Quadriceps muscle activity during commonly used strength training exercises shortly after total knee arthroplasty: implications for home-based exercise selection

Key points

1. 01This study measured muscle electrical activity (EMG) in the operated leg during 6 strength exercises in 24 patients 4 to 8 weeks after total knee arthroplasty.
2. 02Contrary to the researchers' hypothesis, simple exercises beat machines for quadriceps activation.
3. 03Knee extension with an elastic band activated the quadriceps more than the knee extension machine.
4. 04A one-legged squat (and sit to stand) activated the quadriceps more than the leg press machine.
5. 05Combining elastic-band knee extension with sit to stand or a one-legged squat is a sensible minimal home set targeting both the quadriceps and hamstrings.

How it was conducted

Design

Descriptive cross-sectional electromyographic (EMG) study following STROBE guidelines

Participants

24 patients recruited 4 to 8 weeks after unilateral primary total knee arthroplasty, from four outpatient rehabilitation centers in the Copenhagen area

Exercises tested

6 exercises at a pre-determined 10 repetition maximum load: knee extension in machine, knee extension with elastic band, leg press in machine, sit to stand, one-legged squat, straight leg raise

Primary outcome

Voluntary peak muscle activity (%EMGmax) of the quadriceps and hamstrings in the operated leg, normalized to maximal voluntary isometric contraction

Procedure

Familiarization session set the 10 RM load, then an experimental session at least 72 hours later with 4 repetitions per exercise in randomized order; the first repetition was omitted from analysis

What they found

• Knee extension with elastic band produced higher voluntary peak quadriceps activity than the knee extension machine: 93.3 vs 74.9, mean difference 18.3 %EMGmax [95% CI 11.7 to 24.9], P < 0.0001.
• One-legged squat produced higher voluntary peak quadriceps activity than leg press in machine: 86.7 vs 66.8, mean difference 19.9 %EMGmax [95% CI 14.8 to 25.0], P < 0.0001.
• Sit to stand produced higher quadriceps activity than leg press: 80.7 vs 66.8, mean difference 6.0 [95% CI 0.9 to 11.1], P = 0.02.
• Pooled open versus closed kinetic chain isotonic exercises showed no significant difference (84.8 vs 78.0, P = 0.11).
• Both isotonic exercise groups produced higher quadriceps activity than the isometric straight leg raise (P < 0.0001).
• One-legged squat produced higher hamstring activity than leg press: 51.9 vs 33.9, mean difference 18.0, P < 0.0001.
• No significant difference in knee pain (VAS) or perceived exertion (Borg) between exercises, and no increase in resting knee pain after the experimental session.

Limitations

• Cross-sectional design, so it cannot show how these exercises affect long-term muscle strength or function.
• Maximal voluntary isometric contraction was measured at 60 degrees of knee flexion, which may underestimate normalization for dynamic tasks, and some recorded values exceeded 100 %EMGmax.
• Small single-session sample of 24 patients in the early post-operative window only.
• Muscle electrical activity is a proxy for muscle work and does not directly equal strength gains or clinical recovery.

Why it matters

For patients

If you do not have access to gym machines after a knee replacement, elastic-band knee extensions and squats or sit to stands can work your thigh muscle at least as hard.

For clinicians

Home-based elastic band and bodyweight exercises are a non-inferior alternative to machines for voluntary quadriceps activation early after TKA, with elastic-band knee extension plus sit to stand or one-legged squat as a practical minimal set.

For readers

This is short-term electromyography evidence about muscle activation, not a trial of long-term strength or functional outcomes.