The best replacement for leg extension is leg press.

The best alternatives to leg extension are rated as following:
★★★★★  leg press
★★★  squat
★★★  leg curl
★  lunges
  seated leg curl

Your alternative for leg extension is not listed? Suggest!
Speaking about leg extension, one probably shouldn't forget about shoulder press.

Other search terms related to leg extension and their ratings are:
★★★★★ shoulder press
You have in mind a companion for leg extension and it is not yet here? Propose!



Google Trends

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leg extension
leg press
squat