There are three types of muscular contraction; when muscle fibres shorten as they contract; this is known as concentric contraction. When muscle fibres lengthen, or stretch, as they contract this is known as eccentric contraction. When you hold a position steady and still and there is no movement in the muscles yet they are still contracting, this is known as isometric contraction.
All three muscles contractions are used at various times when we practice asanas in yoga. An example of concentric contraction during yoga would be lifting the torso up from Uttansana, this requires the hamstrings, glutes and muscles in the back to shorten, therefore concentrically contracting. Conversely as you fold forwards into Uttanasana the same muscles are lengthening or stretching and therefore eccentrically contracting to allow you to lower the upper body with control.
Concentric contractions cause a joint angle to decrease while eccentric contractions cause a joint angle to decrease, together concentric and eccentric contractions are known as isotonic contraction. Isotonic contraction always generates movement. When performed overtime concentric contractions tighten and shorten the muscle fibres, building strength and giving the muscle a more bulky appearance. While overtime eccentric contraction lengthens the muscle fibres while building strength and flexibility within the muscle.
Once you are in a yoga position and are required to hold that position, without moving, your muscles will be contracting isometrically. An example of this is when you hold almost any final yoga pose for a number of breaths. Isometric contraction builds strength throughout the muscle without changing the length of the muscular fibres.
In this way yoga provides a combination of isotonic and isometric contractions; we use isotonic contractions when we move into a position and isometric contractions when we hold the position for a period of time. In many forms of yoga since our aim is to hold postures for a considerable amount of time we tend to use more eccentric and isometric contractions rather than concentric contractions. Therefore the resulting muscular definition will be lean and strong rather than bulky and strong.
Gemma (200 Hr TTC May 2017)