The leg extension is a single-joint exercise that works your quadricep muscles on the front of your thigh. Three different types of muscle contractions occur during this basic leg exercise. Although the focus is typically on the concentric contraction when you are lifting the weight, your quadricep muscles are contracting during other phases of the movement.
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The leg extension exercise involves extending and bending the knee. The movement does not occur at any other joint. Holding your thigh still, you extend your knee against resistance then lower the weight and repeat. The basic movement is the same whether you do the exercise on a leg extension machine, cable machine or with an exercise band or dumbbell as the resistance.
The concentric contraction occurs during the lifting phase of the leg extension exercise. The quadriceps muscles shorten when they have enough force to overcome the resistance. At its most basic level, a muscle consists of small pieces of fiber called sarcomeres, which are stacked end to end. Actin and myosin are projections on the ends of the sarcomere. When you extend your knee during the leg extension exercise, the myosin and actin grab onto one another, sliding over each other, resulting in the shortening of your quadriceps.
An isometric contraction does not involve any significant movement of the muscle; this type of contraction occurs when your muscles exert equal force against the resistance. During the leg extension exercise, your quadricep muscles isometrically contract at the top of the movement. After you extend your knee, hold your leg straight for a count before lowering the weight. Although your quadriceps are not actively shortening, they are contracting to hold the weight with your knee fully extended.
The eccentric contraction is the most neglected type of muscle contraction. An eccentric contraction occurs when your muscles do not exert enough force against the resistance, so the muscle lengthens. During the leg extension exercise, your quadriceps muscles eccentrically contract as you lower the weight. Many lifters negate this part of the exercise and simply drop the weight, allowing gravity to pull it down. However, eccentric contractions may increase muscle strength, so you should lower the weight slowly and with control.