Original research
Relationship between hip and core strength and frontal plane alignment during a single leg squat

https://doi.org/10.1016/j.ptsp.2014.05.002Get rights and content

Highlights

  • Core and hip strength is significantly correlated with knee kinematics in females.

  • Hip abductor strength was the best predictor of the Frontal Plane Projection Angle.

  • Clinicians should consider hip strength when assessing single leg squat kinematics.

Abstract

Objective

The purpose of this study was to examine the relationship between frontal plane kinematics of the single leg squat and strength of the trunk and hip in females.

Participants

Forty healthy females participated in this study.

Methods

An isometric “make” test using a dynamometer was used to assess peak force normalized to body weight for hip abduction, hip extension, hip external rotation, and a sidelying plank test. Two-dimensional software was used to analyze the frontal plane projection angle (FPPA) and pelvic angle during a single leg squat to 60°.

Results

All 4 strength factors were significantly correlated with the FPPA, ranging from r = 0.396 to r = 0.466. During multiple regression analysis, hip abduction strength was the greatest predictor of the variation in FPPA at r2 = 0.22, p = 0.002. Thus, hip abduction strength accounted for 22% of the variation in the FPPA during the single leg squat. The only strength factor demonstrating a significant correlation with the pelvic angle was hip extension strength (r = 0.550, p < 0.001).

Conclusion

Clinicians should consider the role of the hip abductors, hip external rotators, hip extensors and core musculature on the impact on the FPPA during a single squat, with focus on the hip abductors.

Introduction

Patellofemoral pain syndrome (PFPS), often described as anterior knee pain involving the patella or retinaculum, has a prevalence of 12–13% in females ages 18–35 (Roush & Curtis Bay, 2012). PFPS is the most common injury among runners, and increased physical activity is a risk factor for PFPS (Oakes, McCandless, & Selfe, 2009). Despite the high prevalence, the exact etiology is still unknown and a multifactorial clinical diagnosis is typically made as other conditions are ruled out. Suggested causes of PFPS include static or dynamic lower extremity malalignment, muscle imbalance, lateral retinacular tightness, cartilage disruption, increased Q-angle, overuse, and abnormal hip mechanics (Cook, Mabry, Reiman, & Hegedus, 2012).

Lower extremity mechanics may be influential in anterior knee pain. Females with PFPS have been found to have greater lateral patellar displacement and medial femoral rotation at varying angles of knee flexion (Souza, Draper, Fredericson, & Powers, 2010). The normal Q-angle of the lower extremity creates a lateral pull on the patella. At 45° of knee flexion, a 10% increase in the Q-angle may increase peak pressure on the patella by 45% because of increased lateral tracking of the patella (Huberti & Hayes, 1984). Also, Souzam and Powers (2009) reported females with PFPS demonstrated greater peak hip internal rotation with running, a drop jump, and step-down than those without PFPS. Willson and Davis (2009), demonstrated greater hip adduction with a single leg jump in females with PFPS. Specific to the SLS, a study by Herrington (2003) reported individuals with PFPS demonstrated a greater FPPA than asymptomatic individuals (16.8 versus 8.4° respectively). Frontal plane angle during a 45° squat in females with and without PFPS revealed individuals with PFPS averaged 11.75° in the direction of knee valgus, while the control group averaged 7.79° (Levinger, Gilleard, & Coleman, 2007). Furthermore, in a prospective study of female athletes, those who developed PFPS had significantly more hip adduction with a single leg squat, suggesting a more proximal cause (Noehren, Hamill, & Davis, 2013).

In addition to mechanics, based on muscle function, it follows logic that several hip muscles would impact both knee and hip mechanics. During activities requiring single leg stance, the hip abductors function to prevent pelvic drop and hip adduction. Hip adduction has been strongly correlated with the frontal plane projection angle (FPPA) of knee valgus (Hollman, Ginos, Kozuchowski, Vaughn, Krause, & Youdas, 2009). Also, the external rotators, including the gluteus maximus, prevent excessive hip internal rotation with dynamic activity (Neumann, 2010). Additionally, a systematic review by Prins and van der Wurff (2009) reported strong evidence for decreased hip external rotator, abductor, and extensor strength for those with PFPS as compared to individuals without PFPS. In those with unilateral PFPS, decreased hip external rotator and abductor strength was apparent compared to the uninvolved limb. Although hip weakness as a cause or a result of PFPS was not established, even weakness as a result of PFPS is relevant to the rehabilitation process. A prospective study by Leetun, Ireland, Willson, Ballantyne, and Davis (2004) found that collegiate athletes who suffered low back or lower extremity injuries had significantly lower hip external rotator and abductor strength compared to those who remained injury free. In fact, among variables studied, hip external rotator strength was the best predictor of injury status.

Since both hip strength and lower extremity kinematics are associated with PFPS, a tool to assess these factors clinically is essential. The single leg squat (SLS) is one tool frequently used in physical therapy for assessment and as a treatment strategy for dynamic hip control and lower extremity kinematics. The SLS has functional relevance in sports to landing, cutting, and running. In fact, a recent study (Whatman, 2011) reported a moderate to strong correlation between peak lower extremity kinematic findings during a single leg partial knee bend and jogging.

