REF-326 · Salatkaitė-Urbonė S, Lasevičius R, Wezenbeek E & Satkunskienė D, 2026 · Frontiers in Physiology, · Paper · PubMed

Mechanical loading and fatigue-induced changes in triceps surae muscle-tendon unit: sex-specific responses and recovery dynamics

After standing calf raises to exhaustion in 22 active adults, peak torque fell and MTU passive stiffness rose immediately, then stiffness decreased by 24 h while contractility had only partly recovered and tendon echo intensity kept falling; sex differences were present but limited

Blimited evidenceMeasured on recreational to trained athletesWritten before October 2026, being re-checked

Participants

n=22 young, physically active adults (10 females, 12 males)

Protocol

Fatigue protocol: repetitive bilateral standing calf raises to volitional exhaustion
Assessed pre-, post- and 24 h after exercise: muscle contractility (peak torque), muscle and tendon structure (ultrasound echo intensity, EI), passive mechanical properties (MTU passive stiffness and hysteresis efficiency), EMG of gastrocnemius lateralis (GL) and soleus (SOL)

Key Results

Significant reduction in peak torque immediately after the protocol
Altered neuromuscular activation: GL showed reduced EMG activity, SOL showed compensatory recruitment
Muscle contractility had partially recovered at 24 h
Tendon echo intensity decreased progressively — suggesting prolonged structural change beyond 24 h
MTU passive stiffness increased immediately post-fatigue but decreased after 24 h (the abstract does not say whether it fell below baseline)
Sex-related differences in MTU responses were present but limited
Authors: recovery after fatigue occurs at different time scales across the tissues of the MTU

Mechanism

The tissues of one muscle-tendon unit did not recover together: force and passive stiffness moved quickly (stiffness up, then down within a day), the tendon's structural signal was still changing at 24 h. Inference, not tested: the tendon being the slow tissue is consistent with its lower metabolic turnover; the study did not measure fascia and did not follow the tendon to recovery.

Practical Application

In 22 active adults, calf raises to exhaustion cut peak torque, raised MTU passive stiffness immediately and lowered it again by 24 h, while contractility had recovered only partly and tendon echo intensity was still decreasing — muscle and tendon were on different clocks
At 24 h the calf may test near normal for stiffness while the tendon's structure is still changing; Inference, not tested: this is a window in which the tendon is loaded on an incomplete recovery — the study measured no injury or performance outcome
Sex differences existed but were limited, so this study does not support sex-specific rest prescriptions for the calf
Not from this paper; the base's own practice: after an unaccustomed high-volume calf/plyometric session, keep the next tendon-loading session (hill sprints, jumps) more than 24 h away; the study only followed 24 h

Limitations

Only 24 h follow-up — full tendon recovery timeline unknown
Calf raises — not sport-specific endurance exercise
Relatively small sample (n=22); no control condition
How this entry was checked and changed
Why this level

regraded 2026-09-27 from B+: uncontrolled pre-post, n=22

Content correction 2026-09-27
"Stiffness decreased BELOW baseline at 24 h / stiffness overshoot / transient vulnerability period" -> the abstract says stiffness decreased after 24 h, not that it fell below baseline; corrected.
"Recovery hierarchy muscle → fascia → tendon" -> fascia was not measured; removed. Sex-specific recovery patterns "important for individualizing rest" -> the abstract calls sex differences present but limited.
Level B+ -> B (uncontrolled pre-post, n=22).