A linear relationship between skeletal muscle venous (C T ven ) and oxygenated (ΔHbMbO 2,N ) or deoxygenated (ΔHHbMb N ) near-infrared spectroscopy (NIRS) signal suggests a main Hb contribution to the NIRS signal. However, experimental, and computational evidence supports a significant contribution of Mb to the NIRS. Venous and NIRS measurements from a canine model of muscle oxidative metabolism (Med. Sci. Sports Exerc. 48(10):2013-2020, 2016) were integrated into a computational model of muscle O 2 transport and utilization to evaluate whether the relationship between venous and NIRS oxygenation can be affected by a significant Mb contribution to the NIRS signals. The mathematical model predicted well the measure of the changes of C T ven and NIRS signals for different O 2 delivery (blood flow, arterial O 2 content) in muscle at rest (T1, T2) and during contraction (T3). Furthermore, computational analysis indicates that for adequate O 2 delivery, Mb contribution to NIRS signals was significant (20-30%) even in presence of a linear C T ven -NIRS relationship; for a reduced O 2 delivery the non-linearity of the C T ven -NIRS relationship was related to the Mb contribution (50%). In this case (T3), the deviation from linearity is observed when O 2 delivery is reduced from 1.3 to 0.7 L kg -1 min -1 (C T ven textless 10 mLO 2 100mL -1 ) and Mb saturation decreased from 85 to 40% corresponding to an increase of the Mb contribution to ΔHHbMb N from 15 to 50% and the contribution to ΔHbMbO 2,N from 0% to 30%. In contrast to a common assumption, our model indicates that both NIRS signals (ΔHbMbO 2,N, ΔHHbMb N ) are significantly affected by Hb and Mb oxygenation changes.