Learn chair pose (Utkatasana) properly: how deep to sit, whether your knees may pass your toes and why the pose mainly challenges the front of your thigh.
How deep do you have to go in chair pose for it to count? The honest answer: not as deep as the photo suggests, but as deep as you can stay clean. Chair pose (Utkatasana, "powerful pose") is a deep squat with the arms stretched overhead. You sit on a chair that does not exist, and that is exactly what makes it honest.
Chair pose is less stretch than strength. In a lab study of 11 yoga instructors it showed the highest activation of the rectus femoris (the part of the quadriceps that also flexes the hip) among five standing poses [1]. In 20 people around age 70 it was among the poses with the largest combined extension moment from hip, knee and ankle [2]. So it trains the leg strength you need to get off the sofa, climb stairs and hike.
How hard is it? Doable for beginners as long as you choose the depth yourself. The only mistake is treating it as a pose you simply endure.
Stand hip-width (feet together is the classic variation but harder). Weight evenly on both feet, toes spread, knees soft.
Push your hips back as if sitting on a chair and bend your knees. Your heels stay on the floor, your weight sits more in the heels than in the toes. Your knees point over the middle of your feet, not inward.
Milestone: You hold 5 breaths with bent knees and hands in front of your chest, heels down, knees over your feet.
Lift your breastbone, let your ribs soften down (no hollow back) and stretch your arms up, shoulder-width. Keep your shoulders away from your ears.
Stay for 5 calm breaths. In lab studies chair pose was held for only 10 to 15 seconds [3]. That is short, and for beginners it is a good yardstick: better short and clean than long and shaky.
Milestone to move on: 5 breaths with arms stretched overhead, heels down, without the back tipping into a hollow back.
On an inhale straighten both legs and lower your arms. Shake your legs out briefly.
The big question is: may the knees pass the toes? The answer has nuances. In a small study with 7 trained men, a wooden barrier that stopped the knees from passing the toes lowered the knee torque from about 150 to about 117 N·m, but pushed the hip torque from about 28 to about 303 N·m, roughly ten times as much. The authors suspect the forces are "offloaded" onto the hips and lower back [4]. That was a barbell squat, not yoga, so it is only a guide: knees drifting slightly past the toes are not automatically dangerous, while forcing them to stay back can shift the load.
The knee remains a topic in yoga anyway: in US emergency-room data it was the most frequently affected region of the lower limb (9.4% of yoga injuries) [6], and most injuries affected the trunk [5]. Stabbing or pinching in the knee is a stop signal, not a stretch sensation. This guide explains and motivates; it does not replace supervised training or medical advice.
Squeeze a block between your thighs. It reminds the inner thighs to stay active and stops the knees from collapsing without you having to think about it. Practice trick: if the block holds, the leg axis is stable. If it slips, you are too deep or too tired. For advanced practitioners the wall variation (back against the wall) becomes a strength-endurance exercise.
Deep enough that your heels stay down, your back stays long and your knees stay over the middle of your feet. For many beginners that is a knee angle well above 90 degrees, so still a half-height squat. The studies give no target value for this; lab hold times were 10 to 15 seconds [3].
Slightly, yes. A squat study with 7 men showed that when the knees were stopped from travelling forward, the hip torque rose to about ten times as much [4]. That is a transfer from the barbell squat. What matters is that your heels stay down and your knees do not fall inward.
Mainly the quadriceps. The rectus femoris was particularly active in chair pose [1][3], while the calf, the hamstrings and the gluteus medius were rather little involved [3]. In older people chair pose was among the poses with a high combined leg extension moment [2].
For beginners 5 breaths are enough, then build slowly toward 8 to 10. Time frames like "in 3 weeks" depend on your starting strength and are not backed by evidence. In studies the pose was held for 10 to 15 seconds [3].
Low Lunge (Anjaneyasana): Stretch the Hip Flexors Without Collapsing Into Your Lower Back