I talked a lot about in my "Basic Strength Training" article on why my preference for beginners are full body routines. As a certified personal trainer I feel the split is far too often relied on and the answer in the months of lack of results actually relies in the loss of frequency.
See, when you perform exercise, you have hormonal release that is influenced by the stress, sleep, pre and post nutrition, and tension to the body as a whole; this is why you often hear myself and other instructors say "it's not all about how much you lift".
The greater the stress is to the body as a whole the better hormonal release. Does hitting your chest, triceps, and anterior delts put more tension to your body than hitting your quads, glutes, hamstrings, calves, chest, triceps, back, lats, rhomboids, trapeziums, posterior delts, anterior delts, and core? I seriously doubt it. When you workout your whole body, the body demands more and more anabolic hormones like testosterone.
Another reason the full body routine beats the split is because splits often require working out more than three times per week. Now, there's nothing wrong with a 2 day on/1 day off "upper/lower" split but there is, on the other hand, a problem with training 5 days if you're natural. When you workout, 24-48 hours later cortisol remains very high and is released much more during the exercise. Cortisol is a catabolic hormone that eats muscle and is the opposite of what you want; testosterone.
Frequency is probably the most misunderstood concept of the full body routine. People far too often do not understand what over training is. It is a psychological condition not caused by a single workout but a variety of workouts over a period of time. You either have to be too strong for your own good or extremely stupid for a long period of time. Overreaching happens before over training and gives you the warning signs. Sometimes overreaching is thought to be over training but it is not. A week off usually allows overreaching recuperation times, while over training can take many months. It refers to the CNS; it doesn't refer to the muscles directly.
You can train your body every 48 hours; it is not really as damaging as people like to assume. If a beginner attempts to train, he is capable of making linear progression on all his compound lifts. With the linear progression in mind; will you progress faster training a movement once per week or three times per week? Your progression will be in a 3:1 ratio.
If you haven't been gaining results with your split; perhaps it'd be a good idea to give "basic strength training" or another full-body routine a try.
