Effect of knee flexion angle on infrapatellar fat pad hemoglobin during isometric quadriceps exercise

[Purpose] This study aimed to elucidate the optimal knee flexion angles and contraction intensities for isometric quadriceps exercises to enhance the hemoglobin concentration in the infrapatellar fat pad. [Participants and Methods] This observational, cross-sectional study included 13 healthy young adults. Oxygenated hemoglobin in the infrapatellar fat pad was measured using near-infrared spectroscopy, and the change was calculated by subtracting the value after isometric quadriceps exercise from the value during exercise. Infrapatellar fat pad hardness was measured using shear wave elastography with ultrasonography, and the change was calculated by subtracting the value at rest from that during isometric quadriceps exercises. The changes were assessed under six combinations of knee flexion angle and isometric quadriceps exercise intensity based on maximum voluntary contraction (5°-10%, 5°-50%, 5°-70%, 30°-10%, 30°-50%, and 30°-70%), and were analyzed using a linear mixed model for angles and contraction intensities. [Results] Changes in oxygenated hemoglobin concentration and infrapatellar fat pad hardness at 50% and 70% intensities were significantly higher at 5° knee flexion than at 30°. At 5° of knee flexion, the change in infrapatellar fat pad hardness increased significantly with exercise intensity. [Conclusion] Isometric quadriceps exercise at 5° of knee flexion significantly increased the oxygenated hemoglobin concentration in the infrapatellar fat pad.