The Effect of Hip Joint Position and Velocity on Quadriceps Peak Torque in Children: An Analysis of Varying Knee Flexion Angles

Background:  The hip and knee joints are crucial for movement and are subjected to stress while walking, running, and jumping. The quadriceps muscles play an essential role in knee extension and peak torque production.
Objectives: This study aimed to investigate the relationship between the knee extension peak torque of the quadriceps muscle and the hip angle at different knee angles and different angular velocities while performing concentric knee extension.
Patients and methods:In this cross-sectional study, forty typically developed children of both genders with mean age, weight, and height of 10.22 ± 1.41 years, 40.12 ± 7.97 kg, and 140.65 ± 6.56 cm participated. The inclusion criteria for the study were children who could understand and follow instructions during the testing procedure. The study tested the knee extension peak torque at different knee angles, hip angles, and angular knee velocities.
Results: The results showed no significant difference in knee extensors’ peak torque between supine and sitting positions at 180⁰/s but a significant increase in the sitting position at 60⁰/s. However, a significant increase was found in knee extensors’ peak torque at 60⁰/s compared to 180⁰/s in both supine and sitting positions (p < 0.001). In the supine position, the mean knee extensor peak torque was 36.45 ± 20.66 Nm at 60⁰/s and 40.1 ± 22.17 Nm at 60⁰/sitting position. Meanwhile, the mean knee extensor peak torque was 20.95 ± 14.09 Nm at 180⁰/s in the supine position and 22.05 ± 15.2 Nm at 180⁰/s in a sitting position. The hip angle, velocity, and knee angle all affected the muscle strength with a significant interaction between the hip angle and velocity. Also, there was a significant increase in knee extensors torque in supine and sitting positions with knee 30⁰, 50⁰ and 70⁰ at 60⁰/s  (p=0.001) compared with that at 180⁰/s, while with knee 90⁰, there was increase in knee extensors torque in sitting position only at 60⁰/s compared with that at 180⁰/s (p = 0.002).
Results demonstrated that the knee extensor torque is higher in the slow speed rather than the high speed. Furthermore, at the slow speed (60°/s) across all the knee angles it was found that the extensor knee torque is higher at the sitting position compared to supine lying. Additionally, a significant increase was observed when the knee angle increases from 30° to 70° while it declines when the knee angle reaches 90° in both supine and sitting positions.
Moreover, across all the knee angles, the knee extensor torque is higher at slow speed compared to the fast speed regardless of the hip position. 
Conclusion: The study provides insight into the impact of hip and knee position on the knee extensor peak torque production, which has an important effect on rehabilitation. The relationship between knee angle, velocity, and torque is an important area of study that can modify the development of rehabilitation and exercise programs to improve muscle performance.