Have you ever wondered what the difference between doing your donkey kicks with the knee bent vs straight is? Well, you're not alone! . The goal of an exercise like this is to target the HIP EXTENSORS, specifically the gluteal and hamstring muscles. Both muscles cross the hip joint and extend the hip; however, the hamstring muscles ALSO cross the knee joint and FLEX THE KNEE. Thus, by altering the position of the knee, you can preferentially bias either the hamstrings or glutes. . ➡️Knee Bent = Increased Gluteus Maximus activation ➡️Knee Straight = Increased Hamstring activation . When we bend the knee, shout out to Jon Snow all my GOT friends, we shorten the length of the hamstrings, which reduces its ability to generate force. By reducing the ability of the hamstrings to generate force, we preferentially bias the gluteus maximus over hamstrings with the knee bent. This happens for two reasons. . 1️⃣When the fascia or connective tissue surrounding the muscles is lengthened, it creates passive tension which helps aid in generating force during active contractions. So when the muscle is shortened, it looses this passive tension. . 2️⃣All muscles have an optimal length at which they produce the most tension, as described by the sliding filament model. With the hamstrings on slack, there is LESS overlap between fibers in a phenomenon called active insufficiency. Think about how hard it is to make a fist if you bend your wrist forward first - try it. That's active insufficiency - the inability for a muscle to produce it's maximal torque at shortened ranges. . So you know, Kwon et al 2013 found that a knee flexion angle less than 60 deg was required to preferentially bias the glute max! We hope you enjoyed this science lesson!
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