A new study published in GeroScience by Juliette Lancelot of Nantes Université and colleagues set out to answer that question with unusual rigor. The research team compared the behavior of the gastrocnemius medialis muscle–tendon unit in young, physically active adults and in active older adults who were classified as non-frail or pre-frail. Crucially, the investigators matched the two groups on physical activity levels, a design choice that matters enormously. Most previous studies comparing young and old participants did not control for activity, making it impossible to distinguish the effects of aging itself from the effects of simply moving less. By ensuring comparable activity between groups, the French team could isolate what aging, in the presence of habitual exercise, actually does to the muscle–tendon machinery.

The technical execution of the study was demanding. Participants performed eccentric calf exercises under two very different conditions: highly controlled maximal and submaximal contractions on an isokinetic dynamometer, which holds the ankle at a fixed angular velocity while measuring torque, and a simple heel-drop exercise that anyone can perform at home on a step. Throughout each contraction, the researchers used ultrasonography to simultaneously track the length of individual muscle fascicles, the bundles of muscle fibers that run through the gastrocnemius, and the strain of the Achilles tendon, meaning how much the tendon stretched relative to its resting length. At the same time, they recorded ankle joint angle, the torque produced at the ankle, and the electrical activation of the calf muscles using surface electromyography, following established consensus guidelines for electrode placement and force estimation.

This simultaneous imaging and force monitoring allowed the team to examine what biomechanists call muscle–tendon decoupling. During many movements, the tendon stretches and recoils while the muscle fascicles stay nearly isometric, operating close to their optimal length for force production. This division of labor is one of the secrets of efficient human locomotion: the cheap, elastic tendon does much of the mechanical work while the metabolically expensive muscle fine-tunes its position. If aging disrupted this coordination, for example by making the tendon stiffer so that fascicles had to lengthen more, or by altering neural activation patterns, the consequences could ripple through balance, walking economy, and fall risk in older adults, since ankle muscle strength and triceps surae function are known to discriminate fallers from non-fallers.

The results, however, delivered a surprise. Across every condition tested, whether maximal or submaximal effort on the dynamometer, or the practical heel-drop exercise, and across every measured variable, the researchers found no statistically significant differences between the young and older groups. Tendon strain was similar. Fascicle length and fascicle velocity during the lengthening phase were similar. Achilles tendon stiffness, assessed separately, did not differ between the age groups either. Every comparison returned p values above 0.05, the conventional threshold for statistical significance. In other words, the older adults, all of whom maintained substantial habitual physical activity, produced muscle–tendon behavior during eccentric exercise that was essentially indistinguishable from that of their younger counterparts.

The finding challenges a body of earlier work that reported age-related alterations in tendon properties and fascicle–tendon interaction. Previous investigations had documented stiffer Achilles tendons, altered gastrocnemius behavior during walking, and changed fascicle–tendon coordination during hopping and drop jumps in older compared with younger participants. Reviews had also linked age-related changes in Achilles tendon stiffness to modifications in functional activities. But those studies, like much of the aging literature, typically compared sedentary or freely living older adults with young controls, leaving physical activity as an uncontrolled confounder. The new results suggest that many of the so-called aging effects on the muscle–tendon unit may in fact be effects of disuse, and that they are avoidable.

The implications for rehabilitation are potentially significant. Eccentric exercise is prized in geriatric rehabilitation because lengthening contractions generate high force with low energy expenditure, making them tolerable for people with limited cardiovascular capacity, and because they reliably stimulate tendon adaptation. Mechanically, tendons respond to repeated loading by adjusting their material properties and morphology, a plasticity that has been demonstrated in systematic reviews of exercise interventions across the lifespan. If active older adults already exhibit tendon strain, fascicle dynamics, and stiffness comparable to young adults, then eccentric training prescriptions may not need to be fundamentally re-engineered for age. The same loading principles that remodel tendon and muscle in the young appear to remain available to the older neuromuscular system, provided the person has been keeping the machinery in working order.

The study also carries a methodological lesson for the field. By testing both a laboratory dynamometer protocol and a home-based heel-drop exercise, the researchers bridged the gap between controlled biomechanics and real-world training. The heel-drop, in which a person stands on a step, rises onto the balls of the feet, and slowly lowers the heels below the step level, is among the most commonly prescribed eccentric exercises for calf and Achilles rehabilitation. Demonstrating that the underlying muscle–tendon mechanics behave similarly in the laboratory and at home strengthens the ecological validity of the findings and gives clinicians confidence that what is measured on a dynamometer translates to the living room floor. The team’s use of semi-automated ultrasound fascicle tracking software and careful monitoring of muscle activation further guards against the measurement artifacts that have complicated this literature.

