The relationship of knee flexor and extensor muscle strength and tightness with squat performance in children with cerebral palsy

The rectus femoris and hamstring muscles show increased muscle activation during squats to maintain knee stabilization (1,2). Due to the multi-joint nature of the squat activity, not only lower extremity muscle strength and mobility of the hip, knee, and ankle were required, but also muscle flexibility is an important factor (3). Therefore, muscle weakness and tightness of the lower extremity may limit squat performance in children with CP. Is there a relationship between squat biomechanics and knee flexor-extensor muscle strength and tightness in children with CP? 11 children with spastic CP (GMFCS Level I-II; 6 males; 8 bilateral, 3 unilateral; age: 11.27±3.95 y.o., weight: 36.87±12.35 kg, height: 140.14±13.47 cm) and 11 healthy peers (12.10±3.41 y.o., weight: 40.90±11.62 kg, height: 143.13±15.01 cm) were included in the study. Squat kinetics and kinematics were evaluated with a computerized 3D gait analysis system (VICON, 6xVantage 5, 2xAMTI force plates). Participants were asked to perform a 2-legged squat while positioned with their feet shoulder-width apart on two separate force platforms. Evaluation of the muscle strength around the knee was measured with a hand-held dynamometer (Lafayette, US) in a sitting position with 60 degrees of hip flexion and 30 degrees of knee flexion, with the dynamometer placed at 75% of the segment length (4). Knee flexors muscle shortness was measured with popliteal angle (5) while knee extensors’ was with the Duncan-Ely test (6). All interested squat parameters were statistically compared with Pearson’s correlation coefficient (Cohen d)(p<0.05)(7). Peak knee flexion, sagittal plane hip, and knee ranges were strongly correlated with popliteal angle and Duncan-Ely angle (p<0.05). The knee extensor/flexor muscle strength ratio had a strong correlation with the hip (p=0.002, r=0.554) and knee (p=0.001, r=0.559) ascending time from squat. All interested correlation results were given in Table-1.Download : Download high-res image (339KB)Download : Download full-size image As expected, sagittal plane hip and knee ranges were highly correlated with both knee flexor and extensor muscle strength and tightness during squat (6). Peak knee flexion angle, which is the key component of squat activity (2), was highly correlated with both knee flexor and extensor tightness. Accordingly, knee extensor muscle strength and flexibility may have a major effect on squat performance. Squat ascending time at knee and hip level was highly associated with knee extensor/flexor muscle strength ratio which supports that squat performance predicts functional strength of the lower extremity (8). Therefore, studies examining the relationship between muscle activation patterns, muscle architecture parameters, and squat performance may be designed in the future.