Rest‐to‐work and work‐to‐rest transients of interstitial PO₂ 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 PO 2 transients during rest‐work transitions, identify age‐specific differences in oxygen delivery/utilization balance, and examine the relationship between interstitial and arterial oxygen tension (PO 2 ). Interstitial PO 2 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 ) PO 2 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 PO 2 's were higher for young than for old (66.7 ± 13.7 vs. 60.2 ± 13.0 mmHg; p < 0.05). Work reduced PO 2 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 PO 2 (11 mmHg amplitude for >200 ms), which was absent in old. Further, sustained exercise in young showed a rising trend in PO 2 , while old remained at nadir. The missing PO 2 spike in aged muscle contributes to reduced PO 2 during work and may explain age‐related loss of endurance.