5 Safe Exercises to Do Alongside Your CPM Machine
Your CPM machine is one half of the early recovery equation. It provides the passive range of motion your knee needs to prevent scar tissue and maintain flexibility. But the other half, the active half, comes from targeted exercises that rebuild muscle strength, improve circulation, and teach your brain to control the new joint.
These five exercises are the foundation of every post-knee replacement exercise program. They are safe to perform from the first days after surgery, require no equipment, and can be done in your bed or on a firm surface at home. When combined with consistent CPM machine usage, they set the stage for a complete and timely recovery.
Each exercise below includes step-by-step instructions, the optimal timing relative to your CPM sessions, common mistakes to avoid, and a week-by-week progression plan. These exercises are not a substitute for professional physiotherapy; they complement it. Your physiotherapist will add more advanced exercises as your recovery progresses through the rehabilitation timeline.
Why Exercises Matter Alongside CPM
The CPM machine moves your knee passively, meaning your muscles do not engage during the motion. This is by design; passive motion is ideal for preventing scar tissue, maintaining joint lubrication, and reducing swelling in the early weeks when active movement is too painful.
However, passive motion alone does not rebuild the muscle strength that surgery depletes. Research shows that quadriceps strength drops by 60% to 80% in the first week after knee replacement. Without targeted exercises to counteract this, the weakness persists for months and limits your functional recovery.
The combination of CPM and active exercises produces measurably better outcomes than either alone. CPM maintains the range of motion while exercises build the strength to use that motion in daily life. They are complementary, not competing, interventions.
What These Exercises Achieve
- Prevent blood clots: Ankle pumps and other lower-leg exercises pump blood through the veins, reducing DVT risk during the period of reduced mobility.
- Rebuild quadriceps strength: Quad sets and straight leg raises target the muscle most affected by surgery, beginning the long process of strength recovery.
- Improve active range of motion: Heel slides and short arc quads help bridge the gap between passive ROM (what the CPM achieves) and active ROM (what you can do on your own).
- Reduce swelling: Muscle contractions act as pumps for the lymphatic system, helping clear fluid from around the knee.
- Improve neuromuscular control: Each exercise requires your brain to send signals to specific muscles, retraining the neural pathways disrupted by surgery.
When to Exercise: The Optimal Sequence
The timing of your exercises relative to CPM sessions and ice significantly affects their effectiveness and your comfort level. Here is the optimal sequence, recommended by physiotherapists across Dubai's leading orthopedic hospitals.
- Ice for 20 minutes before the CPM session
- CPM session (2 to 3 hours)
- Rest for 30 minutes with leg elevated
- Perform exercises (15 to 20 minutes)
- Ice for 20 minutes after exercises
Why This Order Works
The CPM session warms up the joint, increases blood flow, and improves flexibility, making the subsequent exercises more comfortable and effective. The 30-minute rest allows any CPM-induced swelling to subside before you ask the muscles to work. The post-exercise ice controls inflammation from the active work.
Avoid exercising immediately before a CPM session, as fatigued muscles may cramp or spasm during the passive motion. Also avoid exercising when pain medication is wearing off; time your exercises for when you feel most comfortable.
Exercise 1: Ankle Pumps
1 Ankle Pumps
Ankle pumps are the first and most important exercise you will do after knee replacement. They are safe to start on day 1, even while still in the hospital, and should be performed frequently throughout the day.
How to perform:
- Lie on your back or sit in a reclined position with your legs extended.
- Slowly point your toes away from you (like pressing a gas pedal). Hold for 2 seconds.
- Slowly pull your toes toward you (like lifting your foot off the pedal). Hold for 2 seconds.
- Repeat 20 times. Perform 3 sets with a short break between sets.
Why it matters: Ankle pumps activate the calf muscle pump, which is the body's mechanism for pushing blood back up through the leg veins toward the heart. After surgery, when you are less mobile, this pumping action is critical for preventing deep vein thrombosis (DVT), one of the most serious post-surgical complications.
Common mistakes:
- Moving too fast. Slow, deliberate motions are more effective than rapid pumping.
- Not going through full range. Pull the toes as far toward you as possible and push as far away as possible.
- Forgetting to do them regularly. Set a phone alarm to remind you every 1 to 2 hours while awake.
Progression: Add ankle circles (clockwise and counter-clockwise) by week 2, and alphabet tracing with your foot by week 3.
Exercise 2: Quad Sets
2 Quad Sets (Isometric Quadriceps Contractions)
Quad sets are the single most important strengthening exercise in the early weeks after knee replacement. They target the quadriceps, the large muscle on the front of your thigh, which is the muscle most weakened by surgery and most critical for walking, standing, and stair climbing.
How to perform:
- Lie on your back with your legs extended. You can place a small rolled towel under the surgical knee for slight support.
