The muscle in my left forearm always aches and hurts after I perform the manoeuvre on my fretting hand.
The reason is because I hold my hand at an angle and away from the fretting board to avoid muting the other strings. It's always the pad of the ring finger that mutes the string below the one I'm hammering on to.
I distance my hand away from the neck because my palm-pads always mute the 1st E string, if too close.
Which makes my wrist ache, but I guess with practice, my left hand, arm and wrist will grow stronger over time.
Am I the only one who plays multiple instruments? Let's be inspirational and find our muses everyday!