Near infrared spectroscopy-derived interstitial hydrogen ion concentration and tissue oxygen saturation during ambulation

The objective of this study was to determine whether walking and running at different treadmill speeds resulted in different metabolic and cardiovascular responses in the vastus lateralis (VL) and lateral gastrocnemius (LG) by examining metabolite accumulation and tissue oxygen saturation. Ten healthy subjects (6 males, 4 females) completed a submaximal treadmill exercise test, beginning at 3.2 km h⁻¹ and increasing by 1.6 km h⁻¹ increments every 3 min until reaching 85% of age-predicted maximal heart rate. Muscle tissue oxygenation (SO₂), total hemoglobin (HbT) and interstitial hydrogen ion concentration ([H⁺]) were calculated from near infrared spectra collected from VL and LG. The [H⁺] threshold for each muscle was determined using a simultaneous bilinear regression. Muscle and treadmill speed effects were analyzed using a linear mixed model analysis. Paired t-tests were used to test for differences between muscles in the [H⁺] threshold. SO₂ decreased (P = 0.001) during running in the VL and LG, but the SO₂ response across treadmill speeds was different between muscles (P = 0.047). In both muscles, HbT and [H⁺] increased as treadmill speed increased (P < 0.001), but the response to exercise was not different between muscles. The [H⁺] threshold occurred at a lower whole-body VO₂ in the LG (1.22 ± 0.63 L min⁻¹) than in the VL (1.46 ± 0.58 L min⁻¹, P = 0.01). In conclusion, interstitial [H⁺] and SO₂ are aggregate measures of local metabolite production and the cardiovascular response. Inferred from simultaneous SO₂ and [H⁺] measures in the VL and LG muscles, muscle perfusion is well matched to VL and LG work during walking, but not running.