What is the most likely reason the same five subjects were used for each study?
Researchers studied sprinting ability to better understand differences between individuals in performance. After completion of the first study, researchers performed two follow up studies to explore the movement economy of and physiological response to sprinting. The same five subjects were used for each study.
Five healthy adult subjects with similar body weight and height were familiarized with the sprinting technique. Each subject was instructed to sprint as fast as they could in a linear path for 20 yards. Infrared timing gates were placed at 5 yards (G1), 10 yards (G2), 15 yards (G3), and 20 yards (G4) into the route to record timing and later extrapolate speed.
The five healthy adult subjects from Study 1 were instructed to sprint as fast as they could in a linear path for 15 yards. An in-ground force plate was inserted on the route at the 10-yard point with the capacity to measure peak propulsive and braking forces in newtons. Propulsive forces have vectors in the +y direction and contribute to acceleration while sprinting. Braking forces have vectors in the -y direction and contribute to deceleration while sprinting.
The five healthy adult subjects from Study 1 and Study 2 were instructed to sprint as fast as they could in a linear path until they could no longer do so. Immediately after completion of their route blood samples were extracted from subjects. Blood samples were analyzed to measure concentration of blood lactate: larger quantities are a known biomarker for muscular fatigue.
It was more convenient given the subjects were already familiarized with the sprinting technique from Study 1
(C) Using the same five subjects for each study allows for within-subjects comparisons across studies. If different subjects were used variability between individuals could confound cross analysis of studies. As an example, it can be observed the slowest 20-yard sprinter had the lowest net peak propulsive force (peak propulsive force - peak braking force) although they had the highest peak propulsive force. If that subject did not return this observation could not be made.