Effects of knee angle on lower extremity extension force and activation time characteristics of selected thigh muscles

The aim of the present study was to examine the effect of knee joint angles on maximal voluntary contraction (MVC) of leg extensor and electromyographic (EMG) activities of rectus femoris (RF), vastus medialis (VM), and biceps femoris (BF) during a maximal isometric leg press task. Eighteen males, aged 18-23, performed maximum bilateral voluntary isometric contractions at five knee joint angles (90 ◦ ,8 0 ◦ ,7 0 ◦ ,6 0 ◦ , and 50 ◦ ;0 ◦ = full extension). The MVC of leg extensors, the force exerted during the first 100 ms of the contraction (F100), the time achieved the MVC, and the average integrated EMG activities (AEMG) of RF, VM, and BF were measured at five knee joint angles. MVC was significantly higher ( p< 0.05 )a t 60 ◦ in comparison to the other knee joint angles, whereas AEMGs of RF and VM were significantly lower ( p< 0.05) at more extended position (50 ◦ ) than at more flexed positions (90 ◦ ,8 0 ◦ , and 70 ◦ ). AEMG of BF did not present significant changes among the five different knee joint angles. These findings suggest that MVC of leg extensor are considerably influenced by the mechanical force-length properties of skeletal muscle. The reduced activation of RF at more extended knee positions indicates its multiple function along a closed chain multi-joint task. On contrary, the similar activation of biceps femoris muscle at all five knee joint angles indicates its importance for the knee joint stability.