How Testosterone's Enhances Muscle Power
    Theoretically, testosterone should improve strength by increasing muscle mass (via increased protein synthesis, nitrogen retention and anti-glucocorticoid actions), as well as by increasing exercise motivation. Moreover, recent studies in humans indicate that testosterone may increase muscle protein synthesis, possibly through stimulation of intramuscular insulin-like growth factor-I (IGF-I) gene expression31.  Testosterone may also enhance strength by altering the way neurotransmitters are produced for muscle contraction.  Contraction of muscle is a duty of the central nervous system comprised of brain and spinal cord. The connection between a motor neuron (A neuron that conveys impulses from the central nervous system to a muscle) and muscle fiber is a specialized synapse called the neuromuscular junction. Upon adequate stimulation, the motoneuron releases a flood of neurotransmitters that bind to receptors and triggers muscle contractions.  The first step in the sequence of events causing contraction of a muscle is the chemical messenger from a nerve (in the form of the neurotransmitter molecules, acetylcholine) to the muscle.  Testosterone has been shown to alter neurotransmitter acetylcholine within motor neurons which could result in a more efficient muscle contraction.  For example, castration causes a decrease in the activity of an enzyme called choline acetyltransferase, resulting in less acetylcholine which technically could result in lower force output, however if the rats are supplemented with testosterone the levels of acetylcholine return to normal, furthermore if the levels are supraphysiological elevated as with testosterone abuse the levels of the neurotransmitters are increased even further30.   

Both motor neurons and muscle have androgen receptors located on them and are, therefore, potential sites of androgen action.  The significance of how androgen receptors affect muscle strength can be exhibited by patients with Kennedy's disease, a degenerative disease in which androgen receptors are defective.  The disease is characterized by severe muscle weakness.  How testosterone interacts with the receptor is like a lock and key system.  If you have lots of locks (testosterone) and no keys (receptors), it not going to do you a whole lot of good.  A study in the Journal of Steroid Biochemistry and Molecular Biology reported that when resistance trained men had muscle biopsies taken the greatest predictor of males 1-RM strength was not testosterone but the androgen receptor content in the thigh muscle36.   It appeared that the quantity of androgen content in muscle was a better predictor of muscle strength than circulating androgens in the men studied.  Motor neurons and skeletal muscle both have androgen receptors. Testosterone causes enlargement of motor neurons causing greater force enhancement.  Males have larger motor neurons than females due to the actions of testosterone.  Research in rats has shown that testosterone administration causes an increase in motor size while a castration causes a decrease in the motor size of neurons29.  The data suggest that testosterone may increase strength by increasing motor neuron size.   

     Although some studies have shown no changes in resting testosterone changes after resistance exercise more recent research has shown that the importance of acute testosterone responses and mechanical stimuli from resistance exercise as upregulators of the androgen receptor which influence muscle strength and size.  Other factors as nutrition, overtraining, intensity, and rest duration all influence the secretion of testosterone during exercise.  The acute increases in testosterone are associated with changes in lean muscle mass and strength. 
Key Points:
•    A decrease in testosterone as little as 10% below normal will impede strength gains in response to a strength training program.
•    Acute elevations in testosteone are a potent stimulator of muscle mass and strength.
•    Testosterone produces a dose dependent effect on muscle strength.
•    Motor Neurons contain androgen receptors and increase in size in response to testosterone which may mediate muscle strength.
•    Testosterone increases the neurotransmitters firing potential of muscle with may enhance force production.