Quadriceps

The quadriceps, or quads, are the group of four muscles that make up most of the front of the thigh: vastus lateralis, vastus intermedius, vastus medialis and rectus femoris. All four insert into the kneecap through the quadriceps tendon, and the kneecap is tied to the shin bone below by the patellar ligament. Together they form one of the most powerful muscles in the body (TeachMeAnatomy).

What do the quadriceps do?

Their main job is to straighten the knee. The three vastus muscles also help keep the kneecap stable (TeachMeAnatomy).

Rectus femoris is the odd one out. It starts on the pelvis rather than the thigh bone, so it crosses both the hip and the knee. As well as straightening the knee, it bends the hip, which makes it one of the hip flexors too (TeachMeAnatomy).

All four are supplied by the femoral nerve, which comes from the lumbar spine at the L2 to L4 levels. Doctors test quadriceps strength when they suspect that nerve has been damaged (TeachMeAnatomy).

What happens to the quadriceps when you sit all day?

Very little, and that is the point. Researchers gave 84 adults shorts with built-in sensors that recorded the electrical activity of the thigh muscles through about 11 hours of a normal day (Tikkanen et al., 2013):

• The quadriceps and hamstrings were inactive for about 68% of the time, roughly 7.5 hours.
• The longest unbroken spells of inactivity averaged about 14 minutes and reached 38 minutes in some people.
• On average, the thigh muscles worked at only about 4% of their maximum.
• Climbing stairs produced much higher activity than brisk walking, and the authors noted that replacing sitting with standing could considerably increase the day’s total muscle activity.

The study measured ordinary people through an ordinary day, not office workers specifically, and it shows inactivity rather than harm. It fits the idea the authors started from, that long spells with the big leg muscles switched off are a health risk of their own: see sedentary behaviour. Ideas for breaking it up are in microbreaks, exercise snacks and the standing desk schedule.

Why quadriceps strength matters for your knees

Osteoarthritis. A meta-analysis of five cohort studies followed 5,707 people with no knee osteoarthritis for between 2.5 and 14 years. Those with weaker knee extensors, meaning mainly the quadriceps, had higher odds of developing painful knee osteoarthritis (odds ratio 1.65) (Øiestad et al., 2015). This is an association: it does not show that strengthening the quads prevents osteoarthritis, only that weakness is a risk marker.

Pain at the front of the knee. Patellofemoral pain is pain behind or around the kneecap. It mostly comes on when the quadriceps mechanism is loaded: climbing stairs, squatting, running, cycling, or sitting with the knees bent (van der Heijden et al., 2015). So the front of the knee can ache after a long spell at a desk with the knees bent.

A Cochrane review of 31 trials with 1,690 people found very low quality but consistent evidence that exercise therapy may reduce this pain and improve function, with a possible small extra benefit when hip exercises are added to knee exercises. There was not enough evidence to say which type of exercise is best (van der Heijden et al., 2015).

When to see a doctor

See a GP if knee pain doesn’t improve within a few weeks. Get advice from NHS 111 straight away if your knee is very painful, you can’t move it or put weight on it, it is badly swollen or has changed shape, it locks, gives way or clicks painfully, or you have a very high temperature with redness or heat around the knee, which can be a sign of infection (NHS).