- Tighten the muscle on the top of your thigh by pushing the back of your knee firmly into the bed or floor.
- You should see or feel the kneecap move slightly upward (toward your hip) as the quad engages.
- Hold the contraction for 5 seconds. Focus on squeezing as hard as you can without causing sharp pain.
- Relax for 5 seconds.
- Repeat 10 times. Perform 3 sets.
Why it matters: The quadriceps is the primary muscle responsible for straightening the knee and controlling it during weight-bearing activities. After TKR, many patients struggle to activate this muscle at all, a phenomenon called quad inhibition. Quad sets retrain the neural connection between your brain and the muscle, which is even more important than the strengthening effect in the early days.
Common mistakes:
- Holding your breath. Breathe normally throughout the contraction.
- Not contracting hard enough. Push firmly; the muscle should feel tight under your hand if you place it on the top of your thigh.
- Giving up too early. If you cannot feel the muscle engage initially, keep trying. It may take several days to re-establish the brain-muscle connection.
Progression: Increase hold time to 10 seconds by week 2. By week 3, perform quad sets with the leg slightly elevated (pillow under ankle, not knee).
Exercise 3: Straight Leg Raises
3 Straight Leg Raises (SLR)
Straight leg raises are a progression from quad sets. They require the quadriceps to work against gravity while keeping the knee straight, building functional strength without stressing the healing joint.
How to perform:
- Lie on your back with the non-surgical leg bent (foot flat on the bed) and the surgical leg straight.
- Perform a quad set first: tighten the quadriceps and push the back of the knee into the bed.
- While maintaining the quad contraction, slowly lift the surgical leg approximately 15 to 20 centimetres (6 to 8 inches) off the bed. Keep the knee straight.
- Hold for 3 to 5 seconds.
- Slowly lower the leg back down.
- Rest for 3 seconds, then repeat.
Why it matters: Straight leg raises strengthen the quadriceps in a functional pattern (lifting against gravity) without requiring knee bending, which is protective of the healing joint. They also engage the hip flexors and core, building the integrated strength needed for walking.
Common mistakes:
- Bending the knee during the lift. If you cannot keep the knee straight, continue with quad sets for a few more days before attempting SLR.
- Lifting too high. 15 to 20 cm is sufficient. Lifting higher shifts the work to the hip flexors rather than the quad.
- Dropping the leg quickly. The lowering phase (eccentric contraction) is as important as the lift. Lower slowly and with control.
Progression: Increase hold time to 10 seconds by week 3. Add a light ankle weight (0.5 to 1 kg) by week 4 if tolerated. By week 5 to 6, perform standing straight leg raises.
Exercise 4: Heel Slides
4 Heel Slides
Heel slides are the active equivalent of what your CPM machine does passively. They require you to bend and straighten your knee under your own muscle power, building active range of motion and neuromuscular control.
How to perform:
- Lie on your back with both legs extended on a smooth surface (bed sheet, towel on a hard floor).
- Slowly slide your heel toward your buttock by bending the surgical knee. Use a smooth, controlled motion.
- Slide as far as you comfortably can. You should feel a stretch, not sharp pain.
- Hold the bent position for 3 seconds.
- Slowly slide the heel back to the starting position, straightening the leg completely.
- Rest for 3 seconds, then repeat.
Why it matters: While the CPM machine provides passive flexion, your muscles need to learn to actively produce and control knee bending. Heel slides train this active flexion, bridging the gap between what the CPM achieves and what your knee can do independently. The active ROM from heel slides should progressively approach the passive ROM from the CPM.
Tip: If the heel drags on the bed sheet, try wearing a sock on the surgical leg or placing a plastic bag under the foot to reduce friction. Some patients find it easier to perform heel slides on a smooth tile or hardwood floor with a sock.
Common mistakes:
- Pulling with a strap or towel too aggressively. A strap around the foot can help if you need assistance, but do not force the knee beyond a comfortable stretch.
- Not straightening fully at the end. Complete extension at the bottom of each rep is as important as the bending.
- Rushing through repetitions. Slow, controlled slides are more effective than fast, choppy movements.
Progression: By week 3, aim to match your CPM flexion angle with your active heel slides. By week 4, add a 3 to 5 second hold at the end of range. By week 5 to 6, perform heel slides while seated on the edge of a chair for an added gravity challenge.
Exercise 5: Short Arc Quads
5 Short Arc Quads (SAQ)
Short arc quads are an advanced quad exercise that targets the VMO (vastus medialis oblique), the inner portion of the quadriceps that is most affected by knee surgery. A strong VMO is essential for knee stability and for the final degrees of knee extension.
How to perform:
- Lie on your back and place a rolled towel or a foam roller (approximately 15 cm diameter) under the surgical knee so that the knee is bent at approximately 30° to 40°.
- Tighten your quadriceps and slowly straighten the knee by lifting the foot until the leg is completely straight. The back of the knee should remain on the towel roll.
