Practically all of the graphs show that the trends they attribute to new racquets were already evident before that introduction. Given the size of the pro tour,

Practically all of the graphs show that the trends they attribute to new racquets were already evident before that introduction. Given the size of the pro tour, it wouldn't take too large a cadre of young hot shots to explain the results independently of racquet technology, and those players' ever advancing age would better explain the U shape of those curves. Indeed this is when McEnroe, Connors, Lendl, and Wilander were all coming to the fore. The evidence for the authors' hypothesis seems extremely weak.