The biceps brachii muscle in the upper arm forms a noticeable bulge during concentric exercises when the elbow is bent and the muscle is shortening in order to move the forearm, according to Sports Injury Clinic. A muscle contraction also occurs when the biceps muscle is relaxing and moving with gravity, but at this point, the muscle is lengthening, and there is no bulge in the biceps area.
There are two types of isotonic exercises that work the biceps brachii muscle, concentric and eccentric. Concentric exercise forces the muscle to move against gravity, as happens in a biceps curl. Eccentric contraction occurs when the muscle must control the downward pull of the arm caused by gravity or weights.
Eccentric exercises are more challenging than the concentric type, explains the University of California at San Diego on its muscle physiology site. One example of the greater capacity of an eccentric exercise is the fact that humans are able to lower a much heavier load than they can lift. Eccentric contractions, although they do not produce the bulging biceps associated with masculinity and strength, may be the best exercises for strengthening a muscle. However, research has shown that eccentric contractions are more likely to lead to soreness and injury, according to Muscle Physiology.