Rest‐to‐work and work‐to‐rest transients of interstitial <scp>PO₂</scp> in spinotrapezius muscle of young and old male rats

Muscle function declines with age. Since the primary energy source for contraction is aerobic, this study investigated age-related changes in muscle oxygenation dynamics to: characterize PO₂ transients during rest-work transitions, identify age-specific differences in oxygen delivery/utilization balance, and examine the relationship between interstitial and arterial oxygen tension (PO₂). Interstitial PO₂ was measured with a high-resolution stroboscopic phosphorescence quenching technique to map intra-contractile dynamics during changes in muscle activity-rest-to-work (RtW) and work-to-rest (WtR) in rats aged three (young) and 23 (old) months. RtW (τ_w) and WtR (τ_r) PO₂ transitions had lag periods and mono-exponential time constants. In young muscles, lag was 4 s, τ_w = 9.0 ± 3.7 s, and τ_r = 15.4 ± 3.9 s. For old, lag was also 4 s with increases to τ_w = 15.9 ± 3.5 s and τ_r = 41.4 ± 8.3 s. Resting PO₂'s were higher for young than for old (66.7 ± 13.7 vs. 60.2 ± 13.0 mmHg; p < 0.05). Work reduced PO₂ with a greater effect on old (42.5 ± 14.0 vs. 28.3 ± 16.5 mmHg; p < 0.05). Intra-contractile measurements revealed a spike in PO₂ (11 mmHg amplitude for >200 ms), which was absent in old. Further, sustained exercise in young showed a rising trend in PO₂, while old remained at nadir. The missing PO₂ spike in aged muscle contributes to reduced PO₂ during work and may explain age-related loss of endurance.