As you go about your daily routine, your muscles are hard at work orchestrating the movements that are necessary to complete physical tasks. The human body contains three kinds of muscle — striated, cardiac and skeletal. When you lift a dumbbell, striated skeletal muscle contractions initiate the movement.
When muscles are at work, they generate force. Produced by contractions, this force occurs whether the muscles are shortening or lengthening. According to Michigan State University, the capability of muscles to generate force is termed “muscle strength,” while “muscle endurance” is defined by the muscles ability to maintain this force. Muscles perform two major types of contractions — isometric and isotonic.
There are two types of isotonic muscle contractions — concentric and eccentric. If you lift a dumbbell and do a biceps curl, for example, your biceps muscle shortens as you lift the dumbbell up through the curl. This shortening action of the biceps is called concentric contraction. Less external force from the dumbbell and a decrease in muscle tension produces the concentric action. Springfield Technical Community College describes this shortening of muscle as an isotonic contraction.
Muscle contractions are also in control when you bring the dumbbell back to the starting position. Lowering the dumbbell produces an isotonic muscle contraction that is different from that used in raising the dumbbell. According to Michigan State University, lowering the dumbbell from the shoulder position to complete the curl causes your biceps muscle to lengthen. This contraction while the muscle lengthens is called an eccentric contraction, and it is initiated when the external force, or in this case the weight of the dumbbell, is greater than that produced by the muscle.
Although lifting a dumbbell is an isotonic movement, if you lift a dumbbell and complete only part of a curl, holding your arm still for several seconds, your biceps remains static, meaning it does not change length. This is an isometric exercise. The University of Nebraska Medical Center explains that even though the muscles do not shorten during isometrics, a buildup of tension provides force.