Yes, but the authors didn't know how. They are mistakenly suggesting that if two dependent variable were to correlate, then one would necessarily be the cause of the other. Whether they showed it or not isn't the point. The point is that it is a false premise logically. They didn't even look for the other dependent effects that could be at work there. Neither did they vary the levels of vibration transmission through a single frame to test the effect on rider performance. Ideally you would start with one frame and a machine that would impart vibration to it. You would vary the level of vibration applied. Consistent with the frame's properties that would vary the level of vibration transmitted. Then you would measure rider performance at each applied and transmitted vibration level. Finally, if you found a cause-effect between the vibration independent variable and the performance dependent variable, you could develop the "hypothesis" that transmitted vibration levels caused by frame differences (keeping the input vibration constant) would also cause differences in performance and test that with different frames.

This is horrible science pure and simple.