The Longevity of Muscle: Evaluating the Efficacy of Strength vs. Endurance Training in Preserving Muscle Function into Old Age

As the global population increasingly skews older, the study of aging and its impact on muscle composition becomes not just a scientific curiosity but a pressing public health concern. The transformation that our muscles undergo with age is both subtle and profound, marked by a decline in strength and a decrease in muscle mass. This gradual loss of muscle mass and strength has far-reaching longevity implications.

As we get older we see a transformation in muscle composition. Specifically, we see a reduction in both size and prevalence of type II, or fast-twitch, muscle fibers. Simultaneously, we see an increase in type I, "slow-twitch," or endurance muscle fibers [1, 2].

The shift has many implications for longevity and healthspan. As we transition from power towards endurance we become predisposed to sarcopenia—the age-related loss of muscle mass and function [3]. This muscle loss not only affects mobility, increasing the risk of falls and dependence among the elderly but has a number of dangerous consequences for our metabolic health.

As muscle composition changes we see a diminishment of neuromuscular function—a phenomenon known as muscle fiber denervation. Every muscle fiber connects to the central nervous system through motor neurons, which initiate muscle contractions. Aging brings about a decrease in neural input to muscle fibers, leading to denervation. This process contributes to the atrophying of muscle. It can be primarily attributed to either a decrease in the number of motor units available or a diminished efficiency in their recruitment.

The impact of denervation is especially severe in type II fast-twitch muscle fibers, which require regular neural stimulation to maintain their size and strength. Without consistent physical activity, the neuromuscular system may reduce the maintenance of these motor units, leading to decreased muscle mass and strength [4].

While the detrimental effects of muscle fiber denervation and the consequent atrophy, especially in type II fibers, are well-documented, the potential for specific exercise regimens to reverse or mitigate these changes offers some hope of decelerating and potentially reversing these age-related muscle dynamics.

The prevailing wisdom suggests strength training as a potent countermeasure to the decline of muscular strength and mass with age. However, the intricate dynamics of how different exercise modalities—strength versus endurance training—affect neuromuscular function and overall muscle health remain largely unknown.

A recent study featured in the Journal of Applied Physiology set out to address these gaps in our understanding [5]. By comparing the long-term effects of strength and endurance training on muscle composition, strength, and neuromuscular function—among other parameters—the study provides insights that question current understanding and suggest new strategies for managing aging's effects on muscles. The study not only provides the specific benefits and limitations of each training modality, it underscores that "normal" aging is not an immutable fact but rather a modifiable outcome deeply influenced by lifestyle choices.

The Study's Design

This study was designed to understand the differential impacts of long-term strength versus endurance training on muscle composition and function in older men. The study's cohort comprised 42 men, including 10 older strength-trained masters athletes and 8 older endurance-trained athletes, with both groups averaging 75 years of age. These cohorts were benchmarked against a control group of older, generally active adults not engaged in specific training routines, and a younger reference group, averaging 25 years old, to provide a comparative baseline across age and training specificity.

To dissect the nuances of muscle health and performance, the researchers employed a multifaceted testing approach.

A pivotal component was the assessment of maximal strength through the 1-repetition maximum test, coupled with evaluations of force generation speed in the legs. These measures offer insights into the functional capabilities of the participants, reflecting both the power and rapid response of their muscles.

Beyond functional testing, the study delved into the microscopic evaluation of muscle tissue. Through biopsies taken from the thigh muscle, researchers conducted a detailed analysis of muscle fiber composition. This included quantifying the types and sizes of muscle fibers, alongside markers indicative of muscle repair and damage.

Two critical markers examined were NCAM (Neural Cell Adhesion Molecule)-positive fibers and neural clumps. NCAM-positive fibers serve as indicators of the muscle's regenerative capacity, while neural clumps signal muscular deterioration linked to neuropathic issues. These microscopic assessments are instrumental in uncovering the cellular and molecular underpinnings of how aging and consistent exercise influence neuromuscular health.

Additionally, the study extended its scope to encompass overall physiological fitness by measuring thigh muscle mass and conducting VO2 max tests. The latter, a gauge of peak oxygen uptake during exertion, provides a holistic view of cardiovascular and pulmonary efficiency in tandem with muscular health.

