Flexion and extension of the legs, or other parts of the body, involve two different joint movements. Thigh flexion, more commonly referred to as hip flexion, occurs when a exercise’s movement decreases the angle between the thigh and hip. In contrast, an extension exercise increases the joint’s initial angle. Flexion and extension exercises involving the same joint often target different muscles.
Thigh Flexion
In thigh flexion exercises, your body typically begins in a straight or nearly straight position, whether you stand erect or lie down. You either move one or both thighs toward the upper body, or bend the upper body down toward the thighs. Depending on the exact movement, and whether you’re holding a weight, thigh flexion exercises can work a variety of muscles. A barbell front squat, for example, work muscles from your shoulders to your ankles. But you can count on getting a good quadriceps workout from a thigh flexion exercise.
Leg Extension
Leg extensions involve a variety of exercises, depending on whether the hip or knee is being extended. In hip extensions, for example, you might bend over for the starting position, then rise to an erect posture. These exercises most often target the hamstring muscles, though quad muscles may come into play as stabilizers. Knee extensions begin with the knee bent. You then extend the leg by straightening the knee. Extension exercises are likely to target the quads.
Flexion Exercises
Squats and lunges -- including their variations -- are common exercises that flex the hips. The targeted muscles vary, depending on the type of squat or lunge you’re performing. Body-weight front lunges and barbell squats, for example, target the quads, other front thigh muscles and the glutes. Front squats target the quads but also work the deltoids in your shoulders, a variety of back and core muscles, as well as the gastrocnemius in your calves. Leg raises can flex the hips from lying, standing, seated or hanging positions and generally target inner or front thigh muscles such as the ilopsoas. Situps are among the hip flexion exercises in which the upper body moves toward the thighs, rather than vice versa. Don’t confuse situps with crunches, in which only the upper torso moves, so the hips don’t flex.
Extension Exercises
Deadlifts and good mornings are examples of exercises in which you begin bent over, with your upper body about parallel with the floor, then rise to an erect position while lifting a weight. These exercises target the hamstrings but also work the glutes and several back muscles, while the quads serve as stabilizers. Knee extensions often begin in a seated position with the knees bent around 90 degrees. You then lifts your lower legs to straighten the knees. These exercises target the quads. Leg presses are also extension exercises that target the quads, although other thigh and calf muscles -- including the hamstrings -- are stabilizers. Knee extensions can also be performed while standing.
