sleds

The more you can train without exceeding your capacity to recover the better your results.
If the squat is troublesome for you, the best alternative exercises to consider are exercises that mimic either the joint action or muscle groups involved in the squat.
The sled is an incredibly versatile and effective training tool. If used properly, the sled can be used for metabolic conditioning, strength training, and hypertrophy.
What makes the concentric movement less effective in building muscle and strength is the same thing that makes it great as a conditioning tool.
There are plenty of ways to work on your legs that don't involve a squat or a barbell.
Beyond weightlifting and powerlifting there is another plane of weight training. The ultimate approach to raw strength is strongman.