REF-1373 · Yu, Carlsson & Thornell, 2004 · Histochemistry and Cell Biology · Paper · PubMed

Is the soreness after a new eccentric workout a sign of damaged muscle or of muscle rebuilding?

One Umeå team read the fibre changes days later as remodelling, not damage; others read them as damage. Unresolved (C+, abstract).

Main caveat: Abstract only. The full text is behind the publisher's paywall, with no Europe PMC copy and no repository copy listed by OpenAlex.
thin evidenceMeasured on non-athletes to trained athletes

Yu, Carlsson and Thornell (Umeå University) start from the usual reading of the myofibrillar and cytoskeletal alterations seen in delayed-onset muscle soreness (DOMS) after eccentric exercise as damage, against which their own recent immunohistochemical studies (Yu and Thornell 2002; Yu et al. 2003) had suggested remodelling.

Here they re-examined the same human muscle biopsies by transmission electron microscopy and immunoelectron microscopy. The ultrastructural hallmarks of DOMS — Z-disc streaming, smearing and disruption — were present, and were significantly more frequent in biopsies taken 2-3 days and 7-8 days after exercise than in control biopsies and those taken 1 h after exercise.

Four main types of change were seen: amorphous widened Z-discs, amorphous sarcomeres, double Z-discs and supernumerary sarcomeres. Immunoelectron microscopy confirmed that alpha-actinin, the main Z-disc protein, was absent from the Z-disc alterations and from the links of electron-dense material between Z-discs in longitudinal register.

The alterations went with an increase of F-actin, which lay within the strands of amorphous material, and of desmin, which lay in their less dense regions. The authors conclude that their results "strongly support" that these hallmarks of DOMS reflect an adaptive remodelling of the myofibrils.

Our reading: this is the base's named source for the remodelling school on PHYS-22's "damage" — a primary histology study, not a review, whose observations (more Z-disc changes days after exercise, with new F-actin and desmin and without alpha-actinin) are compatible with sarcomeres being added or rebuilt; that this is "adaptive remodelling as opposed to damage" is the authors' interpretation of images at three time points.

The size of the sample and the exercise cannot be read from the abstract.

Protocol

Design: histology of human muscle biopsies after eccentric exercise, compared with control biopsies; transmission electron microscopy and immunoelectron microscopy (alpha-actinin, F-actin, desmin). The biopsies are those of the authors' earlier immunohistochemical studies (Yu and Thornell 2002; Yu et al. 2003).
Time points: 1 h, 2-3 days and 7-8 days after exercise, and controls.
Sample: n = not given in the abstract (subjects, biopsies per time point and sex not stated).
Not given in the abstract: the eccentric exercise used, the muscle biopsied, how the controls were obtained, how frequencies were counted and tested, soreness or strength data, and funding (PubMed indexes the paper as "Research Support, Non-U.S. Gov't" without naming the funder) and competing interests.

Key Results

Observation (abstract)Result
Z-disc streaming, smearing and disruptionpresent; significantly more frequent at 2-3 days and 7-8 days than in controls and at 1 h
Main types of changeamorphous widened Z-discs; amorphous sarcomeres; double Z-discs; supernumerary sarcomeres
Alpha-actinin (main Z-disc protein)absent from the Z-disc alterations and from the electron-dense links between Z-discs in longitudinal register
F-actinincreased; present within the strands of amorphous material
Desminincreased; seen in the less dense regions of the alterations
Authors' conclusionthe alterations considered hallmarks of DOMS "reflect an adaptive remodeling of the myofibrils"

Mechanisms

Measured: the time course of Z-disc changes (more at 2-3 and 7-8 days than at 1 h or in controls), their four morphological types, and the distribution of alpha-actinin, F-actin and desmin in them.
Authors' interpretation: the pattern — new actin filaments and desmin, Z-disc material laid down between sarcomeres in register, supernumerary sarcomeres — is a remodelling of the myofibrils, adding or rebuilding sarcomeres, rather than mechanical damage.
Inference: static images at three time points cannot show whether a given change started as a mechanical disruption and was then repaired, or was built as such; both readings predict more changes days after exercise. The two readings may also both hold: strain may trigger the change and the change may be remodelling.
Inference: that changes were less frequent at 1 h, when mechanical strain would have acted, than days later fits a cellular process more than an immediate tear; the abstract does not give the frequencies.

Practical Application

Is the soreness after a new eccentric workout a sign of damaged muscle or of muscle rebuilding? One Umeå team read the fibre changes days later as remodelling, not damage; others read them as damage. Unresolved (C+, abstract).
What it is good for as a source: naming the remodelling school on "eccentric damage" with its own primary data: Z-disc changes peaking days after exercise, built with new F-actin and desmin and without alpha-actinin.
What it does not show: that no damage occurs after eccentric exercise, that soreness means adaptation, or anything about strength, recovery time or performance; the sample size is not in the abstract.
How a principle should cite it: "whether the fibre changes after voluntary eccentric exercise are damage or adaptive remodelling is disputed (Yu 2004, the authors' interpretation of biopsy histology)".
What it means for training: Not from this paper; the base's own practice does not change with the label: after unaccustomed or downhill work, space hard sessions and keep sprints and tests off sore legs (SYNTH-70, SYNTH-11); the repeated-bout effect protects whatever the label (PHYS-24).

Limitations

Abstract only. The full text is behind the publisher's paywall, with no Europe PMC copy and no repository copy listed by OpenAlex. The number of subjects and biopsies, the exercise, the muscle, the control group, the counts behind "significantly more frequent", and the funding and competing interests could not be read.
Re-analysis of earlier biopsies by the same group whose earlier work proposed the remodelling reading; no independent replication is part of this entry.
Static histology. Electron microscopy samples a tiny volume at fixed times; it cannot follow a structure over time or link it to function.
No functional outcome: no strength, soreness or performance data in the abstract.
How this entry was checked and changed
Why this level

a mechanistic laboratory study with no functional or performance outcome (the rubric's line); read from the abstract, which does not give the number of subjects, the exercise, the muscle or how controls were chosen. The full text was not read (paywalled; no open copy)