Muscle metabolism from near infrared spectroscopy during rhythmic handgrip in humans

Abstract

The rate of metabolism in forearm flexor muscles (MO₂) was derived from near-infrared spectroscopy (NIRS-O₂) during ischaemia at rest rhythmic 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 (O₂) [flow × (oxygen saturation in arnterial blood-oxygen saturation in venous blood, S _aO₂−S _vO₂)], and with the ³¹P-magnetic resonance spectroscopy-determined ratio 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 _vO₂ [71 (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 _vO₂ was similar [29 (SEM 3) vs 31 (SEM 4)%]. Accordingly, MO₂ as well as P_I:PCr increased twofold, while V˙O₂ increased only 30%. During PEMI after 15% and 30% MVC, NIRS-O₂ fell to 9 (SEM 1)% and “0”, but the use of oxygen by forearm muscles was not reflected in S _vO₂. During reperfusion after PEMI, the peak NIRS-O₂ was lowest after intense exercise, while for S _vO₂ the reverse was seen. The discrepancies between NIRS-O₂ and S _vO₂, 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 _vO₂ 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.