One or the other: Why strength training might come at the expense of endurance muscles. The neurotransmitter brain-derived neurotrophic factor (BDNF) acts in the muscle, so that during strength training endurance muscle fiber number is decreased. Researchers have more closely investigated this factor, from the group of myokines, and demonstrated that it is produced by the muscle and acts on both muscles and synapses. The results also provide new insights into age-related muscle atrophy. Source 3x.