Age-Related Changes in Skeletal Muscle Oxygen Utilization

Abstract

The cardiovascular and skeletal muscle systems are intrinsically interconnected, sharing the goal of delivering oxygen to metabolically active tissue. Deficiencies within those systems that affect oxygen delivery to working tissues are a hallmark of advancing age. Oxygen delivery and utilization are reflected as muscle oxygen saturation (SmO2) and are assessed using near-infrared resonance spectroscopy (NIRS). SmO2 has been observed to be reduced by ~38% at rest, ~24% during submaximal exercise, and ~59% during maximal exercise with aging (>65 y). Furthermore, aging prolongs restoration of SmO2 back to baseline by >50% after intense exercise. Regulatory factors that contribute to reduced SmO2 with age include blood flow, capillarization, endothelial cells, nitric oxide, and mitochondrial function. These mechanisms are governed by reactive oxygen species (ROS) at the cellular level. However, mishandling of ROS with age ultimately leads to alterations in structure and function of the regulatory factors tasked with maintaining SmO2. The purpose of this review is to provide an update on the current state of the literature regarding age-related effects in SmO2. Furthermore, we attempt to bridge the gap between SmO2 and associated underlying mechanisms affected by aging.

Keywords: aging; muscle metabolism; muscle oxygen saturation (SmO2); near-infrared resonance spectroscopy (NIRS).

Figure 1

Chromophores present in human tissue plotted against the absorption spectra (natural logarithm base). Water, fat, collagen, deoxygenated hemoglobin (Hhb), oxygenated hemoglobin (O₂Hb), melanin, and cytochrome oxidase (Cyt_ox) are observed in the 100 to 3000 nm region. The isosbestic point, the point at which the extinction coefficient of oxygenated and deoxygenated hemoglobin is equal, is plotted at 800 nm. Hhb and O₂Hb are highlighted as being the primary sources of obtained NIRS signals around the isosbestic point. Y-axis absorption coefficient units: μa [cm⁻¹]; X-axis wavelength units: λ [nm]. Reprinted with permission from ref. [45]. Copyright 2014 Elsevier.

Figure 2

Mean SmO₂ from rest to maximal exercise in younger (open circle) and older (solid circle) individuals. Statistically significant differences in SmO₂ were noted at rest and at each point up to VO_2max over the course of standard ramp exercise test. ♦ p < 0.01 between older and younger groups. Reprinted with permission from ref. [13]. Copyright 1999 Taylor & Francis.

Figure 3

Oxygen delivery to muscle is tightly regulated by blood flow. With advancing age, decreased vasodilation, endothelial cell dysfunction, reduced nitric oxide production, and decreased capillary supply and function are each responsible for reduced blood flow observed with aging and are attributed to mismanagement of ROS. Furthermore, content and function of the cellular machinery tasked with utilizing oxygen, mitochondria, are also impaired with aging. These age-related impairments in the oxygen delivery cascade ultimately lead to reductions in SmO₂ levels at rest, during exercise, and recovery from exercise.