-to-bodyweight ratios between the low and high wall squat performance groups were significant (knee extensors: p=.002, hip extensors: p=.038). The wall squat is a closed kinetic chain exercise that combines various movements, including knee extension and hip extension (Dionisio et al, 2008). Although the individual strengths of the knee extensors and hip extensors depending on weight are important, the combined strength of the two is also important for successful squat performance, because it is difficult to perform a squat successfully with only one of these strengths. Thus, composite strength values that combine knee extensor and hip extensor strength should also be considered. In this study, there was a significant difference in the normalized composite value by weight between the two groups (p=.003).

We found no significant difference in knee extensor or hip extensor strength between the low and high wall squat performance groups (knee extensors: p=.393, hip extensors: p=.066). However, there were significant differences in the knee extensor and hip extensor strength-to-bodyweight ratios between the low and high wall squat performance groups (knee extensors: p=.002, hip extensors: p=.038). Claiborne et al (2006) also normalized strength to bodyweight. Although the weight-bearing is less than conventional squat, the wall squat also was a weight-bearing exercise. Therefore knee extensor and hip extensor strengths have limitations to explain the dif ferences in muscle strength between the low and hig h wall squat performance groups before the values are normalized by weight. Clinically, when the lower extremities are strengthened for wall squat performance, exercise programs should be designed with knee extensor and hip extensor strength normalized by weight, rather than considering only the knee extensor and hip extensor strengths.

There were several limitations to the current study. Firstly, we only considered the knee extensors and hip extensors, and did not investigate the activities of other muscles that also may have affected ankle control, such as the ankle dorsiflexors and ankle plantarflexors. Secondly, we did not investigate hip, knee and ankle kinematics during the wall squats, although the joint angles were controlled. Future studies need to be conducted to evaluate hip, knee and ankle kinematics during wall squats for the purpose of investigating asymmetrical movement. Thirdly, only healthy and relatively young subjects (21∼32 years old) were recruited for our study. Thus, our findings cannot be generalized to individuals with lumbar dysfunction or to other patient populations. And this study was a cross-sectional study. Muscle strength was improved by exercise over 4 weeks, and in a study of Bazyler et al (2014), a squat exercise for 8 weeks or more was reported to improve RM. Therefore future studies need to be evaluated to long term effects of wall squat exercise.

This is the first study to investigate and compare the strengths of the knee extensors and hip extensors between low and high wall squat performance groups. The knee extensor and hip extensor strength-to-bodyweight ratios were greater in the high wall squat performance group than in the low wall squat performance group. The results suggest that knee extensor and hip extensor strength are needed for high wall squat performance.