Prescribing an appropriate adaptation period is an important consideration when using treadmills for locomotion studies. The present study investigated within-trial accommodation to running on a force measuring treadmill. Force measures were derived from vertical reaction force records of 16 runners; 8 were experienced in running on a treadmill. Three dependent measures, the peak impact force (F1), the loading rate of the impact force (LR), and the peak active force (F2) were tested for significant differences (p < 0.05) every 2 minutes of a continuous 20-min run using a two-factor ANOVA (group × time) with one repeated measure (time). Coefficients of variation (CV) for each dependent measure were tested for statistical significance in the same way. There were no significant differences in F1, LR, or F2 over any samples for the 20-min running trials. There were no significant changes in CV values for the duration of the run. The results from the present study suggest that after 30 seconds of treadmill running, there were no significant within-day accommodation effects on vertical force measures over a 20-min treadmill run. Variability between individuals in the consistency of force measures, however, could be a confounding factor. This lack of consistent response for individuals should be considered when exposing participants to experimental designs involving treadmill locomotion.