REF-1153 · Arampatzis et al., 2007 · Journal of Experimental Biology · Paper · PubMed

Achilles tendon adaptation has a strain threshold

14 weeks of isometric plantarflexion at matched frequency and volume stiffened the tendon, raised its modulus and thickened it regionally only in the leg loaded to ~4.6% tendon strain (90% MVC), not in the leg loaded to ~2.9% (55% MVC)

Blimited evidenceMeasured on recreational people

Arampatzis and colleagues asked whether the Achilles tendon needs a minimum strain to adapt. Eleven adults trained both legs with isometric plantarflexion for 14 weeks: one leg at low tendon strain (2.85 ± 0.99%) and the other at high strain (4.55 ± 1.38%), with the same contraction frequency and volume; ten controls did not train.

Only the high-strain leg changed: strain at a given tendon force fell, tendon-aponeurosis stiffness and elastic modulus rose, and the tendon thickened in one region. The low-strain leg did not change. The authors conclude that there is a threshold or set-point of strain for tendon adaptation, and that the low-strain exercise was no more of a stimulus than daily life.

In REF-517's extraction, stiffness rose 36.0% in the high-strain leg and 7.9% (not significant) in the low-strain leg.

Protocol

Participants: 21 healthy adults: 11 exercise, 10 control. Sex and age are not in the abstract; REF-1152's table lists the exercise group as 3 men and 8 women, 29.5 ± 5.0 y, 64.1 kg.
Design: each exercise participant trained one leg at low strain and the other at high strain; the abstract states the two protocols had similar frequency and volume.
Exercise: isometric ankle plantarflexion, overcoming (pushing against a fixed dynamometer; our reading of a repetitive isometric protocol, the abstract does not use this term). Repetitive contractions, not long holds.
Low-strain leg (from [REF-517]'s Table 2): 55% of MVC, 3-s contractions, 7 repetitions × 5 sets.
High-strain leg (from [REF-517]'s Table 2): 90% of MVC, 3-s contractions, 4 repetitions × 5 sets.
Frequency and length: 4 sessions a week for 14 weeks (REF-517 and REF-1152 tables).
Strain achieved (abstract): 2.85 ± 0.99% vs 4.55 ± 1.38%.
Not available from the abstract: joint angle, rest between contractions (the same group's later use of the high-strain programme specified 3 s on, 3 s off, REF-1156), the measurement methods, and the control-group values.

Key Results

Outcome (14 weeks)Low-strain leg (55% MVC, ~2.9% strain)High-strain leg (90% MVC, ~4.6% strain)Source
Strain at a given tendon forceno changedecreasedabstract
Tendon-aponeurosis stiffness+7.9%, not significant+36.0%, p < 0.05abstract (direction); REF-517 Table 2 (%)
Elastic modulus-1.6%, not significant+22.9%, p < 0.05abstract (direction); REF-517 Table 2 (%)
Tendon cross-section+4.3%, not significant+9.6%, p < 0.05, region-specificabstract (direction); REF-517 Table 2 (%)
n = 11 exercising participants (both legs) and 10 controls; the abstract gives no p-values or control values.

Mechanisms

Measured: the tendon adapted (stiffness, modulus, regional size) only when the training strain was high; matched frequency and volume at low strain produced nothing.
Authors' interpretation: there is a threshold or set-point of strain magnitude at which the mechanical stimulus changes the tendon's tensional homeostasis; loading below it is no more than what daily activity already provides.
Inference: because %MVC and tendon strain move together here, the trial cannot say whether strain or relative force is the cause; the later meta-analysis REF-1152 found relative intensity alone did not separate protocols, which favours strain.

Practical Application

Is easy calf loading enough to stiffen the Achilles? In an 11-person leg-vs-leg trial, no: 14 weeks of isometric calf work stiffened the tendon only in the leg loaded near 90% of max (~4.6% strain), not at 55% (~2.9%). Small; tendon only.
The high-strain programme, as later papers report it: isometric calf push at 90% of maximal voluntary force, 3-s contractions, 4 per set, 5 sets, 4 sessions a week, 14 weeks. The same programme was followed by about 4% lower running cost in two later runner trials (REF-1155; in REF-1156 the fall was within-group, not significant against controls).
Inference, not tested: moderate isometric calf work (around half of maximum) is unlikely to change the tendon beyond what running already does; dynamic calf raises were not tested here.
Not from this paper; the base's practice: without a dynamometer, 90% of max means a near-maximal single-leg push against an immovable bar or strap, built up over 2-3 weeks rather than from the first session.

Limitations

Small: 11 exercising participants; the within-leg design controls for the person but not for cross-education between legs.
Read from the abstract: no p-values, control values or measurement details; the protocol and percentage changes come from the tables of two later meta-analyses (REF-517, REF-1152), not from this paper's text.
Strain and relative intensity were changed together (55% vs 90% MVC), so the trial cannot separate them.
Tendon mechanics only; no strength, running or injury outcome in the abstract.
Mostly women (8 of 11 per REF-1152's table); non-athletes.
Funding and conflicts of interest are not stated in the abstract.
How this entry was checked and changed
Why this level

within-subject comparison in n=11 with a parallel control group of 10; tendon outcomes only, no performance outcome; from the abstract (protocol details from the tables of two later meta-analyses, labelled)