despite the fact that some runners can run through pain or feeling like shit they are notb referred to as grinders. Why? Because if you have good form, stay relaxed while kicking, have any speed, are a normal runners weight, etc. Then you are not a grinder. But if you weigh more than you should, have shitty form, grunt and make ugly faces and are forced to push the middle of a race because you cannot kick,and you are untalented then you are a grinder.