# Oxygen delivery and consumption in aging skeletal muscle: Insights from an electric analogy model of PO 2 transients

## TL;DR

This paper uses an electrical circuit model to study how oxygen levels change in aging muscle during exercise, revealing age-related differences in oxygen delivery and metabolism.

## Contribution

The study introduces an explicit electrical circuit model to quantitatively analyze oxygen transport and metabolic limitations in aging skeletal muscle.

## Key findings

- The model successfully predicted asymmetric PO2 transition kinetics during rest-work transitions.
- Older muscle showed a 2.5-fold increase in oxygen delivery resistance compared to younger muscle.
- Older muscle exhibited a 5.4-fold compensatory reduction in metabolic resistance during exercise.

## Abstract

Mathematical models are essential for understanding oxygen transport and utilization during metabolic transitions. An electrical analogy concept proposed that exponential PO2 transients arise from interaction between oxygen storage capacitance and transport conductances, but lacked explicit circuit representation limiting quantitative predictions and experimental testing. We developed an explicit electrical circuit model with discrete resistive and capacitive components in physiologically defined topology to generate testable predictions for interstitial PO2 transition dynamics during rest‐work transitions in skeletal muscle. Circuit topology was constructed based on established physiological relationships in rat spinotrapezius muscle. The model equated oxygen partial pressure to voltage, oxygen flux to current, delivery and metabolic barriers to resistances, and tissue oxygen storage to capacitance. The model predicted that transition time constants should equal the product of capacitance and equivalent circuit resistance. Predictions were validated using interstitial PO2 measurements during rest‐work‐rest transitions. The model successfully predicted asymmetric transition kinetics, with time constant ratios matching steady‐state PO2 ratios. Application to young (3‐month) and old (23‐month) rats quantified age‐related changes: 2.5‐fold higher delivery resistance in old muscle with compensatory 5.4‐fold metabolic resistance reduction during exercise versus 3.1‐fold in young muscle. An explicit, validated electrical circuit model confirmed that PO2 transition kinetics are governed by capacitance‐resistance interactions and quantitatively separated delivery versus metabolic limitations in aging muscle.