Roger Fielding directs a muscle physiology laboratory at Tufts University. In a research study he collaborated on with Walter Frontera and others, the team conducted muscle biopsies on the same cohort of older volunteers on two separate occasions, 8.9 years apart, and assessed both whole-muscle strength and the contractile characteristics of individual muscle fibers. At the second assessment, whole-muscle strength and specific force were significantly reduced. As stated in a Tufts University news announcement discussing this research, Fielding noted that the individual fast-twitch and slow-twitch fibers were producing approximately the same force as they had nearly a decade prior. This is an account of research involving humans, not a regimen for training, and the information provided below should not be interpreted as medical guidance for individuals with specific conditions.
This result was at odds with a well-known fact: older individuals experience a decline in muscle power. If the fibers themselves weren’t weakening, some other factor must be at play.
What really changes
The conclusion researchers consistently reach is that individuals are not losing fiber quality but rather losing fibers, particularly a specific type at a greater rate. Skeletal muscle is composed of slow-twitch fibers, designed for prolonged, low-intensity exertion, and fast-twitch fibers, tailored for rapid, powerful contractions. As people age, fast-twitch fibers tend to atrophy and vanish more quickly than slow-twitch fibers.
Fielding’s team quantified part of this in an earlier, independent 12-year longitudinal investigation of older men, spearheaded by Walter Frontera and featured in the Journal of Applied Physiology. Over that span, isokinetic strength in both the knee and elbow declined by approximately 20 to 30 percent, across both slow and rapid movement speeds, accompanied by noticeable reduction in thigh muscle cross-sectional area. Such losses accumulate over a lifetime and do not affect every type of movement uniformly.
It’s primarily the muscle, not the signal directing it
A 2024 study published in the Journal of Applied Physiology, led by David Wrucke and Christopher Sundberg’s team at Marquette University, sought to determine how much of the power loss can be attributed to the muscle itself versus the nervous system’s capacity to activate it, across older and much older adults of both genders. Their findings suggested that the muscle itself was the key factor. Voluntary activation, or how effectively the brain can engage the muscle during maximal exertion, remained close to its upper limit in all age categories within their study, explaining only a small, inconsistent portion of the variance in power, and primarily in women. In contrast, what aligned closely with the loss of power was the muscle’s contractile properties: the speed at which an electrically induced twitch generates force, reflecting the tissue characteristics rather than the strength of the incoming signal.
A noteworthy detail from that study deserves attention. The researchers recorded daily step counts using accelerometers, as a rough proxy for the physical activity levels of each participant in their everyday lives. Step count accounted for only about 3 percent of the differences in peak power. Merely maintaining general activity did not significantly contribute to preserving this specific capability.
A distinct line of inquiry hints at the wiring
This raises an unresolved question: if the muscle’s contractile function is what’s changing, what is driving this change, and why is it occurring more in fast-twitch fibers? Another strand of research, including the master athlete comparison outlined below, has pointed to motor unit remodeling, the gradual diminishment of specific, larger motor neurons that control fast-twitch fibers, as a likely upstream factor. Fast-twitch fibers that cease to consistently receive signals are destined to disappear or potentially be supplanted by adjacent slow-twitch neurons, regardless of their own tissue properties in the interim.
What distinguishes those who retain their fast fibers
An additional study, conducted by Tiril Tøien and collaborators and published in the Journal of Applied Physiology, compared master athletes over the age of 70 who engaged in lifelong strength training versus those who participated in lifelong endurance training against both recreationally active older adults and younger active individuals, revealing a clear divide. The strength-trained group exhibited a distribution and clustering of fast-twitch fibers that closely resembled that of the younger comparison cohort, along with