I like to have a secondary leg day for a couple of reasons. For starters volume is typically measured on a weekly basis. Meaning that whatever the suggested volume is for a muscle group or specific movement can be done over the course of a week. To avoid fatigue you split the total volume out over the week. (Volume is total weight and repetitions). In addition to my secondary leg day I also do some split squats on Thursday but I'll talk more about those another time. Secondary leg day consists of box squats: I do these for a couple of reasons. They do help with lower body fatigue for somewhat obvious reasons meaning they reduce range of motion and give you a position to rest and reset. Additionally, much like paused squats they prevent the bounce out of the bottom forcing you to rely on your strength to stand back up and not just the momentum and the stretch reflex. You need to be mindful of your form and ensure you're not just dropping onto the box. The movement also helps with your explosiveness. Box squats can also be used to help with depth. Full range of motion has shown in studies to help joints by strengthening tendons and ligaments (I'm not a doctor so I won't dig too deep into that). You can increase your depth by starting with a taller box then reducing the height over time until you can reach your full range of motion. One more movement that helps with this that I also do on secondary leg day is goblet squats. Box squats were a big part of my squat PRs a couple of years ago. Next I do Jump Squats: These help with explosiveness. I've neglected plyometric type movements in my younger years and I've paid for it as I got older. Explosive movements once again help with joint health. Just because you aren't trying to be a professional athlete in your older age doesn't mean you don't need this type of training...you do If you want to be all around healthy. Walking Lunges: I've made an adjustment to my walking lunges, I hold the dumbbell over my head rather than down by my sides. This helps train balance through isometric bracing (once again strengthening your core). The first two or three times doing this you'll probably find that you have to keep catching yourself but eventually you develop the strength and motor skill to stay upright...that's how you know it's working.