# Suppose the peak propulsive force of Subject 3 had been lesser than their peak braking force between G3 and G4, what would this signify?

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.

Study 1

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.

Study 2

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.

Study 3

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.

The subject would be traveling backwards.

The subject would be traveling at a constant velocity.

The subject would be accelerating.

The subject would be decelerating.

(D) Subject 3 would have been decelerating. Regardless of the point in the sprint it occurred, having a greater peak braking force than peak propulsive force means that in that moment deceleration is occurring. Because the subject was still sprinting between G3 and G4 it can be assumed they were traveling in the +y direction at the time this occurred. If they were standing still they would be accelerating in the -y direction. Because they were in motion net force in the -y direction must first decelerate them until they reach a velocity of 0 yds/s before they are considered to be accelerating in the -y direction.

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