A straight leg raise looks like nothing: lie on your back, lock the knee, lift the leg a few inches. Yet in the first weeks after a knee injury or surgery it is often the main strength exercise on the sheet, because squats are off the table and the thigh muscle has gone quiet.
Why the quad switches off
After a knee is injured or operated on, the quadriceps often cannot contract fully even though the muscle is intact. Clinicians call this arthrogenic muscle inhibition: swelling and altered signals from the joint reduce the nervous system's drive to the quad. A 2024 review in Osteoarthritis and Cartilage Open reports that people lose roughly 60 to 83 percent of knee extension strength in the operated leg in the first days after total knee replacement, and that failure to activate the muscle explains nearly twice as much of the loss at one month as muscle wasting does. A systematic review in the Journal of Athletic Training pooled 352 people with a torn ACL and found activation failure in 57 percent of injured legs, and in about a third of the uninjured legs too. The straight leg raise asks the quad to hold the joint locked, many times a day, before the knee can take load through bending.
Which muscles do the work
- Quadriceps: hold the knee fully straight for the whole lift.
- Hip flexors: iliopsoas and rectus femoris lift the leg. They can raise a leg with a weak quad, which is the shortcut the exercise tries to prevent.
- Trunk: a bent opposite knee and a braced abdomen stop the lower back arching.
How to do it
- Lie on your back with the other knee bent and that foot flat.
- Pull your toes toward you and press the back of the knee down, tightening the thigh.
- Lift the straight leg until the heel is a few inches off the bed.
- Hold for 5 to 10 seconds, then lower slowly, heel first.
- Repeat until the thigh feels tired, about 3 minutes per set according to the American Academy of Orthopaedic Surgeons.
The quad lag problem
Watch the knee as the leg comes up. If it bends slightly while the leg rises, that is an extension lag, or quad lag: the quad cannot hold the last few degrees of straightening. Therapists check by asking you to straighten the knee fully on your own, then comparing that with how straight it goes when they move it. A lag is information, not failure: it tells your therapist to step back to quad sets, short lifts over a rolled towel, or electrical stimulation before loading the lift again.
Progressing with ankle weights
Once a set is easy and the knee stays locked, resistance goes up. After a knee replacement, the AAOS guide says light ankle weights can usually begin 4 to 6 weeks after surgery, starting at 1 to 2 pounds. Add weight only when every rep still shows zero lag; a weighted lift with a bent knee is just hip flexor training. Your therapist sets the timing.
When to move on
The straight leg raise builds a floor, not a ceiling. Walking and stairs load the quad through a bending knee, so once you can lift with weight and no lag, the plan shifts to standing work. If that is next, see our guide to the sit-to-stand exercise
Doing this exercise in a guided session
At home you cannot see your own knee bend while you lift. In a KineTrue session the camera tracks knee angle through each rep and records hold time and rep count, so your therapist can see whether a lag is creeping in between visits and adjust the plan.
Stop and contact your clinician if the knee swells noticeably after a session, if you feel sharp or catching pain in the knee rather than tiredness in the thigh, or if a new lag appears. Those are signs the dose is wrong, not signs to push through.