Higher VO2 max values are associated with a more efficient heart and circulatory system, which can pump more blood and deliver oxygen more effectively to the muscles and other tissues. This efficiency is crucial for reducing the risk of cardiovascular diseases, including heart attack, stroke, and hypertension. Numerous studies have established VO2 max as a powerful predictor of all-cause mortality. Individuals with higher VO2 max levels tend to have a lower risk of death from all causes, including cardiovascular diseases and cancer.

Together the comprehensive approach of the study design provides a strong foundation to understand the interplay between muscle integrity, functional capacity, and the overall systemic health status across different exercise modalities and age groups.

The Comparative Impact on Muscle Performance

The findings from the study paint a compelling picture of the impact of long-term strength and endurance training on muscle performance in aging. The data showed that older adults who have dedicated themselves to strength training not only outperform their endurance-trained and non-athlete counterparts in terms of muscle strength and the rate of force development but intriguingly, their performance in these metrics closely mirrors that of significantly younger, moderately active adults.

This observation underscores the potent "anti-aging" effect of consistent strength training on muscle function, suggesting that such an exercise regimen can significantly counteract the decline in muscular strength and responsiveness typically associated with aging.

On the other hand, the endurance-trained older adults present a fascinating profile themselves. The older endurance-trained cohort showed enhanced muscle strength and a faster rate of force development compared to their recreationally active counterparts, underscoring the unexpected muscle function benefits of endurance training, even in an age group traditionally focused on aerobic fitness.

Yet, when compared to the younger adult group, these endurance-trained seniors match in rate of force development but fall short in absolute strength. This distinction provides some insight into the exact adaptions of endurance training—promoting rapid force application but not necessarily maximizing strength to the levels achieved through strength training.

Rate of Force

VO2 Max

The analysis of VO2 max results from the study provides insights into the varying aerobic capacities among different age and training demographics. As a reminder, VO2 max, the definitive measure of an individual's peak oxygen consumption during maximal exercise, serves as an essential indicator of cardiovascular fitness.

The data from the study reveal a notable superiority in aerobic capacity among the older endurance-trained adults, with VO2 max readings averaging 47.5 ml/kg/min. This figure significantly exceeds those observed in both their strength-trained counterparts and the recreationally active older adult group, who recorded VO2 max values of 33.2 ml/kg/min and 35.2 ml/kg/min, respectively. These disparities highlight the targeted benefits and adaptations of prolonged endurance training, particularly its capacity to significantly boost cardiovascular efficiency and stamina.

When these findings are contrasted with the VO2 max levels of the younger, moderately active adults, who achieved an average of 63.1 ml/kg/min, the complexity of interpreting aerobic fitness across age and activity levels deepens. This superior performance by the younger cohort, despite not being engaged in specialized endurance training, points to a higher innate aerobic capacity among younger individuals.

This disparity could be attributed to age-related physiological enhancements or perhaps an underappreciation of the younger group's actual level of physical activity. This could indicate a limitation of the study in precisely accounting for the level of physical activity among the younger cohort. However, the elevated VO2 max in this group suggests that youth may naturally possess a higher baseline for aerobic fitness, which is further modifiable through targeted training.

Muscle Fiber Composition

The analysis of muscle fiber composition across different groups in the study further reveals the impact of long-term exercise modalities on the aging musculature. The distribution of muscle fiber types, particularly between type I and type II fibers, is a key determinant of muscular function, with type I fibers being more resistant to fatigue and type II fibers being associated with higher force production and faster fatigue rates.

The findings reveal that older adults engaged in strength training have preserved a higher percentage of type II, fast-twitch, muscle fibers (41%) compared to their endurance-trained peers (34%) and the recreationally active older control group (30%). This preservation is notably similar to the percentage observed in younger adults (42%), suggesting that strength training plays a critical role in maintaining the muscle fiber composition akin to that of a younger physiological state.

This correlation between strength training and the maintenance of type II fibers supports the notion that exercise specificity can influence the aging muscle's structural composition, potentially offsetting the natural decline in fast-twitch muscle fibers associated with aging.

This is an important insight for longevity. It is important to note that muscle serves as a critical reservoir for glucose. Maintaining muscle mass not only enhances strength and mobility but also improves metabolic rate and insulin sensitivity.

Conversely, the endurance-trained older adults exhibited a higher proportion of type I muscle fibers (61%), which is consistent with the adaptations expected from prolonged endurance training. This adaptation aligns with the endurance training's focus on enhancing muscular endurance and oxidative capacity, traits that are characteristic of type I fibers.

