Are You UNDER Training?
If you want to gain muscle and strength then the reason that you're not is that you're either training too much or too little. Although there aren't any specific side-effects from undertraining, it certainly can take a huge toll on your own motivation to push harder.
Although anyone starting with weight training for the first time will gain muscle with a generous 90-sec rest between each set and still get bigger, they can and should only train three times a week using higher than 6 reps. Beginners also get away with doing 6 sets for each body-part and still grow, fast.
The problem with those of us who have been training for more than a year is that Adaptation = Stagnation. This simply means that if you don't increase your workload overall, your body will have no reason to adapt. Training at a sub-intense workout for 20 minutes as an afterthought is not going to build muscle, it never has.
Progressive resistance builds muscle, that means that your muscles need to be stressed at a progressively more difficult "heavy" weight. Without manipulating your loads, your rest times, your tempos and the duration and frequency of your workouts, you won't gain muscle on a sustainable basis.
Periodization is the key to muscle hypertrophy, muscle is planned because you don't start high volume training, you start off with a low-volume training, 4 to 6 weeks later you change to a medium-volume workout and then ending the volume cycle by doing a high-volume workout for 4 to 6 weeks.
Undertraining is when you're training without progress. The only way that you will ever gain more muscle is by coming up with a plan that you implement by training on a regular basis with a specific objective to each workout. Doing the same amount of reps and sets with the same weight for months, or even years, won't add muscle.
But you don't need an article on-line to know that, you can just look at yourself in the mirror at least once a day, you'll know if you gaining or losing muscle. Below are a few tips to those who are reading this who think they may be suffering from undertraining signs:
Train your weakest body-part twice a week. For example, doing a five-day split, you would train this weak body-part on Monday and Friday.
Train with a stopwatch so that you can reduce rest periods by 15 seconds. If you do that on each set, you'll see a big difference.
Commit to a tempo, plan ahead by selecting a weight that you do a 3-4 second negative with an explosive concentric, it will improve muscle hypertrophy and instantly extend the sets you can do.
Learn periodization and how to move from training each body-part twice in a day doing an intensification workout before breakfast and then the accumulation workout around sunset, all doing the same body-part. It's part of a training protocol taught at Hypertrophy M.A.X.
Either get help from someone who knows or learn everything you can about periodization and create your own program. Your research into periodization will reveal that undertraining is actually acceptable when doing a specific planned de-loading-and-recovery for a few weeks. Other than that, undertraining is certainly NOT recommended for muscle gain.