Assuming the same number of exercises and sets over a period of a week or two is it better to work out longer but less frequently, or shorter but more frequently?
I am frequently asked a lot of variations of this question, usually along the lines of whether I recommend a full body or split routine, and if so how far to split up the workout and in what way. Like most things, the answer depends on the individual.
Recovery from and response to exercise involves multiple, interrelated local and systemic processes which can take more or less time depending on the individual, the muscles, how intensely they work them, and other factors. Local processes include repair of microtrauma and synthesis of new muscle tissue, and the microtrauma results in an inflammatory response which affects the rest of the body. The harder the training, the more muscles worked, or the higher the volume of exercise the greater the inflammatory response. If adequate time is not allowed for recovery and inflammation becomes chronic you will enter an overtrained state, and depending on the degree of overtraining either stop progressing or even regress and lose muscle mass. (Cytokine hypothesis of over training: a physiological adaptation to excessive stress?,” Medicine & Science in Sports and Exercise, Vol 32, No. 2, pp 317-331, 200).
Some people prefer to split up exercises into workouts organized by muscle group or body area, under the assumption you can work some muscles without affecting the recovery of others. However, regardless of how you split up exercises or what equipment you use there can be a lot of overlap in the muscles affected. You can do exercises that target different muscle groups in different workouts, but even when performing relatively isolatory exercises you will still use other muscles to maintain proper positioning and/or alignment and some of these may be under considerable tension depending on the loads used. A good example of this is the standing barbell curl. While it targets the biceps and other arm flexors, you use many other muscle groups to maintain proper positioning, most notably, your chest and shoulders maintain your upper arm position preventing your shoulders from extending (unless you are doing drag curls) and your back and hip extensors maintain your posture while the weight is held in front of you.
Although assuming a constant volume of exercise differences in distribution may have little effect on systemic recovery, training too frequently may still interfere with local recovery depending on the degree of overlap in muscular involvement between workouts. Just because two or more workouts target different muscle groups doesn’t mean you don’t need to rest between them.
There are downsides to training too infrequently, however. While there appears to be very little difference in muscular strength increases between training once, twice, or three times per week for most people (and some may require an even lower lower volume or frequency to avoid overtraining), metabolic conditioning appears to start to suffer at frequencies below twice weekly.
While individual recovery ability, goals, and situations vary considerably, I have found two full-body workouts a week to be a good starting point for most people. It is infrequent enough that most people will not overtrain if they keep the workouts relatively brief (only one set of around ten exercises including work for smaller muscle groups like neck, forearms, or calves) while frequent enough that metabolic and cardiovascular conditioning is not compromised. Some people do appear to get better results from split routines, though, possibly due to local recovery requiring longer than systemic (which may be the case for predominantly fast-twitch muscles).
If your recovery ability allows it you can distribute the exercises over slightly more frequent workouts, but I do not recommend working out more than three, non-consecutive days per week and it is better to err on the lower side with volume and frequency to avoid overtraining. If you need more time for recovery between workouts or circumstances prevent you from working out more frequently, you can also still make good progress on even very infrequent training provided when you do work out you do so very intensely.
Also, consider the optimal volume and frequency of training may vary depending on your body’s response to exercise and goals. You may get stronger faster with extremely brief and infrequent training, but your conditioning may improve more quickly with a slightly higher volume and frequency.
The best way to determine what you should do is to clearly define your training goals and track measurements specific to those goals along with your workouts, experiment, and adjust your workouts based on how your body responds.