How Muscles Work & Respond to Resistance Exercise (cont.)

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Where a muscle attaches to the bone closest to the center of the body is called its origin. The insertion of a muscle is where it attaches to bone farthest from the center of the body. The biceps origin is in the scapula of the shoulder, and its insertion is in radius bone of the forearm.

When a muscle contracts or shortens, it pulls on both its origin and insertion in bone and causes the joint to move. To return the joint to its original position, the reciprocal muscle on the other side of the joint must contract and shorten. Muscles don't push joints, they only shorten and pull. It is up to both reciprocal muscle groups to work together to move the body. For instance, your biceps shortens and bends your elbow, while the triceps on the other side of the arm shortens and returns the elbow to its original position. This "reciprocal" synergy between muscle groups is sometimes called the agonist/antagonistic system.

Concentric and eccentric contractions are two types of contractions that you use every time you lift weights. Concentric contractions are when a muscle shortens, and eccentric contractions are when the muscle shortens and lengthens at the same time. It sounds confusing, but here's how it works.

Consider the lat pull-down exercise. Pulling down the bar uses the following muscle groups: biceps, latissimus dorsi, posterior deltoids, and rhomboids. All these muscles located in the back and arms contract and shorten, moving the shoulder and arm. These are concentric contractions.

To return the bar and lower the weight stack requires all those muscles to next lengthen and allow the bar to return to its starting position over your head. You don't just let go and allow the bar to fly up and the weight stack to crash down. Instead, you return the bar slowly by both contracting those muscles and allowing them to lengthen. This is an eccentric contraction, where there is shortening and tension within the muscle associated with lengthening.

Eccentric contractions are also called "negative" work. For example, suppose you lift the final biceps curl of your set with the assistance of your spotter and then lower it slowly on your own. During this lowering, or negative eccentric phase, the biceps is contracting to lower it slowly and prevent the dumbbell from falling, but it's lengthening at the same time to allow your arm to straighten and return to the starting position.

Eccentric contractions can generate more force and strength than concentric contractions. Eccentric contractions can also make your muscles more sore than concentric contractions, probably because of the greater force generated and because of the simultaneous lengthening and shortening of the muscle. Walking down stairs or going downhill is an eccentric stress on the quadriceps muscles of the thigh while going up is concentric. That is why your quads hurt more going downhill.

Skeletal Muscle Control