I am going to have to respectfully disagree:
1. Looking at someone's hand/arm muscles will give you little useful information, you really can not tell how hard someone is gripping, and even if you could, it won't tell you how easy or difficult a disarm may be as there are a other factors that come into play, as in 2.
2. The idea that people who are fast with a knife don't grip really tightly is inaccurate at best. Perhaps SOME people who are fast also don't grip really tightly, however after 35 years of training, sparring, and professional teaching of edged weapon offense/defense, I can tell you that anyone with significant experience grips their weapon plenty tight. Even those without formal training who are determined and experienced in the street (Criminals) tend to grip quite tightly and their attacks can be quite fast - one need look no further than youtube for examples.
Physiologically there is no reason why a very tight grip need inhibit the speed of one's weapon arm as the muscles which control the movement of the arm (articulation at the shoulder and elbow) are separate from those in the forearm and hand which determine grip. Granted, some people tense everything from hand through the back and chest when gripping hard, but trying to determine how much of that is going on and how likely your chances of getting a disarm are based on that........well, not something you want to risk your life on.