Mixed martial arts (MMA) performance is characterised by high lactate production (9-20mmol∙L-1), heart rate (HR) >90%HRmax in between rounds and effort-pause ratio (E:P) ~1:1-4:1 over 9-25 mins(1,2). Performance is therefore likely influenced by the athlete’s ability to maintain energy resynthesis at a high percentage of their aerobic capacity (V̇O2max). There is currently no accepted method for measuring the physiological performance of MMA bouts. This study aimed to determine the relationship between the external load of MMA simulated competition and markers of cardiorespiratory fitness. Six human, male participants (25±3 years; 75±8kg; 178±9cm) were recruited for this study following institutional ethical approval. Each participant completed the following procedures at least once. Two participants completed each procedure twice, separated by a 7-week training program thus providing 8 distinct data points. Participants completed a treadmill based graded exercise test (GXT) to volitional failure to determine V̇O2max (ml∙kg∙min-1), ventilatory thresholds (VT1 and VT2, ml∙kg∙min-1) and running velocity at V̇O2max (vV̇O2max, km∙hour-1). Gas exchange variables were recorded via direct gas analysis using a Metalyzer 3B (Cortex Medical, Germany). Each participant also completed a 3x5mins simulated MMA bout within 4 days of the GXT. External load of simulated bouts was measured via Catapult Optimeye S5 accelerometers (Catapult Innovations, Australia) worn on the T3-4 vertebrae for the full duration of each simulated bout to record Playerload (PLdACC) and Playerload per minute (PLdACC∙min-1) in AU(3). Internal load in AU was calculated using sessional rating of perceived exertion (sRPE) for the overall simulated bout(4). Relationships between cardiorespiratory and performance variables were calculated using Pearson’s r correlation (Bayes factor [BF10] ≥3)(5) using JASP 0.16.0 (JASP Team, Netherlands). PLdACC (158±26 AU) was found to have a strongly supported, very large correlation (r = .824, BF10 = 12) with vV̇O2max (16±2km∙hour-1), a strongly supported, very large correlation (r = .828, BF10 = 13) with VT1 (28±5 ml∙kg.min-1); a moderately supported, very large correlation (r = .734, BF10 = 5) with VT2 (39±6ml∙kg.min-1); a very strong, very large correlation (r = .886, BF10 = 30) with VT1% of V̇O2max (49±11%), but not VT2% of V̇O2max (78±8%). PLdACC∙min-1 (11±2AU) shared a strongly supported, very large correlation (r = .820, BF10 = 12) with vV̇O2max; a strong, very large correlation (r = .819, BF10 = 12) with VT1; a moderately supported, very large correlation (r = .743, BF10 = 5) with VT2; a strong, very large correlation (r = .879, BF10 = 27) with VT1% of V̇O2max, but not VT2% of V̇O2max. There were no statistically relevant correlations between V̇O2max (50±5 ml∙kg.min-1), sRPE (119±24AU) and any other variable. Results indicate that MMA athletes capable of higher intensity work at their V̇O2max are also capable of performing more total work in simulated bouts. This is potentially influenced by enhanced metabolic thresholds. Cardiorespiratory training for MMA may be improved by aiming to achieve VT1 and VT2 at a higher % of V̇O2max.  The strong relationships presented support the in-field use of accelerometry in monitoring improvements in sport specific fitness of MMA athletes in response to training.