When you're looking for a split, you want to be sure how many days a week you're ready to workout.
It is also important to have a balanced split to avoid overworking important muscles which can easily lead to injuries.
Wether you want to gain mass or strength makes no difference, only the sets and exercises will differ.
It is recommended to workout a minimum of 3 days a week to get valueable results.
***Cardio can be incorporated anywhere in the splits, even rest days!***
3 Day splits:
Considering the limited number of days you're willing to workout, you will have to pack more than one muscle in a day. With these splits, it is very easy to screw up and work muscles that shouldnt be worked together for optimal results.
Mistakes like pecs/shoulders in the same day are often made. Front deltoids being involved in both pectoral exercises and shoulder exercises, this can lead to poor results and even injuries. Just because you enjoy working certain muscles, doesn't mean it's ok to do them together. It is important to always keep in mind that any muscle you work on, indirectly works on other muscles. Those muscles, in need of recovery just like the muscles you worked on can be wrongly targetted with a bad split and end up lacking behind or injured.
Here's an exemple of a good 3 day split:
Mon; Pecs/Biceps
tue; Rest
Wed; Rest
Thu; Legs/Shoulders/Triceps
Fri; Rest
Sat; Back/Traps
Sun; Rest
Notice how in this split we try to spread apart the muscles that can be involved indirectly and can be weak or still in recovery on a different day. There are different alternations in days for this split that you can work with, but it is best not to mix the muscles too much. There are different variations that can be done with 3 day splits, but finding a decent balance is difficult.
4 Day splits:
4 day splits give you the opportunity to get nice workouts without having to work too many different things on the same days.
Here's an exemple of a good 4 day split:
Mon; Pec/Light Triceps
Tue; Back/Light Biceps
Wed; Rest
Thu; Legs/Shoulders/Traps
Fri; Heavy Biceps/Triceps
Sat; Rest
Sun; Rest
This split allows heavier work on the arms during the week, but is mainly for those attemting to get a body for the looks rather than power.
People concentrating on powerlifting may prefer a different one:
Mon; Legs
Tue; Rest
Wed; Back/Triceps
Thu; Rest
Fri;Pec/Biceps
Sat; Rest
Sun; Shoulders/Traps
This allows the 3 primary compound lifts (Squat, deadlift, Bench Press) to be full or near full energy level. It also spreads apart back day and shoulder day considering lower back is indirectly involved in all standing presses.
5 Day splits:
5 day splits are the most common ones. It gives you the opportunity to concentrate on different muscles every day, optimizing the results.
Here's an exemple of a good 5 day split:
Mon; Pecs/Light triceps
Tue; Back/Light biceps
Wed; Rest
Thu; Shoulders/Traps
Fri; Heavy Biceps/Triceps
Sat; Legs
Sun; Rest
This is one of the most balanced splits I know. It allows for sufficient work on every muscle group and keeps them from being overworked with the way they're spread apart.
ABs
Some people seem to think doing 50 000 crunches is going to make them swole. The fact is, your core, obliques and abs are heavily worked on during exercises such as Squats and Deadlifts. While weighted ab exercises are excellent to gain mass/visibility in the abdominal section, primary compound lifts are more important. Is it therefor better to work on your abs on days that are far apart from Back and Leg days or incorporated after Back and Leg workouts.
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Considering rest periods are key to results, I do not recommend 6 and 7 days splits.