Muscular tissue has three main jobs: create motion, maintain posture and balance, and create heat. There are three basic kinds of muscular tissues:
- Skeletal muscles which move our limbs
- Cardiac muscles which pump our hearts
- Smooth muscles which line our blood vessels and other tubes
The active, yang, styles of yoga specifically target the skeletal and the cardiac muscles. But again, the whole body does benefit from an active yoga practice. However, the major benefits of a yang style of asana practice are the strengthening and lengthening of our muscle tissues.
Muscles are connected to the bones via tendons. That is a simplification; in reality there is continuum of tissues from the muscles that turn into tendons, which in turn gradually become bone. The body is not made up of discontinuous pieces. Everything is connected and a part of everything else. But it does help us to understand the body if we employ simplifying models. So let’s go with this simplification of bones, tendons, and muscles for now.
If we look closer at the muscle itself, we find it is composed of tubes nested within tubes within tubes. This is a common feature of the body. Most of the body can be considered a system of tubes; the muscles are just one example. If we zoom in, we eventually find a single fiber of muscle tissue, which is called a “myofibril.” Again, this tube of myofibril is made up of smaller fibers. One unit of the fibers, called a “sarcomere,” contains interconnected filaments of actin, myosin, and titin. The actin fibers are thin filaments; the myosin fibers are thick filaments. The titin fibers connect the myosin fibers to the wall of the sarcomere (called the “Z-line,” which is not named in the diagram).
Around the myofibril, the single contracting fiber, but still within the muscle itself are connective tissues or fascia (which are described in detail later). Connective tissue can make up as much as thirty percent of the mass of the muscle. It contributes to the muscle’s ability to stretch and allows the neighboring fibers to slide along each other. Any program of exercise trying to lengthen the muscles will also affect the muscle’s fascia.