The best replacement for forearm is underarm crutches.

The best alternatives to forearm are rated as following:
★★★★★  underarm crutches
★★★★★  elbow
★★  arm
★  forearm
★  palm
★  wrist
★  foot length
★  upper arm

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Speaking about forearm, one probably shouldn't forget about weights exercises.

Other search terms related to forearm and their ratings are:
★★★★★ weights exercises
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forearm
underarm crutches
elbow