Although traditional Yoga is a spiritual discipline, many people come to Yoga expecting to improve their flexibility and reduce feelings of stress and anxiety. As most people age, muscles shorten and tendons (the tissue that connects muscles to bones) become stiffer. The result of this is a reduction in range of motion, of knees, shoulders, elbows, spine, and other joints. Losing flexibility is also associated with an increased risk of pain and injury. Tight muscles increase the likelihood that you could suddenly move past your safe range of motion and damage ligaments, tendons, and the muscles themselves. With increased practise of yoga muscles become more flexible and joints and tendons become less stiff.
Whilst achieving a Yoga posture requires flexibility, it also requires strength in terms of muscle strength (how heavy an object you can lift) and muscle endurance (how long you can lift it). Without exercise muscle mass is lost as we age, and this can eventually result in weakness and loss of balance and coordination.
Muscles act in an antagonistic way, one muscle contracting and the other either relaxing or acting as a control mechanism. This contraction requires ‘muscle strength’ and the length of time that you can hold the posture depends on your ‘muscle endurance/stamina’. So to bend forward as in Paschimottanasana (seated forward bend) requires the muscles along the front of the body to contract in order to pull the torso down over the legs. To hold the body in Lolasana (Pendant pose) requires great upper body strength as well as flexibility in the legs and hips. Similarily in Plank pose, strength is required throughout the whole body and particularly the core muscles of the abdomen to keep the body in a perfectly straight line.
Strength and flexibility are intertwined and dependent on each other. You might be very flexible but that would be no good if you could not pull yourself into the posture for lack of strength. You might be very strong and have the physical ability to force your body into a posture – but without flexibility that posture will be uncomfortable and even painful.