Rugby league is a collision sport that is intermittent in nature. Players require highly developed maximal aerobic power, muscular strength and power and speed and agility, to perform through an eighty minute game. In contrast to starting players, interchange players often perform 2 short (∼20 minutes) and more intense play. The aim of the investigation was to further develop a rugby league match simulation protocol for interchange players and determine the external validity and test – retest reliability among semi-professional rugby league players. Our modified Rugby League Movement Simulation Protocol for Interchange players included more realistic contact elements during the ‘Ball in Play’ portion of testing. Eight semi-professional Championship Rugby League players (age=26±3 years, height=180±3.4 cm, body mass=102.3±11.5 kg) performed the RLMSP-i twice, separated by one week. The protocol involved walking, jogging, running, sprinting and contact elements, based upon GPS data recorded during Super League matches. Movement was recorded using 5 Hz GPS device and heart rate (HR) was recorded by HR monitor and later calculated as a percentage of each participant's age-predicted maximum. Based on previous research, the threshold for smallest worthwhile change (SWC) was 0.2 SDs. The coefficient of variation (CV±95% CL) of the RLMSP-i for Heart Rate Peak (SWC=2.54) was 0.5 (±0.7), RPE (SWC=0.22) 4.5 (±6.6), peak speed (SWC=0.31) 8.6 (±11.0), distance (SWC=1.58) 15.6 (±16.0), and ≥HIT running m/min (SWC=0.47) 23 (±20). In only one case (Heart Rate Peak) case was the CV smaller than the smallest worthwhile change, whilst in only Heart Rate Peak was very large change (10 x SWC) larger than the CV observed. These data suggest that our modified protocol produces comparable responses in heart rate and ≥HIT m/min as observed during Super League match performances, but lower values of total m/min and peak speed. Due to the presence of an enhanced contact element, contributing to the overall physiological load of the participant, it provides a reliable measure of physical performance for Heart Rate Peak, Heart Rate and ratings of perceived exertion, but not for distance, ≥HIT m/min or peak speed.