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The straight leg raise exercise can be done from either a lying or standing position. Either way, it develops a muscle at the front of your hips. To determine what muscle is responsible for lowering the leg back down to the floor, you've got to understand the difference between a concentric and eccentric contraction.
During a lying or standing leg lift, your hip performs flexion, which means that your upper thigh is lifted up forward and toward your chest. According to ExRx.net, the iliopsoas muscle is responsible for handling this movement. The iliopsoas is a muscle group composed of the illiacus and psoas. The iliopsoas is the largest and strongest of the hip flexor muscle group.
A synergist muscle is one that contributes to a movement, assisting the primary muscle. During the leg lift exercise, the other hip flexor muscles serve as synergists. The hip flexor muscle group consists of the tensor fasciae latae, rectus femoris, sartorius, pectineus, adductor longus and adductor brevis. All of these muscles are located near the front of your hip joint.
Types of Contractions
During the lying leg lift exercise, as you lift your leg up off the floor, your iliopsoas and other hip flexors are performing a concentric contraction. This means that they contract and shorten, causing your leg to be lifted up to the ceiling. However, as you lower your leg back down to the floor, it's still your iliopsoas and hip flexor muscles that are handling the work. Gravity pulls down on your leg, so your hip flexors contract eccentrically to control your leg to the floor and prevent it from slamming into the ground. When a muscle eccentrically contracts, that means that it contracts while it lengthens.
When a muscle is contracting eccentrically, it doesn't require the recruitment of as many muscle fibers. As a result, your muscles are able to handle more resistance during the eccentric component of exercise. This is why during the leg lift, lowering your leg down to the floor feels easier than when you pick your leg up. The eccentric component of the leg lift can be beneficial for those who have injured one of their hips and are yet able to do rehabilitation. There's evidence that completing eccentric contractions on one limb can develop strength in the limb that's not being used. Therefore, the leg lift exercise on one leg has the capability of neuromuscular development and thus strength improvements in the injured leg.