There are, of course, boundaries to what the study can claim. The older participants were active and at most pre-frail, so the results speak to successful or well-maintained aging rather than to frail individuals or those with sarcopenia, whose muscle–tendon behavior may still differ. The measurements focused on the gastrocnemius medialis and Achilles tendon during a single class of movements, and other muscle groups or faster, more ballistic tasks could reveal age differences that slow eccentric exercise does not. The authors also note that their findings suggest, rather than prove, that maintaining high physical activity with increasing age mitigates the effects of aging on muscle–tendon behavior and muscle–tendon interactions; a longitudinal study that follows sedentary adults as they adopt or abandon exercise would be needed to establish causation directly.

Even with those caveats, the message is striking and, for an aging global population in which insufficient physical activity remains widespread, quietly urgent. The muscle–tendon unit does not appear to be doomed by the passage of time itself. In people who keep moving, the elastic partnership between calf muscle and Achilles tendon, the elegant decoupling that lets a cheap spring do the work of an expensive engine, persists into older adulthood largely intact. For clinicians designing eccentric rehabilitation protocols, the findings suggest that age alone should not dictate a fundamentally different mechanical approach. And for everyone else, the study adds to a growing case that the neuromuscular consequences we often blame on aging are, to a meaningful degree, negotiable, and that the currency for negotiating them is simply sustained, habitual movement.

Subject of Research: Age-related differences in gastrocnemius medialis muscle–tendon behavior during eccentric exercise in physically active young and older adults

Article Title: Gastrocnemius medialis muscle–tendon behavior during eccentric exercise in young and active older adults

Article References: Lancelot, J., Souron, R., Chapelet, G., Boureau, A.-S., Demoy, E., Rampal, M., Piriou, M., Cornu, C., & Crouzier, M. (2026). Gastrocnemius medialis muscle–tendon behavior during eccentric exercise in young and active older adults. GeroScience. https://doi.org/10.1007/s11357-026-02547-7

Image Credits: AI Generated

DOI: 10.1007/s11357-026-02547-7

Keywords: eccentric exercise, muscle–tendon unit, Achilles tendon, gastrocnemius medialis, aging, ultrasound imaging, tendon stiffness, fascicle dynamics, muscle–tendon decoupling, rehabilitation, older adults, GeroScience

Tendon-bone insertion injuries, which occur where soft tendon tissue anchors into hard bone, are among the most frequently encountered injuries in daily life and sports. Their clinical burden is growing annually, and restoring normal physiological function remains remarkably difficult. The reason lies in the anatomy: the tendon-bone junction is a graded transition zone that includes a specialized fibrocartilage layer, and unlike many tissues, it cannot regenerate itself faithfully after injury. Instead, the body fills the gap with disorganized scar tissue that has low biomechanical strength. That weakness translates directly into high postoperative re-rupture rates, making rapid, high-quality regeneration an urgent challenge in orthopedics, sports medicine, and tissue engineering.

The quality of tendon-bone healing depends heavily on how well the fibrocartilage layer regenerates at the interface, and macrophages, the versatile immune cells that flood any wound site, orchestrate much of that process. In the early phase of healing, M1-polarized macrophages dominate, secreting pro-inflammatory factors such as IL-1β and IL-6 that intensify local inflammation and recruit fibroblasts to the damaged site. Later, macrophages shift toward the M2 phenotype, releasing anti-inflammatory signals like IL-10 and Arg1 that dampen inflammation and promote tissue reconstruction. Previous work has shown that this transition from a pro-inflammatory to an anti-inflammatory microenvironment is crucial for moving from the inflammatory phase into the proliferative phase of repair, and that biologically inducing early M2 aggregation at the interface actively improves healing outcomes.

What makes the new study distinctive is its focus on macrophage metabolism. M2 macrophages rely on oxidative phosphorylation and fatty acid oxidation (FAO) for their energy supply, whereas activation of PPAR signaling and its regulatory genes increases FAO capacity. The research team, led by Xiaojun Ma and colleagues, hypothesized that if a compound could push macrophage metabolism toward FAO and oxidative phosphorylation, it would favor M2 polarization and thereby improve tendon-bone healing. Didymin was a natural candidate: earlier reports had documented its purgative, anti-inflammatory, and antioxidant properties, and prior metabolic tracing experiments had revealed that the compound enhances FAO rather than glycolysis. Crucially, didymin had already been shown to modulate the M1/M2 balance by converting pro-inflammatory M1-like macrophages into anti-inflammatory M2-like ones, without altering baseline M2 polarization.

To test the hypothesis, the researchers established a mouse model of tendon-bone healing by surgically severing the Achilles tendon near the calcaneal bone, scraping away the residual fibrocartilaginous layer, drilling a bone channel, and suturing the tendon stump back to the bone, a procedure that mimics the changes seen after rotator cuff reconstruction. Thirty-six male C57BL/6J mice were randomly assigned to six groups: a sham-operated control, an untreated model group, three didymin dose groups receiving 1, 2, or 4 milligrams per kilogram per day by oral gavage, and a positive-control group receiving the drug disulfiram at 50 milligrams per kilogram per day. Treatment began one day after surgery and continued daily for four weeks, with all protocols approved by the animal ethics committee of the People’s Hospital of Ningxia Hui Autonomous Region.

