John Kelly hits hard, gets hit hard, but doesn't get injured.
Yes. Kelly is durable like Dobbs. Both of those men play so hard it's the player on the other team who will get hurt before they do. I always thought Jennings showed that same toughness but even the toughest can get that fluke injury that just requires recovery time to get back healthy.
I do think a focus on low rep strength training combined with explosiveness intensity training on higher volume days combined with an emphasis on mass-building on those higher volume days can provide that bigger, stronger mass of muscle that can shield the body from injury underneath it and around the joints it protects. Sometimes that benefit is lost when the emphasis on speed neglects the importance of power. Ideally, you get the best of strength, explosive power (fast-twitch muscle intensity training), speed and size (mass).
You can usually see signs of training imbalances with the injuries, softer, more injury prone, getting dominated at the line of scrimmage, and a general lack of strength and mass for the kind of aggression needed to truly dominate the other team at the LOS (line of scrimmage).
All speed and no attention to either strength or mass, means you get pushed back at the line of scrimmage and gashed with the run. You might not think the muscle building is as important as speed or strength for football until the rash of injuries appear to remind you why it is.
All size and strength but no speed, your secondary gets beat and opposing backs can hit holes for a few yards before anyone can even get a hand on them.
When you're squatting and deadlifting consistently, you'll naturally build muscle above the knee that can protect it by absorbing force and impact that it otherwise wouldn't have there to serve as a buffer to the joints.
Sometimes people see opposing running backs consistently getting past the linemen for a few yards where the linebackers have to step up to make the tackle and they assume it's a speed problem when it may be the opposite.
It could be too much emphasis on speed while neglecting strength and size means our linemen are getting pushed back for those yards instead of penetrating to stop them behind the line of scrimmage.
You can be fast as all get out but it does you no good if you're pancaked while even a slower running back runs right past you.
Then again, you don't want big slow lumbering linemen who get beat either. That's where the optimal balance comes in to help. Another imbalance would be an emphasis on explosiveness with higher volume intensity training without the heavy days for strength training so that it's not just weak muscles that are explosive.
The same can be said for a neglect of mass-building training in the mix on higher volume days because explosive muscles that are too small to make a difference isn't optimal for you either.
So there are four different types of training there that can't be neglected at the expense of any of the other before suffering results of that neglect that show up on the field where it counts. If you mix up the training with heavy days early in the week for linear progression gains of strength and intensity higher volume days that give you explosiveness and mass, you can get the best of those four weapons working for you. It's like a stool with four legs. Sure, you can dangerously teeter on less than all four legs, but eventually, that imbalance below the seat will give way to gravity under the person depending on it to hold their weight and begin to fall in the direction of the missing stool leg. In the same way that gravity acts as the force that exposes which stool leg is missing, the demands of plays on the body of the players during the course of the football game will act as the force that exposes which pillar of training is missing the work it needs so that the benefits of complete training are available to the player on the field on game day when he needs it most.