Patellar tendon load progression during rehabilitation exercises

Key points

1. 0135 exercises were ranked by a composite loading index combining peak force, loading impulse, and loading rate into three tiers (low, moderate, high).
2. 02The single-leg decline squat had the highest loading index (0.747) with a peak force of 5.2 bodyweights, making it unsuitable as a starting exercise despite its common use.
3. 03Tier 1 (low load) exercises included walking, step-ups, and double-leg 60-degree squats with peak loads of 0.8 to 2.7 bodyweights.
4. 04Most jumping and landing exercises fell into Tier 2 (moderate load), except single-leg maximal hops and single-leg countermovement jumps, which were Tier 3.
5. 05Progressing from double-leg to single-leg squats before deepening squat angle may better control patellar tendon load than the reverse.

How it was conducted

Design

Cross-sectional biomechanical laboratory study

Participants

20 healthy adults (10 female, 10 male), mean age 25.9 years, no history of lower-limb or spinal injury in the prior 6 months

Exercises tested

35 rehabilitation exercises including squat variations, step-ups, lunges, jumps, hops, landings, running, and sport-specific tasks

Instrumentation

12-camera motion capture at 100 Hz and 3 force platforms at 1000 Hz; inverse kinematics and dynamics used to estimate patellar tendon force

Loading index

Weighted sum: 50% loading peak, 30% loading impulse, 20% loading rate; normalized 0 to 1 and divided into three tiers

Primary outcome

Patellar tendon loading index per exercise, used to rank and tier all 35 exercises

What they found

• Loading index ranged from 0.106 (walking) to 0.747 (single-leg decline squat).
• Tier 3 exercises (index 0.66 or above): single-leg decline squat (0.747), running cut (0.725), single-leg countermovement jump (0.711), single-leg repeated forward hops (0.666), single-leg maximal forward hop (0.666); peak loads 4.6 to 5.5 bodyweights.
• Single-leg decline squat peak patellar tendon force: 5.2 bodyweights; loading impulse: 6.8 bodyweights x second.
• Tier 2 exercises (index 0.33 to 0.66): Spanish squat (0.563), running (0.612), double-leg countermovement jump (0.610), single-leg drop vertical jump (0.599), single-leg full squat (0.580), double-leg drop vertical jump (0.563), lunge (0.471), double-leg full squat (0.428), single-leg 60-degree squat (0.411), Bulgarian squat (0.406); peak loads 2.8 to 5.0 bodyweights.
• Tier 1 exercises (index 0.33 or below): 20 cm step-up (0.187), double-leg 60-degree squat (0.224), 20 cm step-down (0.288), 30 cm step-up (0.321); peak loads 0.8 to 2.7 bodyweights.
• Single-leg squats did not double peak loading compared to double-leg squats; 60-degree squats produced approximately half the peak loading of full squats.
• The single-leg decline squat was the only squat variation producing more peak patellar tendon load than running.

Limitations

• Only healthy young adults were studied; results may not generalize directly to patients with patellar tendinopathy or post-ACL reconstruction.
• The knee model was constrained to the sagittal plane and did not account for frontal or transverse plane movements, which may affect patellar tendon forces during single-limb tasks.
• Exercise order was not randomized, so fatigue may have influenced movement patterns and tendon force estimates in later exercises.
• Hamstring co-activation was not modeled; net knee moment was used to estimate quadriceps and patellar tendon loading, which may overestimate actual tendon force.

Why it matters

For patients

Patients with patellar tendinopathy or recovering from ACL surgery with a bone-patellar tendon-bone graft should know that starting rehabilitation with the single-leg decline squat may be too aggressive, and gentler exercises like step-ups and partial squats are better early options.

For clinicians

The three-tier loading framework gives clinicians an objective, biomechanics-based tool to sequence patellar tendon loading progression, with a modifiable spreadsheet available to re-weight tiers for individual patients.

For readers

This is the first study to rank 35 common rehabilitation exercises by a composite patellar tendon loading index, providing a practical evidence base for structured load progression in knee rehabilitation.