Caroline PT, DPT, NCS, CSCS on Instagram: "With @arynasabalenka playing the US Open tonight, it seems like the perfect time to answer the question, what the heck is up with the string?! She is using a Brock string to train eye movements called convergence and divergence, motions that help us change focus depth and her case, doing so with speed and accuracy. #physicaltherapist #neurology #tennis #usopen @tennischannel"