Unfortunately, many people of all ages in our sedentary society have weak quads. In fact, this can be true even for people who do a fair amount of walking or running. Although these forms of exercise have many benefits, adequately strengthening the quads isn't one of them. Other activities, including riding a bicycle, lifting weights, and doing yoga, do a much better job. If you do choose yoga as your primary quad-strengthening activity, be sure to practice poses that target these muscles about three times a week, and use long holds and/or multiple repetitions to build endurance as well as strength.
Along with strengthening the quads, it's important to keep them flexible. They will become short and tight unless you give them a good long stretch at the end of any exercise session in which you've worked them. Even if you don't work to strengthen your quads, they will become short if they're never taken through their full range of motion; the soft tissues of the body simply conform to the shapes in which we spend the most time. If you rarely straighten your elbow or stretch your arm fully over your head, for instance, your elbow and shoulder will gradually lose those portions of normal movement. In the case of shortened quads, the knee and hip will suffer. If you rarely stretch the quads, you will lose the ability to fully flex (bend) the knee. (Of course, other factors, including injuries and arthritis, can also inhibit knee flexion.)
This loss of full flexion is especially noticeable in Virasana (Hero Pose). It can prevent you from being able to sit on your heels, let alone between your heels, as the complete pose demands. Loss of full knee flexion also limits your ability to pull your foot up into your groin in Vrksasana (Tree Pose), and it creates difficulties in seated poses and forward bends in which one or both knees must flex deeply, such as Janu Sirsasana (Head-to-Knee Pose) and Padmasana (Lotus Pose).
If the rectus femoris is short, it can limit not only knee flexion but also full extension at the hip. Combined with shortness in other hip flexors, like the psoas and the iliacus, shortness in the rectus femoris causes the pelvis to tip forward and the lower back to curve excessively when you are standing. Shortness in the hip flexors also contributes to lower back pain in backbends, such as Setu Bandha Sarvangasana, Ustrasana (Camel Pose) and Urdhva Dhanurasana (Upward-Facing Bow Pose). A sedentary lifestyle plays a part in the shortening of the hip flexors as well. If you spend long hours sitting, a position in which the hip flexors are shortened, your body will adapt to that shape unless you regularly stretch it in the other direction.
Balance Strength with Flexibility
Fortunately, it's simple (though often not easy) to stretch the quadriceps: Because their action is to extend the knee, to stretch them you just have to flex the knee. (A word of caution: If you have knee injuries or arthritis, check with your health care provider before working on deepening your knee flexion.) You can gently flex the knees by lying on your back and drawing your knees toward your chest. Wrap your hands around your shins and pull them down toward your thighs. To flex the knees more deeply, practice Virasana for two to three minutes on most days. If your knees are stiff and your quads tight, you will need to sit on a support, like a yoga block. As your quads' flexibility improves, gradually lower the height of the prop.