Using too much weight, too soon; always start lower than your expected ability and work your way up that first workout. If your form suffers, you are swinging the weight, or using momentum, this indicates you may be using too much weight. Greater momentum increases the potential for injury and reduces the effectiveness to the muscle group being targeted.
Other supplements could easily have been included here, but these are considered the most useful and effective for the majority of bodybuilders and athletes. Although food should always come first, supplements offer an effective alternative for getting nutrients that either aren't available in sufficient quantity in food or are in foods that you may not be eating.
Those micro-tears that are such a key factor for muscle-building need rest to rebuild themselves and grow stronger. When do they do that? When you’re asleep! “You have to rest and feed your muscles between workouts or you will tear them down and they will become weaker,” says Olson. “Over time, you run the risk of over-training, which can result in injury, and possibly even more sleep troubles.”
Parashos, S. A., Swearingen, C. J., Biglan, K. M., Bodis-Wollner, I., Liang, G. S., Ross, G. W., Tilley, B. C., and Shulman, L. M. Determinants of the timing of symptomatic treatment in early Parkinson disease: The National Institutes of Health Exploratory Trials in Parkinson Disease (NET-PD) Experience. Arch Neurol. 2009;66:1099-1104. View abstract.
Some bodybuilders use drugs such as anabolic steroids and precursor substances such as prohormones to increase muscle hypertrophy. Anabolic steroids cause hypertrophy of both types (I and II) of muscle fibers, likely caused by an increased synthesis of muscle proteins. They also provoke undesired side effects including hepatotoxicity, gynecomastia, acne, the early onset of male pattern baldness and a decline in the body's own testosterone production, which can cause testicular atrophy. Other performance-enhancing substances used by competitive bodybuilders include human growth hormone (HGH), which can cause acromegaly.
Kilduff, L. P., Georgiades, E., James, N., Minnion, R. H., Mitchell, M., Kingsmore, D., Hadjicharlambous, M., and Pitsiladis, Y. P. The effects of creatine supplementation on cardiovascular, metabolic, and thermoregulatory responses during exercise in the heat in endurance-trained humans. Int J Sport Nutr Exerc Metab 2004;14(4):443-460. View abstract.
Some other cytokines and hormones may increase the receptor activity. These include growth hormone (GH) which acts upon the growth hormone receptor (GHR) to stimulate c-Src which directly increases the activity of the CrT via phosphorylation. This is known to occur with the 55kDa version of c-Src but not the 70kDa version and requires CD59 alongside c-Src.
While most of these muscle building supplements can be taken at any time of the day, some are best to include in your pre-workout routine. Citrulline malate, in particular, is one that should be taken about an hour ahead of your workout. Because this supplement boosts performance, taking it ahead of your workout will maximize its effect, making sure you get the most out of the supplement.
Creatine is a hydrophilic polar molecule that consists of a negatively charged carboxyl group and a positively charged functional group . The hydrophilic nature of creatine limits its bioavailability . In an attempt to increase creatines bioavailability creatine has been esterified to reduce the hydrophilicity; this product is known as creatine ethyl ester. Manufacturers of creatine ethyl ester promote their product as being able to by-pass the creatine transporter due to improved sarcolemmal permeability toward creatine . Spillane et al  analyzed the effects of a 5 days loading protocol (0.30 g/kg lean mass) followed by a 42 days maintenance phase (0.075 g/kg lean mass) of CM or ethyl ester both combined with a resistance training program in 30 novice males with no previous resistance training experience. The results of this study  showed that ethyl ester was not as effective as CM to enhance serum and muscle creatine stores. Furthermore creatine ethyl ester offered no additional benefit for improving body composition, muscle mass, strength, and power. This research did not support the claims of the creatine ethyl ester manufacturers.
A great analogy that I like is that the balance between training and recovery is like digging a hole. Each time you lift, you dig yourself deeper and make it harder to climb out of the hole. To get back out again, you have to fill in the hole to return to ground level, and the only way to fill it is with food and rest. If you overdo it in the gym by pushing too hard, you won’t be able to train as often or at a high capacity. Eventually, you’ll get injured.
Osteoblast cells are known to express creatine kinase. Bone growth factors such as IGF-1, PTH, and even Vitamin D seem to induce bone growth alongside increases in creatine kinase activity. Vitamin D has been noted to work indirectly by increasing the cellular energy state (these hormones increase creatine kinase in order to do so) in order to make bone cells more responsive to estrogen. This evidence, paired with enhanced growth rates of osteoblasts in the presence of higher than normal (10-20mM) concentrations of creatine suggest a role of creatine in promoting osteoblastic and bone growth, secondary to increasing energy availability.
It is suggested [16,37] that another mechanism for the effect of creatine could be enhanced muscle glycogen accumulation and GLUT4 expression, when creatine supplementation is combined with a glycogen depleting exercise. Whereas it has been observed  that creatine supplementation alone does not enhance muscle glycogen storage. Hickner et al  observed positive effects of creatine supplementation for enhancing initial and maintaining a higher level of muscle glycogen during 2 hours of cycling. In general, it is accepted that glycogen depleting exercises, such as high intensity or long duration exercise should combine high carbohydrate diets with creatine supplementation to achieve heightened muscle glycogen stores .
In muscle cells, the creatine transporter is predominantly localized to the sarcolemmal membrane. Western blot analysis of creatine transporter expression revealed the presence of two distinc protein bands, migrating at 55kDa and 70kDa on reducing SDS-PAGE gels. The 73kDa band has been reported to be the predominant band in humans, with no differences based on gender. A more recent report demonstrated that the 55kDa creatine transporter variant is glycosylated, forming the 73 kDa protein. Therefore, the 55 and 75kDa protein bands are actually immature and mature/processed forms of the creatine transporter protein, respectively.