The older control group and the young adult control group had type I fiber percentages of 65% and 49%, respectively, highlighting the variance in fiber type distribution that can result from different levels of physical activity and aging.

These contrasting profiles between strength and endurance-trained individuals highlights the plasticity of the human muscle in response to different types of physical training. Strength training's association with a higher percentage of type II fibers underscores its effectiveness in enhancing aspects of muscular function that rely on power and rapid force generation. On the other hand, the endurance training-induced shift towards a higher percentage of type I fibers reflects an adaptation towards improving sustained muscular endurance and efficiency.

Muscle Fiber

The study also analyzed the distribution and health of type IIa and type IIx muscle fibers, as well as the cross-sectional area of various fiber types, providing insights into the adaptive responses of muscle to different exercise modalities and the aging process.

Type IIa fibers, characterized by their fast oxidative glycolytic capacity, serve as a bridge between the pure endurance capabilities of type I fibers and the explosive power of type IIx fibers.

The observation that older adults engaged in long-term strength training exhibit a higher proportion of type IIa fibers, aligning closely with the percentages found in younger, active individuals, suggests a protective or adaptive effect of strength training against the age-related shift towards slower, more oxidative muscle fiber types. This aligns with findings from studies on the effects of resistance training on endurance capacity and muscle fiber composition in young top-level cyclists, which highlight the role of resistance training in preserving fast-twitch muscle fiber composition in aging populations as well a more general conferred benefit [6].

In contrast, the distribution of type IIx fibers—known for their fast glycolytic properties—remained consistent across all groups examined. This uniformity across age and training types indicates a resilience of these fibers to the effects of aging and specific exercise regimes, or possibly, it suggests a ceiling effect of adaptation, where the intrinsic properties of these fibers limit the extent of change in response to external stimuli.

Furthermore, the study reports no significant differences in the cross-sectional area of type I, type II, type IIa, and type IIx fibers across the groups. This finding is intriguing, as it suggests that despite the variations in fiber type distribution, the overall muscle fiber size remains comparable.

This could imply that the hypertrophic, muscle building, response to different types of training or the atrophic, muscle wasting, pressures of aging may affect muscle fibers uniformly. This is a concept supported by research on muscle composition, which indicates that muscle fiber atrophy and hypertrophy mechanisms may operate across fiber types in a more homogenized manner than previously thought [7]. When we waste muscle we do so uniformly across muscle fiber types.

Muscle mosaic

Interestingly, the analysis of muscle fiber atrophy reveals a more pronounced loss in older endurance-trained and control groups compared to their strength-trained peers and the younger cohort.

This suggests that endurance training, while supremely beneficial for cardiovascular health and metabolic efficiency, may not confer the same protective effects against muscle fiber atrophy as strength training.

The presence of atrophied fibers in these groups underscores the importance of resistance training across all age groups. Previous studies have emphasized the role of mechanical load in maintaining muscle fiber integrity, further corroborating the idea that strength training may offer unique benefits in preserving muscle health [8].

Neuromuscular Impacts

The complexity of muscle aging and adaptation is further underscored by examining markers of muscle denervation, particularly through the lens of nuclear clumps and neural cell adhesion molecule (NCAM)-positive fibers. These markers offer a window into the neural integrity of muscle tissue, providing insights into the consequences of different exercise modalities and the inexorable march of time on muscular health.

Nuclear clumps, aggregates of nuclei that typically emerge in response to denervation and subsequent reinnervation processes, serve as a poignant indicator of ongoing neuromuscular remodeling. The observation that older adults, both those engaged in strength training and those in the control group, exhibit a higher incidence of nuclear clumps compared to their younger counterparts underscores the impact of aging on neuromuscular connectivity.

Contrastingly, the diminished presence of nuclear clumps in older adults who have consistently engaged in endurance training offers an important insight. This group's deviation from the aging trend not only suggests a resilience against the typical neuromuscular deterioration seen with aging but also hints at a protective benefit afforded by regular aerobic exercise.

The findings align with previous research, which posits that endurance training may bolster neuromuscular efficiency, offering a buffer against the harmful effects of age-related denervation. This protective mechanism potentially operates through enhanced circulation, better oxidative capacity, and improved neural plasticity, creating a more robust neuromuscular system even in the face of aging [9].

