Obstructive sleep apnea (OSA) is characterized by recurrent arousals and episodic oxyhemoglobin desaturations during sleep. Patients who suffer from OSA have reduced exercise performance, although the mechanisms for this reduction are not clear. Purpose: To examine the tissue oxygen saturation (StO2) of the tibialis anterior (TA) muscle in adults with OSA during a graded exercise test. Methods: Near-infrared spectroscopy (NIRS) was used to measure StO2. All participants walked at 2.5 mph on a treadmill for six stages of progressively steeper grades (0%, 3%, 6%, 9%, 12%, 15%) or until volitional fatigue. Each stage lasted three minutes and STO2 was averaged over the last minute of each stage. The severity of OSA was determined using an at home sleep testing system and characterized by the apnea-hypopnea index (AHI) and time during sleep spent under 90% oxygen saturation (T90). Body composition was measured using dual energy X-ray absorptiometry. Results: Nineteen adults (men:10; women:9; age=43+9 years, BMI=34.4+8.1 kg•m-2) enrolled in the study. Thirteen subjects completed the exercise protocol, whereas six subjects stopped after completing Stage 4 (2.5 mph @ 9%). Trunk fat was lower (45+21 kg vs. 68+18 kg, p=0.028) and T90 was less (3.9+4.0 min vs. 0.8+2.4 min, p=0.048) in those who completed the protocol compared to the non-completers. No difference between groups in AHI (p=0.486) or STO2 at Stage 4 (p=0.633) were observed. Among the non-completers, those with the lowest STO2 at Stage 4 had greater T90 compared to those with the highest ST O2 at Stage 4 (7.3+2.5 min vs. 0.5+0.4 min, p=0.039). However, no differences in AHI (p=0.513), or trunk fat (p=0.211) were observed. Discussion: These data suggest greater hypoxia during sleep, rather than the total number of apneic events, limits exercise performance and suggests that treating OSA could enhance exercise tolerance in OSA patients.