How often to train quads?

A handful of researchers have been taking swings at nailing down what variables are important for determining training frequency for individual muscle groups. For the most part, muscle size, fiber type, and joint range of motion (ROM) are likely the most important variables. Large muscles, fast twitch-dominant muscles, and muscles that can undergo large ranges of motion usually take the longest to recovery – which mostly describes the quads. However, the quads benefit from a good blood supply and have often been shown to recover much more quickly than almost every other muscle group. This means that the quads can be trained rather frequently; however, adding quad days might require mostly isolation movements as other muscles involved in compound lower body exercises won’t recover as quickly. If you want to boost quad growth, try adding leg extensions or reverse Nordics to other workouts throughout the week to boost your quad frequency. I’d shoot for 2 big quad workouts per week with 1 or 2 smaller ones between the big workouts.
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▪️ Based on: Chen, T. C., Yang, T. J., Huang, M. J., Wang, H. S., Tseng, K. W., Chen, H. L., & Nosaka, K. (2019). Damage and the repeated bout effect of arm, leg, and trunk muscles induced by eccentric resistance exercises. Scandinavian Journal of Medicine & Science in Sports, 29(5), 725-73