The apparent neuromuscular benefits of endurance training underscore its value not just for cardiovascular health but also for maintaining neuromuscular integrity and function into older age. These insights suggest that consistent aerobic exercise could play a crucial role in counteracting the neural challenges of aging

The examination of NCAM-positive fibers across the study's cohorts highlights the dynamics of muscle regeneration and neural plasticity. NCAM-positive fibers are indicative of muscle cells that are actively involved in the regeneration process.

NCAM plays a role in neural plasticity and is involved in the reinnervation of muscle fibers following nerve damage. The molecule facilitates the establishment of neuromuscular junctions, ensuring that regenerated muscle fibers receive appropriate neural input. Proper muscle innervation is essential for muscle contraction, strength, and coordination, highlighting the importance of NCAM in maintaining neuromuscular health.

The consistent detection of NCAM-positive fibers among all groups suggests a fundamental level of ongoing muscular repair and regeneration that is seemingly impervious to the variables of age or specific training types. This uniform distribution of NCAM-positive fibers, irrespective of the participants' age or exercise regimen, underscores the resilience and adaptability of muscle tissue. It suggests that the mechanisms underpinning muscle repair and regeneration maintain their functional capacity across the lifespan, undeterred by the differing demands of strength versus endurance training.

The absence of significant differences in NCAM expression across groups reinforces the concept that muscle tissue possesses an intrinsic ability to respond to injury or stress through regeneration and repair, a capacity that does not diminish significantly with age or vary markedly with different forms of physical activity.

Interpreting These Results

The results of the study highlight the differential impacts of strength versus endurance training on neuromuscular health. While strength training appears to offer significant benefits in terms of muscle fiber composition and atrophy resistance, endurance training may provide a distinct advantage in preserving neuromuscular connectivity and efficiency in older adults.

The evidence suggesting that lifelong strength training offers superior preservation of neuromuscular function and strength, when compared to endurance training and recreational activities, aligns well with the principles of exercise specificity. It makes sense that those who train specifically for strength and size exhibit superior outcomes in these domains compared to their age-matched peers—no news here.

Interestingly, strength-trained older adults not only showcase enhanced outcomes in strength and neuromuscular function but also retain several physiological properties akin to adults half their age. This observation is a testament to the profound benefits of strength training, underscoring its significance in maintaining functional capacity. Given the critical role of strength, rate of force development, and neuromuscular function in everyday activities, it reinforces that strength training as an indispensable component of healthy aging and longevity.

At the same time, the importance of endurance training in sustaining cardiorespiratory fitness into older age cannot be overlooked. While strength-trained seniors, who occasionally engage in aerobic activities, displayed VO2 max levels comparable to their recreationally active counterparts, they did not achieve the heightened aerobic capacity observed in endurance-trained adults.

The study sheds light on the significant advantages of endurance training in maintaining neuromuscular connectivity and efficiency among older adults, which could be attributed to improved circulation and neural plasticity.

These findings underscore the potential of endurance exercises, such as running, cycling, and swimming, to counteract the age-related deterioration of neuromuscular functions.

By enhancing blood flow and fostering a more adaptable neural network, endurance training supports the preservation of muscle function and coordination. This aspect of physical fitness is crucial for sustaining everyday activities and overall quality of life as one ages.

The study suggests that incorporating regular endurance training into exercise routines could be a key strategy for promoting healthier aging, highlighting its role in not just cardiovascular health but also in safeguarding the integrity of the neuromuscular system.

This divergence highlights the specificity of exercise modalities and their benefits. The results reinforce that no single training regimen offers a panacea for the multifaceted declines associated with aging—it’s imperative to engage in both strength and endurance training.

Interestingly, despite their specialized training, none of the older groups, including the endurance-trained athletes, could fully arrest the decline in aerobic capacity, highlighting a universal trend of diminished function compared to younger, moderately active individuals. This serves as a reminder of the inexorable nature of aging, yet it also provides a path forward: a balanced regimen of strength and endurance training appears to be the most efficacious strategy for aspiring "super-agers."

It becomes increasingly clear that "normal" aging is not an immutable fact but rather a modifiable outcome deeply influenced by lifestyle choices. The decline in muscle strength, heart health, and aerobic fitness often attributed to aging is, to a significant extent, a byproduct of decreased activity levels. The narrative emerging from this study and others champions the idea of an active lifestyle during middle and old age as a powerful countermeasure to traditional aging trajectories, redefining what it means to age healthily.