Strength without flexibility creates stiffness. Flexibility without strength creates weakness and instability. The ideal body composition has both strength and flexibility, which is one of the reasons why Hatha Yoga is so popular.
How yoga builds both strength and flexibility
The body weight bearing poses that are part of a standard yoga class are designed to increase whole body strength and flexibility rather than isolating particular muscle groups which is an effect that conventional weight training can have. This ensures that the whole body is balanced and able to support itself. The focus is on core strength, building power in the muscles bracketing the spine. Using body weight and gravity to strengthen the muscles means that our bodies are adaptable to everyday tasks. Yoga practice ensures that strength in the body is proportionate and gradual, with less risk of injury from other conventional strength training practices. A variety of muscle groups are activated during the transition between yoga poses that would not normally be called upon. In this way, Yoga practice stabilizes and supports the whole body.
Flexibility relates to the muscle fascia. Tendons and ligaments are not meant to be stretched as they are designed to hold the muscles and bones in place, but muscle fascia are the connective tissue in the body that are designed to stretch safely. Having flexible muscle fascia has been shown to increase joint lubrication, aid healing, assist circulation, and boost mobility. Hatha yoga poses use prolonged stretching to elongate the muscle fascia, which creates permanent positive changes in flexibility. In this way flexibility is created in the body at the same time as strength is stimulated.
For an ideal body composition, the muscles need to have strength as well as flexibility.