A dancer’s body is her most important asset, and without her muscles properly working, she can never master her craft. When a dancer understands how her muscles work, she’ll be better able to confidently execute moves and combinations. Because of that, dance students all need a basic understanding of kinesiology, the study of human movement. Any study of Dance Kinesiology begins with understanding the Law of Approximation: that muscles can only shorten or contract, pulling their ends toward each other; they cannot push them away. After you grasp that basic concept, it’s time to delve into the basic types of muscular contractions.
Dynamic (or Isotonic) Muscular Contractions
When there’s a change in the muscle involved and observable joint, Dynamic Muscular Contractions occur. Dynamic Muscular Contractions can further be divided into two types: Concentric and Eccentric Contractions. Concentric Contractions, in a nutshell, are those movements directed “toward the center,” whereas Eccentric Contractions move a joint “away from the center.”
Concentric Muscular Contractions
The first type of Dynamic Muscular Contractions are referred to as Concentric Contractions, and they involve shortening of the muscle in the direction of the Primary Muscle (we’ll discuss this and other possible roles of muscles in Part 2); these movements “toward the center” are what most people typically think of when they think of Muscular Contractions.
For instance, if you’re seated in a chair and straighten out your leg, your hamstring shortens, or changes length, in order to lift your foot and straighten your leg. Because in a Concentric Contraction, the torque (or tendency of force) is greater than the resistance (or opposite tendency), these movements allow the dancer to feel stronger the more she performs them!
Eccentric Muscular Contractions
The second type of Dynamic Muscular Contractions, Eccentric Contractions, involve the lengthening of the muscle away from the primary muscle. As the distance between the ends of the muscle become greater, the contraction decreases. When you are seated in a chair, raise your leg and then release the leg to let it back down to the ground; as you do so, the muscle will lengthen until it achieves its resting length.
Static (or Isometric) Muscular Contractions
In contrast to Dynamic (or Isotonic) Muscular Contractions, Static (or Isometric) Contractions involve either partial or complete muscular contraction but produce no visible joint movement; as such, Static Contractions require you to hold the contraction still. No movement occurs, because the torque and resistance are perfectly counterbalanced.
Understanding types of muscular contractions is only the beginning. The next level of Dance Kinesiology involves understanding the various roles muscles play in any given contraction. Different from muscle types, any given muscle can take on different roles, depending on the contractions involved.