If you watch runners before a race or baseball, football and other athletes warming up before a game, you will see that most of them do the butt kick (quadriceps) stretch. In this stretch they maintain an erect position, stand on one leg, and bend the other leg so the thigh is pointed directly downward with the knee bent and the heel of the foot close to the buttocks. The athlete then presses the foot closer to the buttocks to get a stronger stretch of the quadriceps. In some cases they even pull the thigh backward for an even strong stretch of the quadriceps and the hip flexors.
This butt kick stretch is constantly recommended in running and fitness magazines and running and sports training books. The popularity of this stretch has even permeated the fitness field where you can see many recreational athletes and bodybuilders doing this stretch. You are likely to have this stretch recommended to you by trainers and coaches.
But, is the butt kick stretch effective for runners and others? The answer is NO! There is no questioning the fact that this stretch is effective for fully stretching the quadriceps muscle. When you pull the heel of the foot close to the buttocks, it not only stretches the quadriceps through the normal range of motion, but beyond. As a result, if this stretch is continued over a long period of time, you will have overstretched knee joint ligaments that are crucial in maintaining joint integrity.
Understand that the knee joint is an unstable joint and requires strength and relative tightness, of muscles, ligaments and tendons to maintain joint stability. Once the support structures are weakened your knee becomes less stable and more prone to injury.
In addition to overstretching the ligaments, you also overextend the joint. If you look closely at the end position when the heel is close to the buttocks you will see that the knee joint is opened up to its maximum to allow the shin to be right next to the thigh. In some cases large calves act as a fulcrum to create even more joint stretching that causes the support structures to become even more overstretched.
Doing this stretch every so often is not dangerous, especially if you also squat or assume a seated squat position for extended periods. However, in the down position in the squat, the muscles are under some tension to maintain your balance and stability. Thus even though the joint and joint structures get stretched out, you still exhibit muscle activity (tension )to hold the knee fairly stable.
But, if you continue to do the butt kick stretch and do not do exercises to strengthen the knee joint after having it stretched out, you may end up with a much looser joint more prone to injury. This is the main reason why the butt kick is not recommended. This quadriceps stretch is also contra-indicated for runners because you never get into this position in running and it will not help you become a better or safer runner.
Some people maintain that sprinters bring the heel close to the buttocks when they run and thus this stretch duplicates what occurs in running. Although this occurs in good runners with effective technique, when the heel is close to the buttocks the knee is already in front of the body and the quadriceps has some slack. The thigh is not pointed straight down as in the butt kick with the quadriceps and it tendon tight around the knee joint. Thus there is less stretching of the joint structures when the knee is bent in effective running.
Some runners are taught to “kick their butt” when running and as a result, attempt to kick the buttocks with the thigh pointed straight down. This actually slows down running speed and often leads to injury. Because of this, this stretch does not help you in your running. It is of such limited value it surprising that runners consider it a not-to-be-left-out stretch.
Much more effective to stretch the quadriceps are active stretches that engage the muscles during the stretch. A great exercise for this is the unweighted squat. When you go into a squat, the muscles stretch eccentrically and develop greater tension the lower you go. This helps to maintain joint integrity and at the same time stretch the quadriceps through a greater range of motion than seen in any form of running, jumping or kicking. Thus not only is the unweighted squat safe but very effective when used as an active stretch.
For more information on active stretching and the butt kick in particular, read Explosive Running, Build a Better Athlete and Women’s Soccer: Using Science to Improve Speed.