On a block return the ball hits the player, not the racket

A chipped return absorbs the incoming energy of a serve through a nearly static arm, which is a different load entirely.

Most tennis strokes are the player doing something to the ball. A block return is closer to the reverse, and the arm has to be organised accordingly.

When a player swings, contact happens with the racket already moving fast, so the collision is between two objects travelling in similar directions and the change of momentum is spread across a long stroke. Chip a two hundred kilometre an hour serve back with a short punch and there is barely any racket speed to soften the meeting. The ball hits a nearly stationary string bed and reverses, and the entire impulse has to be resisted by a hand, a wrist and a shoulder that are holding a position rather than executing a movement.

Holding a position against a sudden force is a specific kind of muscular work, and it is unforgiving because there is no time to react. The impact is over in a few thousandths of a second, far quicker than any reflex, so whatever stiffness the arm has at contact is the stiffness it uses. Set the wrist a fraction loose on a heavy serve and the racket head is driven backwards through the joint, which is how wrists get hurt on returns rather than on forehands.

The slice return puts the shoulder in an awkward place too. Contact happens out in front and slightly across, with the arm extended and the shoulder blade needing to be pinned back by muscles that are also trying to stabilise against a shove. On a big first serve, that shove is meaningful. Repeat it across a match against a serving specialist and the muscles that hold the shoulder blade fatigue before anything else does, which is why returners get sore between the shoulder blades and not in the racket arm.

There is a piece of received wisdom that the block return is the safe option for a player with an injury, because it is short and it needs no power. That is exactly backwards for anyone with a wrist or hand problem. The short stroke removes your ability to absorb, and absorption is the whole job.

If you want the truly gentle return, take a step back and swing at it. A moving racket meets a fast ball far more kindly than a still one does.