As previously mentioned, females with PFPS have been found to have increased peak knee angles with the SLS (Herrington,, Levinger and Gilleard, 2007). Also, Long-Rossi and Salsich (2010) assessed pain with a single leg squat, gluteus medius, gluteus maximus, and hip external rotator strength, and Anterior Knee Pain Questionnaire (AKPQ) score in females with PFPS. There was a positive correlation between hip external rotator strength and the AKPQ score supporting the findings of the Leetun et al. (2004) study. There was a negative correlation between pain with a SLS and external rotator strength but mechanics of the SLS were not assessed.

Several studies with varying methodologies have investigated the link between hip strength and performance during a SLS in healthy individuals, focusing on frontal plane kinematics of the knee (Claiborne et al., 2006, Crossley et al., 2011, Willson et al., 2006). However, as supported by a recent systematic review, (Cashman, 2012) it is somewhat difficult to draw conclusions from these studies as none of these studies included exclusively female participants and the methodologies varied. Also, it is worthwhile to note that trunk movement was not reported in any of the studies. A trunk lean or a pelvic drop can be a compensation for a weak hip abductor (Neumann, 2010).

Willson et al. (2006) was the most comprehensive study in terms of muscle groups; however, this study used a less challenging 45° squat and only 22 females (along with 24 males). Furthermore, FPPA was captured only at the deepest part of the squat via a digital image, and this may not represent the moment in time when FPPA was the greatest. Thus, the limited muscles tested in other studies must be considered. Only one study assessed trunk strength (Crossley et al., 2011), and a subjective analysis of the SLS was used. Also, only one study (Claiborne et al., 2006) included hip extension strength testing. Hollman et al. (2009) revealed gluteus maximus EMG was moderately correlated (r = −0.451) with the FPPA of knee valgus during a single leg step down. This may be due to the function of the gluteus maximus as a powerful external rotator, controlling internal rotation and secondarily, knee valgus. Thus, future studies should include testing of trunk and gluteus maximus strength.

Finally, in light of differences between male and female pelvic structure and strength, gender differences must be considered. Gender differences in performance of the single leg squat have been examined by two studies of “healthy” individuals, but without a consensus. Claiborne et al. (2006) found no significant differences in frontal plane motion between genders. In contrast, when Zeller, McCrory, Kibler, and Uhl (2003) compared kinematics in both men and women, the women had significantly more hip adduction during the task. Although related to sagittal and transverse planes, increased hip flexion and external rotation also were noted in females. Furthermore, a study by Pantano, White, Gilchrist, and Leddy (2005) assessed the relationship between the pelvic width to femoral length ratio to static and dynamic knee valgus. Pelvic width to femoral length ratio was related to static and dynamic knee valgus (r = 0.47 and 0.48, respectively). This may have implications for data analysis since women are more likely to have a greater pelvic width to femoral length ratio. Lower extremity strength differences between females and males also are evident, particularly in hip abduction and flexion, even when normalized to body weight (Claiborne et al., 2006). Thus, in consideration of the different findings in these studies and the greater incidence of PFPS in females, it may be prudent for future studies to examine data between females and males separately to account for potential differences in kinematics and strength.

In summary, it is evident that the relationship between hip and core strength and mechanics of the single leg squat in females is still unclear, yet potentially clinically relevant to the development of PFPS. Thus, the purpose of this study was to examine the relationship between frontal plane kinematics of the single leg squat and trunk and hip strength in females.

Section snippets

Methods

Forty participants were recruited to participate in this study; the sample was one of convenience. Inclusion criteria were females between the ages of 18 and 30 years. Exclusion criteria was any previous low back or lower extremity surgery, low back or lower extremity pain or injury within the last six months, any neurological disorder, or obesity, as defined by a BMI greater than 30. All participants provided informed consent and the project was approved by the Grand Valley State University

Results

Forty healthy females (average age = 22.88 ± 0.32 years) participated in the study. Average characteristics of the participants were as follows: height = 165.5 ± 0.86 cm, weight = 60.36 ± 1.7 kg, body mass index = 22.03 ± 0.45. During the SLS, the average FPPA was −10.78° ± 0.88. At the time of the peak FPPA, the average angle of the pelvis from horizontal was 4.68° ± 0.57. This angle was not able to be calculated on two individuals secondary to the degree of forward trunk flexion during the

Discussion

The purpose of this study was to examine the relationship between frontal plane kinematics of the SLS and trunk and hip strength. In this study, all four strength factors had a significant correlation with the FPPA. As represented by the multiple regression analysis, the hip abductors were the best overall predictor of the FPPA. This is not surprising as the hip abductors function to eccentrically control hip adduction and thus valgus at the knee during a squat (Powers, 2003). Based on the

Conclusion

The relationship between hip strength and the mechanics of a SLS in females is clinically relevant. Clinicians should consider the role of the hip abductors, hip external rotators, hip extensors and core musculature on the impact on the FPPA during a single squat. The focus, however, should lie on the hip abductors as they were the greatest predictor of FPPA. Although the other muscles were significant predictors individually, they did not contribute anything further to the overall equation

Conflict of interest

None declared.

Ethical approval

This study was approved by the Grand Valley State University, Human Research Review Committee, protocol reference #13-065-H.

Photographs in which the person was identifiable were done with written permission.

Funding

None declared.

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