Introduction: Skeletal muscle fibre typology (MFT) directly influences a muscle's contraction speed and fatigue profile both during and following exercise. MFT of a given muscle can vary dramatically between individuals and is largely determined hereditarily. Furthermore, MFT is associated with an athlete's suitability to a sport/event and is suggested as a determining factor for separating elite athletes, from their sub-elite counterparts in sprint and endurance sporting disciplines. Despite these important characteristics, there is limited research on the MFT of team-sport athletes and the impact this may have on performance. The invasive nature of the muscle biopsy, the gold standard of MFT determination, has shielded research investigating the MFT of professional team-sport athletes. Recently validated, non-invasive measures to estimate MFT, through the use of magnetic resonance imaging (MRI) and proton magnetic resonance spectroscopy (1H-MRS), may renew the interest of MFT in team-sport athletes. Australian football (AF) is a team sport that requires athletes to cover large distances at varying speeds and intensities. External load is a quantification of the work performed by an athlete (i.e. distance travelled, distance travelled at high speeds, etc.). It is important to quantify external loads achieved during match-play as this will provide relevant data on the specific movement profiles of competition and may assist in the development of AFL-specific conditioning programs, testing protocols as well as team recruitment and selection. Numerous contextual factors have an influence on the external loads a professional AF athlete achieves during match-paly; position, physical characteristics, and conceivably, their MFT. Preliminary findings in elite soccer athletes have suggested that athletes with a higher proportion of type II fibres will achieve reduced distance sprinting (> 6.67 m×s-1), have a reduced peak 1-min sprint period, and display a larger decrement in high-speed running (HSR) (> 4.17 m×s-1) from the first to the second half of match-play. Despite these findings stemming from an elite team-sport cohort, more research is required on a larger cohort, across an entire season, and in the sport of AF, due to the difference in the external loads of match-play. Further investigation may determine whether such findings are true across different team sports, such that a particular MFT may achieve higher external loads during match-play of some variable and therefore may be predictive of success in a sport, be better suited to a position, or require a higher degree of fatigue management during match-play. As such, further investigation is required to determine if similar findings can be made in AF, which can provide novel and impactful information to practitioners and researchers alike. Aim: The primary aim of this research study was to investigate the association between MFT and different variables pertaining to the external loads achieved during professional AF match-play. Secondary aims include; quantifying the MFT of professional AF athletes and quantify the external-loads of match-play during professional AF. Methods: Twenty-four professional AF athletes were characterised by position (smalls, talls, and hybrids), differences in anthropometrics (height, body mass, and body composition), and physical (muscular strength, power, and endurance) and physiological characteristics (MFT) were determined. 1H-MRS was used to quantify intramuscular carnosine content in the soleus and gastrocnemius muscles and expressed as a Z-score relative to an age and gender-matched non-athlete population to estimate MFT. A more negative Z-score is predictive of a higher proportion of type I fibres, while a more positive Z-score is predictive of a higher proportion of type II fibres. External load during matchplay was collected using portable global navigational satellite system (GNSS) units (n = 224 matches). Data were extracted following each session from proprietary software, onto a customised statistical software. External load was expressed as a peak period of matchplay, determined using the maximal mean method. Power law was used to calculate the decrement in intensity with time, expressed as intercept (peak value) and slope (decrement over time). R software was used to perform ANOVA and linear mixed model analyses to determine; i) differences in MFT between playing position groups ii) differences in external loads in match-play between positions, and iii) the association between match-play external load and MFT. Microsoft excel was used to collate data and calculate group means, as well as calculate 95 % confidence intervals from linear mixed models results. Results: MFT of the gastrocnemius and soleus was significantly associated with HSR (>4.17 m×s-1) distance (soleus; p = 0.013, gastrocnemius; p < 0.001), and HSR slope (soleus; p = 0.021, gastrocnemius; p < 0.001). The cohort presented with a large range of Z-scores across both gastrocnemius (min -1.83, max 0.81) and soleus (min -2.06, max 1.88), with a greater affinity for negative Z-scores (gastrocnemius; 17/24, 70.1 %, soleus; 19/24, 79.2 %). Talls spent a greater duration on the ground than both smalls and hybrids (p = 0.027), while hybrids averaged 16 m×min-1 greater speed than talls (p = 0.001) as well as ~800 m more distance covered in HSR (p < 0.001). Hybrids had the largest intercept for speed, HSR, and acceleration, however only significantly greater than talls for speed intercept (p = 0.006). Conclusion: MFT appears to influence a professional AF athlete's HSR distance achieved within a match, as well as their HSR slope across peak periods of play. A high proportion of negative Z-scores were found within the cohort. These findings are similar to previous research investigating the influence of MFT on variables describing high intensity running in elite soccer athletes. These cross-sport similarities strengthen the rationale to continue investigations on the effect of MFT on team sport athletes. The high proportion of negative Z-scores reflects the high endurance capacity and load tolerance that the sport of AF demands, conceivably suggesting that athletes with high proportions of type II fibres may not fit the ideal phenotype for the sport. As has been established previously, external loads of match-play exhibited positional differences, with hybrids typically achieving the greatest loads. This information highlights the need for practitioners to individualise training prescription on a positional basis for both load and intensity. Future research should investigate the association between MFT and team sport athlete's ability to recover from exercise, which seems to be the likely cause of the difference observed in MFT for HSR. This information will empower practitioners to better manage athletes' recovery within a match and throughout a pre-season period.