- Hold the straight position for 5 seconds, focusing on squeezing the inner quad (the muscle just above and to the inside of the kneecap).
- Slowly lower the foot back down to the starting position.
- Rest for 3 seconds, then repeat.
Why it matters: Short arc quads isolate the final 30° to 40° of knee extension, which is the range most important for walking with a normal gait and most dependent on VMO strength. Patients who neglect this exercise often retain a slight bend when walking (extension lag), which causes a visible limp and increased energy expenditure.
Common mistakes:
- Using a roll that is too large. A 15 cm (6 inch) diameter is ideal. Too large puts the knee at too much flexion, which may be uncomfortable and changes the exercise mechanics.
- Not achieving full extension at the top. The leg must straighten completely. If you cannot straighten fully, the roll may be too large, or you may need more time with quad sets before progressing to SAQ.
- Lifting the thigh off the roll. Keep the back of the knee in contact with the roll throughout the exercise.
Progression: Increase hold time to 10 seconds by week 4. Add a light ankle weight (0.5 kg) by week 4 to 5. By week 6, progress to seated knee extensions if cleared by your physiotherapist.
Weekly Exercise Schedule
Here is how to introduce and progress all five exercises alongside your CPM protocol:
| Week | Exercises | Sets x Reps | Sessions / Day |
|---|---|---|---|
| Days 1 to 3 | Ankle Pumps + Quad Sets | 3 x 20 pumps, 3 x 10 sets | 4 to 6 (pumps), 3 (sets) |
| Week 1 | Ankle Pumps + Quad Sets + Straight Leg Raises | Add 3 x 10 SLR | 3 |
| Week 1 to 2 | Add Heel Slides | Add 3 x 10 slides | 2 to 3 |
| Week 2 to 3 | Add Short Arc Quads | Add 3 x 10 SAQ | 2 to 3 |
| Week 4+ | All 5 exercises with progressions | Increase holds, add weights | 2 to 3 |
A complete exercise session with all five exercises takes approximately 15 to 20 minutes. This is manageable even on days when motivation is low. Remember, consistency matters more than intensity in the early weeks.
Safety Guidelines
- You experience sharp, sudden pain (not the expected stretching or muscle effort)
- Pain from exercises does not return to baseline within 2 hours
- You notice a significant increase in swelling after exercising
- You hear clicking, catching, or locking in the knee
- You feel unstable or the knee gives way
General Safety Rules
- Never exercise through sharp pain. Muscle effort and a stretching sensation are expected. Sharp or stabbing pain is a signal to stop.
- Follow the "2-hour rule": If your pain or swelling after exercising has not returned to pre-exercise levels within 2 hours, you did too much. Reduce intensity or volume at the next session.
- Exercise after CPM, not before. The CPM warms up the joint, making exercises more effective and comfortable.
- Ice after every exercise session. Apply ice for 20 minutes to control inflammation. Read our complete pain and swelling management guide for detailed ice protocols.
- Do not add exercises beyond these five without your physiotherapist's guidance. These are specifically selected for safety during the CPM phase. More advanced exercises will be added as you progress.
- Maintain good nutrition. Muscles cannot rebuild without adequate protein. Aim for 1.5 to 2 grams of protein per kilogram of body weight daily.
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Book on WhatsAppFrequently Asked Questions
After. The CPM session warms up the joint and improves flexibility, making subsequent exercises more effective and comfortable. The recommended sequence is: ice, CPM session, 30-minute rest, exercises, then ice again. This approach has been shown to produce better ROM and strength outcomes.
This is common in the first few days after surgery due to quadriceps inhibition, a protective neurological response. Keep attempting quad sets even if you cannot feel the contraction. Place your hand on the top of your thigh to feel for any activation. The connection will restore over several days. If you still cannot activate the quad after one week, discuss neuromuscular electrical stimulation (NMES) with your physiotherapist.
A complete session with all five exercises takes approximately 15 to 20 minutes. In the first few days when you are only doing ankle pumps and quad sets, sessions are shorter (5 to 10 minutes). As you add more exercises over weeks 1 to 3, sessions gradually lengthen. Quality and form matter more than duration.
These five exercises are specifically selected for safety during the CPM phase (first 4 to 6 weeks). More advanced exercises such as standing exercises, step-ups, resistance training, and balance work will be added by your physiotherapist as you progress. Doing too much too soon risks increased swelling, pain, and setbacks.
Gentle exercises (ankle pumps and quad sets) are safe and actually helpful even when the knee is swollen, as they promote fluid drainage. However, if swelling has significantly increased compared to the previous day, reduce the intensity of more demanding exercises (SLR, heel slides, SAQ) and focus on ice, elevation, and ankle pumps until swelling improves. If swelling is sudden and severe, contact your surgeon.