A number of ways. First, it isn't just F = MA, the different joints and ligaments apply a different ratio of force depending on how strong you are. For example, biceps and hamstrings can get a lot stronger as a ratio of their typical strength than tendons in the wrist. By analysing known biometric data of the athlete (their weight, resting heart rate (athletes have a much lower rate), how fast they can cycle, perspiration patterns) and combining it with the gyros and accelerometers in both the phone and watch you should be able to accurately reverse calculate the weight and the exercise.
Interesting. I'd love to see how accurate this method could be, because it seems like there's a lot of variables to account for, and even then, it might not fully determine the exercise and weight.