Which of the following is a major flaw in the design of Study 2?

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.

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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.

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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.

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Subjects ran 15 yards instead of 5 yards.

The infrared timing gates were placed at positions different than in Study 1.

Force data was only collected at a single point and was not representative of sprinting ability over the entirety of a 20-yard distance.

Blood samples were taken immediately after sprinting because subjects were still sweating and could contaminate the sample.

(C) Force data was only collected at a single point and was not representative of sprinting ability over the entirety of a 20-yard distance. If force data were collected for every step taken by subjects, then a more complete picture of their movement economy can be painted. Currently, data only represents a 5-millisecond period of the sprint. If a subject produced more braking force at the 15-yard point than 10 yard, or more propulsive force at the 5-yard point than 10 yards, it cannot be observed. Answers B and D do not relate to Study 2, and answer A would not have helped the study.

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