Purpose: Aging causes notable effects on skeletal muscle structure and function which accumulate over time to negatively impact quality of life in the elderly. To better understand how aging impacts the mechanical properties of muscle as a function of age, a series of studies were conducted, using mice as a model system, to assess responses to longitudinal and transversal stresses at a single muscle fiber resolution.

Methods: Muscle fibers isolated from primarily slow-twitch fibered muscle (soleus) and a primarily fast-twitch fibered muscle (extensor digitorum longus, EDL) were extracted from young (3-month, N=5) and old (21-month, N=5) animals. Passive mechanical tests (ramp, relaxation) were performed in the longitudinal direction at 50% strain, and Atomic Force Microscopy (AFM) tests were applied transversally. Longitudinal stresses (dynamic ( ), instantaneous (σ_{i_Ramp}) and static (σ_{s_Relax})) and the Young's modulus were measured in both directions.

Results: For both types of fibers, longitudinal stresses were shown to be significantly decreased (p < 0.001) in aged mice, with accentuated decreases observed for the EDL. Transversal AFM tests were used to elucidate the cartographies of elasticity and revealed no global differences between young and old animals for either fiber type. Regardless of age, decreased mechanical properties were universally observed for EDL fibers compared to soleus fibers.

Conclusion: These findings highlight the potential of AFM to quantify transversal stiffness of the endomysium and have begun to better delineate the impacts of aging on the mechanical characteristics of skeletal muscle at single fiber resolution.