After a knee replacement, an ACL reconstruction, or a bad knee injury, the first exercise most people get is the heel slide. It looks like nothing, yet how far the knee bends in the first weeks shapes the rest of recovery.
Why early bending matters
A knee that sits still after surgery stiffens as scar tissue forms, so rehab guidelines put flexion targets on the calendar. The OHSU Health total knee guideline aims for 0 to 90 degrees by hospital discharge, flags the surgeon if that range is not held at weeks 2 to 3, and considers manipulation under anaesthesia if flexion is still not past 90 degrees at week 6. One surgeon's ACL protocol sets a minimum of 90 degrees by 2 weeks, plus full extension. Both include heel slides from the first phase.
How to do a heel slide
- Lie on your back, legs straight. A sock or plastic bag under the heel cuts friction.
- Slide the heel toward your buttock, keeping it on the bed, until you feel a firm stretch.
- Hold for 5 to 10 seconds, then slide back until the leg is straight.
- Do the sets your therapist set, spread through the day.
Variations your therapist may add
- Towel assist: loop a towel around the foot and use your arms to draw the heel in when the leg is too weak or sore.
- Wall slide: lie with the foot flat on a wall and let gravity slide it down, the other foot resting on top for gentle pressure.
What progress looks like
A 2018 study of 559 knee replacement patients found average flexion of about 100 degrees at 2 weeks and 117 degrees at 52 weeks, with most of the change in the first 12 weeks and a plateau by 26 weeks. Early on, the knee should bend a little further most weeks; later, gains slow. A weekly measurement makes that trend visible.
Swelling and bending
Swelling takes up room inside the joint, so a swollen knee bends less. It also quiets the quadriceps, so a puffy knee feels weak as well as stiff. The OHSU guideline treats a swelling increase under 2 cm after exercise, with soreness lasting under 2 hours, as a sign the dose is right. A knee that balloons after every session needs a smaller dose.
Doing heel slides in a guided session
Heel slides are easy to do badly: the heel lifts or the knee stops short. KineTrue's camera-guided sessions track the knee angle on each repetition and record the best bend reached, so your therapist can see the trend between visits and set the next target.
If you also log pain alongside motion, see our guide to why pain tracking matters
Stop and contact your surgeon or physiotherapist if the knee becomes hot, red, or much more swollen, if it bends less than last week, if you notice calf pain, or if the pain is sharp rather than a stretch.