Understanding skeletal muscle adaptation is key to optimizing training and rehabilitation strategies, yet the causal links between training stimuli and muscle response remain unclear. This gap reflects the difficulty of observing multi-scale adaptations within the same muscle in vivo. Calf muscles, critical for propulsion and postural control, remain relatively under-studied, and long-term outcomes with intermediate stages of adaptation are rarely documented.

We investigated temporal and regional remodelling of the gastrocnemius medialis muscle during 12 weeks of eccentric training in six young, healthy adults. Participants trained on alternate days with progressive overload calf raise exercises. Muscle architecture and function were assessed at baseline, 6, and 12 weeks using 3D ultrasonography and dynamometry.

Group-level analysis revealed a 38% increase in peak plantarflexion torque at 90° (p < 0.01), while muscle volume, PCSA, fascicle length, and pennation angle showed no consistent changes. Individual response profiles varied: some participants showed longitudinal growth with longer fascicles and smaller pennation angles, others displayed radial growth with increased PCSA, while some exhibited minimal architectural change despite torque gains. Trends suggested shifts in fascicle length-angle torque relationships and altered tendon compliance in certain individuals.

By combining regional muscle morphology with functional outcomes over time, this study demonstrates the feasibility of tracking multi-scale adaptation in vivo. The pronounced inter-individual and region-specific variability highlights the need for tailored interventions that consider baseline architecture and regional strain patterns to optimize outcomes in training and rehabilitation.

The authors have declared no competing interest.

This work was supported by the European Research Council under the Starting Grant INTERACT (grant agreement No. 803035) and the Consolidator Grant ROBOREACTOR (grant agreement No. 101123866).

I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained.

Yes

The details of the IRB/oversight body that provided approval or exemption for the research described are given below:

the Ethics Committee of Twente (Enschede, The Netherlands) gave ethical approval for this work

I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals.

Yes

I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance).

Yes

I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable.

Yes

All data produced in the present study are available upon reasonable request to the authors