The spring mass model of running proposes that with increases in running speed there will be decreases in ground contact time and concomitant increases in muscle tendon unit (MTU) stiffness. Previous studies examining these relationships have used either a limited number of running speeds or low numbers of participants. To date equivocal results have been found between changes in MTU stiffness and running speed. The purpose of this study was to examine changes in MTU stiffness with changes in running speed. After institutional ethical approval 18 physically active males (aged 20.8±0.9 years; stature 1.77±0.18 m; body mass 75.4±11.1 kg) completed the maximal anaerobic running test (MART). The MART test began at 11.9 km/h with each stage increasing by 1.37 km/h until participants could no longer complete a 20 s stage. Throughout the test ground contact time (Tc) was recorded at 1000 Hz using the Optojump Next system. Ground Tcs were used to calculate MTU stiffness using the method of Morin et al1. Regression analysis revealed near perfect relationships with running speed for both Tc (F 1, 8=295.101; p<0.001; R2=0.970; SEE=0.007 ms) and MTU stiffness (F 1, 8=725.844; p<0.001; R2=0.988; SEE=0.798 kN/m). This study has used more participants and a wider range of participants than previous literature. These results provide strong supporting evidence for the spring mass model of running.