Aging Muscles Stay Springy: Active Older Adults Match the Young in Tendon Behavior

When an older adult lowers their heel slowly off a step, a remarkable piece of biological engineering goes to work inside their calf. The gastrocnemius medialis muscle, one of the two heads of the calf complex, lengthens under tension while the Achilles tendon stretches and recoils like a spring, absorbing and returning energy with every repetition. This eccentric phase of movement, where muscles actively lengthen while producing force, is a cornerstone of rehabilitation programs for older people, precisely because it builds strength with relatively low metabolic cost. Yet a fundamental question has lingered: does aging fundamentally change how the muscle and tendon interact during these lengthening contractions, and if so, should exercise prescriptions for older adults look different from those for the young?

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