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The use of antibiotics in farm animals is one of the main reasons for the development of resistant bacterial strains (e.g., zoonotic pathogens). Therefore, save alternatives are needed. Here, we examined how post-hatch application (day one to seven of life) of the probiotic <i>Enterococcus faecium</i> AL41 (EF) affects the development and tissue properties of the broiler pectoralis major muscle (PM). Expression of regulators, namely <i>IGF-1</i>, <i>PAX7</i>, and <i>MYF5</i>, was also investigated. At day 1 (<i>n</i> = 6), and days 5, 8, and 12 (<i>n</i> = 10), muscle samples were taken from control and EF supplemented chicks. From day 5 on, myonuclei number per fiber was elevated in EF chicks. Improved capillarization (from day 8), larger myofibers, increased body and PM weights (day 12) were found in the EF group. Part of our findings is explainable by higher intramuscular expression of <i>IGF-1</i> and lower <i>MYF5</i> expression in EF birds. In both groups <i>IGF-1</i> expression decreases with age, thereby increasing the cellular myogenic potential. However, a strong increase in <i>PAX7</i> expression and more PAX7-positive nuclei were found in EF chicks at day 12. We conclude that EF supplementation improves PM growth and health due to positive effects on bioavailability and fusion capacity of SATC progeny and better tissue perfusion.