The 5 day split
A five day split routine is essentially to work out different body parts over a period of five days without working the same body parts on consecutive days. For example the upper body lower body split routine is common or the push pull day routine is common. The scientific literature suggests that the human body only requires a maximum of 48 hours to recover with 36 hours being the typical recovery time. This means at the absolute minimum 24 hours rest time is a requirement which is why split routines came into popularity. It is possible to do a full body routine but again it would require doing the full body workout with 36 hour rest periods in between which would be roughly 3 workouts per week total vs 5 total.
The 7 day split
The 7 day split is the same format as the five day split only with slightly higher intensity at the beginning of the week and reduced intensity toward the end as fatigue is cumulative. If we think of fatigue as being a cup which gets filled over a microcycle or mesocycle then the high intensity routine over 7 days straight will result in a full cup much faster. If we think of an overflowing cup as over training then we could even see regression in lifts if a person trains week after week without what is known as a deload. The ideal is to train and slowly ramp up intensity in terms of overload until the body reaches a physical peak which according to literature typically happens after 3 or 4 weeks of training (toward the end of what is known as a mesocycle).
Which is better?
This really depends on the body and the discipline of the person. If you are the sort of person who feels more alive after time in the gym or who just enjoys working out then the 7 day split may be better for you. One benefit of high frequency training is that you may stop getting DOMs. DOMs seem to be a result of being under trained so for example if you train legs once per week then you'll be much more sore than if you train legs daily. Once your body adapts which can take months or years depending on your genes; you may stop getting sore at all at this point. Soreness is not an indicator of muscle growth or a good training session; it merely indicates that the muscles were sufficiently shocked by a new exercise the body wasn't used to.
The 7 day routine may have the benefit of releasing more natural growth hormone. Depending on the volume, muscle growth (muscle hypertrophy) may be greater or lesser. There seems to be a natural correlation between volume (measured by sets x reps x weight = V) and progress (measured as muscle growth). There also seems to be literature which indicates a minimum effective volume (MEV) and a maximum recoverable volume (MRV). The MEV is the least amount of volume which allows for muscle growth. The MEV can be thought of as the minimum effective dose of training stimulus in order to trigger a response of muscle adaptation.
- The total volume is what matters (per week, per body part) for muscle adaptation.
- Overtraining can happen but it takes weeks of over reaching (training beyond the maximum recoverable volume)
- Occasionally training without enough rest is not bad just as long as after you do see your numbers lagging for a couple of weeks that you deload (simply don't work out that body part for a week or two).
- Training 5 or 7 days a week is up to preference and it is possible to get the same result by compressing it to 5 intense days or you can go 7.
- There may be a hypothetical benefit in terms of growth hormone release by training for 7 days. This depends on how you feel personally after you train. If you feel the pump, if you feel more alive, if you feel strong, then there could be something to it.
- Cardio can be done not just 7 days a week but multiple times a day. At the same time it is much easier to over train doing cardio.
- Cardio + weight lifting for 7 days a week could be a bad idea unless a person is very over weight. This is because if a person does not have enough body fat stores (high enough body fat percentage) they will likely lose muscle and not see much gains by doing cardio and weight lifting on the same day.
References
Schoenfeld, B. J., Ogborn, D., & Krieger, J. W. (2017). Dose-response relationship between weekly resistance training volume and increases in muscle mass: A systematic review and meta-analysis. Journal of sports sciences, 35(11), 1073-1082.