Muscle metabolism from near infrared spectroscopy during rhythmic handgrip in humans

The rate of metabolism in forearm flexor muscles (MO₂) was derived from near-infrared spectroscopy (NIRS-O₂) during ischaemia at rest rythmic handgrip at 15% and 30% of maximal voluntary contraction (MVC), post-exercise muscle ischaemia (PEMI), and recovery in seven subjects. The MO₂ was compared with forearm oxygen uptake (V̇O₂) [flow x (oxygen saturation in arterial blood-oxygen saturation in venous blood, S(a)O₂-S(v)O₂)], and with the ³¹P-magnetic resonance spectroscopy-determined ration of inorganic phosphate to phosphocreatine (P(I):PCr). During ischaemia at rest, the fall in NIRS-O₂ was more pronounced [76 (SEM 3) to 3 (SEM 1)%] than in S(v)O₂ [7] (SEM 3) to 59 (SEM 2)%]. During the handgrip, NIRS-O₂ was lower at 30% compared to 15% MVC [58 (SEM 3) vs 67 (SEM 3)%] while the S(v)O₂ was similar [29 (SEM 3) vs 31 (SEM 4)%]. Accordingly, MO₂ as well as P(I):PCr increased twofold, while V̇O₂ increased only 30%. During PEMI after 15% and 30% MVC, NIRS-O₂ fell to 9 (SEM 1)% and 'O', but the use of oxygen by forearm muscles was not reflected in S(v)O₂. During reperfusion after PEMI, the peak NIRS-O₂ was lowest after intense exercise, while for S(v)O₂ the reverse was seen. The discrepancies between NIRSO-O₂ and S(v)O₂, and therefore between the estimates of the metabolic rate, would suggest significant limitations in sampling venous blood which is representative of the flexor muscle capillaries. In support of this contention, S(v)O₂ and venous pH decreased during the first seconds of reperfusion after PEMI. To conclude, NIRS-O₂ of forearm flexor muscles closely reflected the exercise intensity and the metabolic rate determined by magnetic resonance spectroscopy but not that rate derived from flow and the arterio-venous oxygen difference.