Achilles tendon loading during weight bearing exercises

Key points

1. 01Unilateral jump landing had the greatest Achilles tendon stress of all 11 exercises tested, placing it in a category of its own.
2. 02Squat and Romberg standing had the lowest peak Achilles tendon stress and are suitable as first-stage rehabilitation exercises.
3. 03Peak Achilles tendon stress and force during unilateral heel raise were similar to running, but the stress rate (rate of loading) was over four times greater during running.
4. 04The impulse (total tendon energy transfer) per unilateral heel-raise repetition was over four times greater than per running step, meaning cumulative dose across a session differs substantially.
5. 05A standard rehabilitation dose of 3 sets of 15 heel raises produces a cumulative Achilles tendon impulse that is only a fraction of the total impulse experienced during a 30-minute run.

How it was conducted

Design

Controlled laboratory study using a musculoskeletal model

Participants

18 healthy, physically active males (mean age, height, and weight reported; Tegner activity level 5.9 +/- 0.9)

Exercises tested

11 weight-bearing exercises: Romberg standing, tandem standing, unilateral standing, bilateral and unilateral heel raise, squat, forward lunge, bilateral and unilateral jump-landing, walking, and running

AT loading measurement

Kinematic data at 200 Hz and kinetic data at 1000 Hz fed into a 25-degree-of-freedom Human Body Model (Motek Medical) to estimate triceps surae forces; AT cross-sectional area measured by ultrasound to calculate stress

Primary outcomes

Peak AT stress (MPa), AT force (BW), AT stress rate (MPa/s), ankle range of motion (degrees), and AT force impulse (BW*s)

Statistics

Repeated-measures MANOVA (alpha = 0.05) with Bonferroni pairwise comparisons

What they found

• MANOVA revealed significant differences between exercises (Wilks lambda = 0.001, p-value < 0.05); univariate follow-up tests confirmed differences in all five AT loading variables across exercises.
• Exercises were grouped into four stress categories: Category 1 (lowest) - Romberg and squat; Category 2 - forward lunge, bilateral heel raise, tandem standing, and unilateral standing; Category 3 - walking, bilateral jump-landing, unilateral heel raise, and running; Category 4 (highest) - unilateral jump-landing alone.
• AT loading estimates during walking were reported as approximately 0.4 N/BW and a corresponding MPa value noted as generally larger than prior optic-fiber studies.
• Peak AT force during running at approximately 3.8 m/s was approximately 3.8 BW by musculoskeletal model estimation, comparable to direct in-vivo measurements of approximately 3.9 BW at a similar speed.
• Peak AT stress and force were lower during bilateral heel raise compared to unilateral heel raise; bilateral heel raise values were cited as lower N and lower MPa than unilateral heel raise.
• AT stress rate during running was over four times greater than during unilateral heel raise, while AT impulse per repetition was over four times greater for the heel raise than per running step.
• A standard dose of 3 sets of 15 unilateral heel raises produces a cumulative AT impulse that is only approximately 39% of the total AT impulse experienced during a typical 30-minute run (based on 180 steps/min at 3.8 m/s); approximately 1800 unilateral heel raises would be needed to match the AT impulse of that run.
• Romberg standing had the lowest ankle ROM (reported mean degrees +/- SD) followed by tandem stance; forward lunge, unilateral jump-landing, and bilateral jump-landing had the highest ROM.
• AT stress rate order largely mirrored AT stress order across exercises; running and unilateral jump-landing showed the highest stress rates.

Limitations

• Only healthy males were studied, limiting direct application to patients with Achilles tendinopathy or rupture.
• Musculoskeletal model estimates involve assumptions about muscle insertion geometry and optimization criteria; absolute force values may differ from direct in-vivo measurements.
• Not all exercises from published rehabilitation protocols could be included (e.g., eccentric heel drops from a step edge, which are common in tendinopathy programs).
• The VISA-A questionnaire was not used to screen participants, so subclinical tendinopathy cannot be excluded; only right lower extremity data were analyzed.

Why it matters

For patients

Patients recovering from Achilles tendon injury can expect their therapist to start with bilateral standing and squat exercises that place low stress on the tendon, progressing to running and jumping only in the later stages.

For clinicians

This biomechanical framework provides evidence-based guidance for sequencing weight-bearing exercises during Achilles rehabilitation: begin with Romberg and squat, progress through bilateral heel raise and forward lunge, then unilateral heel raise and walking, and finally running and unilateral jump-landing.

For readers

This laboratory study provides the first direct comparison of Achilles tendon loading across a broad range of rehabilitation exercises using consistent methodology, offering a biomechanically grounded rationale for progressive exercise prescription.