The structural results were striking. Micro-computed tomography scanning showed that high-dose didymin produced new bone volume at the interface comparable to that seen in the positive-control group, with significant increases in bone volume fraction, bone surface, trabecular number, and bone mineral density, along with significantly reduced trabecular separation compared with the untreated model group. Histological staining revealed that model-group animals suffered from disorganized collagen fibers, consolidated fibroblast nuclei, necrotic areas, and inflammatory cell infiltration at the junction, all of which were attenuated by didymin treatment. Serum biochemistry told a matching story: levels of osteocalcin, alkaline phosphatase, and calcium, all markers of bone formation activity, rose significantly in the treated animals.

Fibrocartilage regeneration, the critical determinant of healing quality, also improved. Safranin-O/fast green staining showed localized absence of the cartilage layer and abnormal chondrocyte proliferation in untreated mice, defects that didymin largely corrected. Immunofluorescence assays demonstrated that expression of the cartilage markers Sox9 and Collagen II, which dropped sharply at the injured interface, was significantly restored by the compound. The researchers also measured increased expression of TGF-β1 and TGF-β3, growth factors known to drive chondrogenesis and fibrocartilage formation, further supporting the conclusion that didymin actively promotes regeneration of the specialized transition tissue rather than merely reducing inflammation.

Flow cytometry and molecular profiling of the healing tissue revealed the immune mechanism at work. Untreated model mice showed elevated levels of F4/80-positive CD86-positive M1 macrophages and elevated IL-1β and IL-6 mRNA, while didymin administration at all three doses reduced these pro-inflammatory indicators. Medium and high doses simultaneously raised the abundance of F4/80-positive CD206-positive M2 macrophages and increased expression of Arg1 and IL-10. In parallel cell experiments using bone marrow-derived macrophages, didymin significantly reduced CD86 expression under M1-polarizing conditions and boosted levels of acetyl-CoA, the FAO enzymes Cpt1a and ACS, and the PPAR pathway proteins PPAR-γ and RXRA, indicating a metabolic reprogramming toward fatty acid oxidation.

The team then ran a decisive validation experiment using pharmacological blockers. When the PPAR-γ antagonist GW9662 or the fatty acid oxidation inhibitor etomoxir was added to didymin-treated M1 macrophages, the compound’s effects were substantially reversed: acetyl-CoA levels, CD206-positive M2 populations, and anti-inflammatory Arg1 and IL-10 expression all fell, while CD86-positive M1 populations and pro-inflammatory IL-1β and IL-6 rose again. This loss-of-function evidence strongly suggests that didymin drives macrophage polarization toward the M2 phenotype specifically by promoting the PPAR signaling-mediated FAO pathway, which in turn supports tendon-bone healing.

The authors acknowledge important limitations that temper immediate clinical translation. No biomechanical testing was performed, the antagonist and inhibitor experiments were conducted only in cell culture rather than in living animals, and all mice received local penicillin injections postoperatively, which could theoretically interact with didymin’s anti-inflammatory and antibacterial properties. Only male mice were studied, and sex is known to influence inflammatory responses, bone metabolism, and tendon healing, so verification in female animals is needed. The mouse Achilles model also cannot fully reproduce the biomechanics, vascular supply, and stress conditions of human rotator cuff attachment sites. Even so, the study lays a concrete foundation for didymin as a candidate therapy, linking a dietary flavonoid, immune cell metabolism, and musculoskeletal regeneration in a single mechanistic chain that future research can now build upon.

Subject of Research: The role of didymin in promoting tendon-bone healing through PPAR-mediated fatty acid oxidation and macrophage M2 polarization

Article Title: Didymin may Enhance Tendon‐Bone Healing Partly by Promoting PPAR‐Mediated Fatty Acid Oxidation and Macrophage M2 Polarization

Article References: Ma, X., Shen, J., Ma, J., Li, H., Li, Y., & Wan, J. (2026). Didymin may Enhance Tendon‐Bone Healing Partly by Promoting PPAR‐Mediated Fatty Acid Oxidation and Macrophage M2 Polarization. Immunity, Inflammation and Disease, 14(9), Article e70485. https://doi.org/10.1002/iid3.70485

Image Credits: AI Generated

DOI: 10.1002/iid3.70485

Keywords: didymin, tendon-bone healing, macrophage polarization, M2 macrophages, PPAR signaling, fatty acid oxidation, fibrocartilage, Achilles tendon, rotator cuff, bone regeneration, inflammation, natural compounds