Training legs can be intense. Especially when you have to squeeze in a handful of complex compound movements to try & target the anterior & posterior musculature evenly & efficiently. With that being said, if you have the time & availability to do so, you should be splitting your leg workouts into anterior & posterior days for larger benefit. Here is why:

(Keep in mind some people only have time for one long leg day. From a trainer & fitness professional perspective, I get it. IF you can spare the time, I would 100% recommend it.)

Many beginner liftees will find benefits from full leg days. But, if you are an intermediate-advanced lifter, you will find it necessary to split your leg days if you are trying to reach the next level. 

Focus on more volume on target area (when needed):

• Instead of trying to target everything in one session, splitting can cut the workout duration in half. 
• Higher quality sets & lower induced fatigue.

Recovery: 

• Longer recovery times are required when hitting full leg days. This can limit the amount of high intensity sets you are able to hit each week for legs. 
• Frequent stimulation is a necessity, which can be utilized when recovery time is adequate. 

Intensity

• One of the most important factors to continual progression & development in the gym. By far one of the most important principles in the FITT-VP method. 
• Let’s put this in perspective. Imagine going heavy & all-out on RDLs, and then having to go to a quad focused hack squat & deliver the same approach. Seems inefficient right? Quads will lack required stimulation to ensure intensity, load, & mechanical tension are prioritized.
• Splitting allows you to efficiently allocate your exertion to either anterior or posterior concentration without having to worry about neglected one or the other. 

Overemphasizing

• Many people tend to favor one over the other when doing full leg days (glutes, hams, quads). 
• Make it easy & even by developing balanced split days that target weaknesses & current undertrained areas + imbalances. 

Volume management

• If emphasizing hypertrophy, you will find splitting your training days into anterior/posterior extremely efficient for recovery. 
• Train harder & train less! 
• Focus on heavy loads without having to attack a bunch of exercises in 1 workout. 

Frequency 

• Studies have shown that equated volume, regardless of frequency, does not impact hypertrophy negatively. The main thing we will see impacts on is recovery & performance. Which long term can & will deter further progress. As long as volume its equal, then frequency doesn’t essentially “matter”.
• But, on the contrary, new studies showing the effects of low volume training can show that frequency starts to become more of an important variable in comparison to volume, when it comes to the amount of times times you “train”. Even if that means 3 high intensity sets of squats weekly. You will still be training more frequently! Just food for thought. 

Evidence & peer reviewed articles:

A systematic review and meta-analysis by Schoenfeld et al. (2016) found that training muscle groups twice per week leads to greater hypertrophy than once per week, even when total volume is equal.

https://pubmed.ncbi.nlm.nih.gov/30558493/

This suggests that splitting leg workouts allows for more frequent stimulation of each muscle group, potentially enhancing growth.

https://rd.springer.com/article/10.1007/s40279-018-0872-x?utm_source

Another study by Schoenfeld et al. (2018) concluded that, when total training volume is equated, the frequency of resistance training sessions does not significantly impact muscle hypertrophy. However, splitting workouts can help manage fatigue and maintain high-quality training sessions, which is beneficial for recovery and performance.

https://pubmed.ncbi.nlm.nih.gov/30558493/

Resistance training frequency and skeletal muscle hypertrophy: A review of available evidence https://pubmed.ncbi.nlm.nih.gov/30236847/

Frequency: The Overlooked Resistance Training Variable for Inducing Muscle Hypertrophy?

https://pubmed.ncbi.nlm.nih.gov/27752983/

Effects of Variations in Resistance Training Frequency on Strength Development in Well-Trained Populations and Implications for In-Season Athlete Training: A Systematic Review and Meta-analysis https://pmc.ncbi.nlm.nih.gov/articles/PMC8363540/

Effects of different resistance training frequencies on body composition and muscular performance adaptations in men

https://pmc.ncbi.nlm.nih.gov/articles/PMC8067909/

Effects of resistance training frequency on measures of muscle hypertrophy: A systematic review and meta-analysis

https://www.iat.uni-leipzig.de/datenbanken/iks/power/Record/4042947

Effect of Resistance Training Frequency on Neuromuscular Performance and Muscle Morphology After 8 Weeks in Trained Men

https://journals.lww.com/nsca-jscr/fulltext/2019/08000/effect_of_resistance_training_frequency_on.8.aspx

Effect Of Training Frequency On Muscle Mass, Strength And Power Performance During Reduced Resistance Training

https://journals.lww.com/acsm-msse/fulltext/2016/05001/effect_of_training_frequency_on_muscle_mass%2C.1423.aspx

Resistance training frequency and skeletal muscle hypertrophy: A review of available evidence

https://www.sciencedirect.com/science/article/abs/pii/S1440244018308624