Leg curls are a great accessory exercise for the hamstrings, and unlike their counter-movement – the leg extension – provide no shear forces to the knee. Though leg curls by themselves will not develop the full strength potential of the hamstrings, they are an excellent beginning movement, and are also effective in helping to prevent hamstring pulls during running and sprinting.
The lying leg curl is a single joint exercise that improves the strength of the posterior leg muscles, collectively known as the hamstrings. The semimembranosus, semitendinosus, and the biceps femoris make up the hamstrings. The lying leg curl can be performed bilaterally or unilaterally. This particular version of the exercise is performed bilaterally, and uses a special technique to further overload the hamstrings by including the gastrocnemius muscle in the concentric portion, and eliminating it from the eccentric portion. This type of technique is an example of eccentric overloading.
Eccentric contractions are muscular contractions that occur when a weight is lowered. Eccentric overloading is method to improve strength faster.
To employ eccentric overloading in a strength training exercise, one must change the lowering phase to make it more difficult to perform. In this particular exercise, you use the gastrocnemius, which can act as a partial flexor of the knee, on the concentric contraction. Doing so helps you lift more weight.
When you reach the top of the range of motion, you eliminate the gastrocnemius from the exercise by pointing your toes. As a result, the hamstrings are forced to work harder in the descent. This technique is an example of building the strength deficit.
Anyone who has ever trained with weights knows that lifting a given load is harder than lowering one. This fact exists because of strength deficit, which means that we are stronger in lowering than we are in lifting. Typical, balanced strength deficit is 25%, meaning that if you can lift 100lbs in a maximal effort, you will be able to lower 125lbs in the same maximal effort.
To begin the exercise, lie facedown on leg curl machine as shown. Make sure that the axis of rotation of the machine – the pivot point of the machine – matches up with your own axis of rotation, which is your knee joint. Be sure to adjust the pad of the leg curl so that it rests on the leg just below the gastrocnemius, on the Achilles tendon. Keeping your toes straight, flex the ankles, and inhale.
As you exhale, forcefully flex the knees so that the pad on your legs moves toward your buttocks.
Leg Curls have what is referred to as a descending strength curve, meaning that, as you lift the weight to the top of the range of motion, your mechanical advantage diminishes, making the weight feel as though it is getting heavier. While a properly designed leg curl will eliminate most of this issue, you will likely feel that the top is difficult to reach. Squeeze the hamstrings fully so that you reach your full range of motion. Be sure to keep the toes straight, as they will likely want to fly outward as you go upward.
As you reach the top of the movement and finish your exhalation, allow a natural breath cycle to take place, and begin inhaling as you point your toes and lower the weight. Do not skip out on the transition of the ankles at this point; it is crucial to removing the gastrocnemius from the exercise and eccentrically loading the hamstrings. Your goal during the descent is to lower the weight at a fixed speed throughout, rather than dropping the first few degrees of range of motion quickly, and then catching the weight lower in the range where it is easier to control.
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