Intratendinous pressure changes in the Achilles tendon during stretching and eccentric exercise

Key points

1. 01Intratendinous pressure increased exponentially from 0 degrees of dorsiflexion onward in both the insertion and mid-portion regions (p < 0.001)
2. 02The deep insertion layer consistently showed the highest pressures, up to 36.60 times greater than the proximal portion during straight-knee calf stretch (p < 0.001)
3. 03Eccentric heel-drop produced the highest pressures of all three loading conditions, with deep insertion pressure 5.75 times higher than bent-knee calf stretch (p < 0.001)
4. 04Mid-portion pressure was 7.45 to 9.77 times higher than proximal portion across all loading conditions (p < 0.001), likely driven by torsion-induced internal compression
5. 05A plantar flexion range of -15 to 0 degrees appears to be a low-pressure zone suitable for applying tensile load during rehabilitation

How it was conducted

Design

Cadaveric biomechanical study using 12 matched pairs of fresh-frozen human cadaveric legs (6 male, 6 female donors; mean age 77.9 +/- 10 years)

Intervention

Miniature Millar Mikro-Tip pressure catheter inserted under ultrasound guidance into four Achilles tendon regions: proximal portion, mid-portion, superficial insertion, and deep insertion

Loading conditions

Three conditions: bent-knee calf stretch, straight-knee calf stretch, and eccentric heel-drop (200 N total tensile load, scaled 1:5 from in vivo data)

Ankle motion

Ankle cycled from 15 degrees plantarflexion to 15 degrees dorsiflexion and back for each condition

Primary outcome

Intratendinous pressure (mm Hg) across tendon regions and loading conditions analyzed with linear mixed model on log-transformed data

What they found

• Ankle flexion angle significantly affected intratendinous pressure (F = 9404.88, p < 0.001); pressure increased exponentially from 0 degrees dorsiflexion onward
• Tendon region significantly affected pressure (F = 1269.29, p < 0.001); deep insertion was consistently highest across all conditions
• Loading condition significantly affected pressure (F = 435.98, p < 0.001); eccentric heel-drop > straight-knee stretch > bent-knee stretch
• Deep insertion pressure during straight-knee calf stretch was 36.60 times higher than proximal portion (95% CI 28.22-47.51, p < 0.001), 3.75 times higher than mid-portion (95% CI 2.89-4.86, p < 0.001), and 2.92 times higher than superficial insertion (95% CI 2.25-3.79, p < 0.001)
• Mid-portion pressure was 7.45 times higher than proximal portion during bent-knee stretch (95% CI 5.60-9.65, p < 0.001), 9.77 times higher during straight-knee stretch (95% CI 7.53-12.67, p < 0.001), and 8.52 times higher during eccentric heel-drop (95% CI 6.35-11.42, p < 0.001)
• In the deep insertion, eccentric heel-drop pressure was 5.75 times higher than bent-knee stretch (95% CI 4.46-7.42, p < 0.001) and 1.45 times higher than straight-knee stretch (95% CI 1.13-1.87, p < 0.001)
• Straight-knee calf stretch produced deep insertion pressure 3.96 times higher than bent-knee calf stretch (95% CI 3.12-5.03, p < 0.001)

Limitations

• Cadaveric specimens with unknown injury history and age-related tissue changes that may not reflect the clinical population
• Implanted pressure catheter may create a space-occupying effect that artificially elevates measured pressures compared to true in vivo values
• Soleus muscle could not be separately activated, potentially underestimating mid-portion pressures during the eccentric heel-drop
• Effects of ankle inversion-eversion, variable tendon torsion, and calcaneal morphology were not investigated and may further influence intratendinous pressure

Why it matters

For patients

People with Achilles tendinopathy may benefit from avoiding deep ankle dorsiflexion during stretching and early rehabilitation, and from using heel lifts to limit daily compressive load on the tendon.

For clinicians

Eccentric heel-drop exercises and aggressive calf stretches generate the highest tendon pressures, particularly at the deep insertion; programming rehabilitation within a plantar flexion to neutral range may reduce compressive overload while still providing tensile stimulus.

For readers

This cadaveric study provides the first direct pressure measurements inside the Achilles tendon during common exercises, offering a biomechanical rationale for why insertional and mid-portion tendinopathy occur where they do and how exercise prescription could be modified.