The quads and gluts would be the general reply. Recent research (Morin et al 2015) set out to determine if this was the case. With horizontal force production being the key determinant of acceleration performance Morin set out to determine which muscles were most important in producing these horizontal force's.

They examined 14 male subjects from various sports including sprinters, they compared the strength of various muscle with their performance during acceleration. The most significant relationships they discovered were between;

Over the whole acceleration phase - peak horizontal ground reaction force (GRF) and peak hip extensor torque, especially eccentric hamstring strength.

Over the first 10 steps - between gluteus maximus concentric strength and horizontal GRF.

Taken together these results indicate that strength training to improve acceleration should focus upon concentric hip extensor strength particularly the gluteus maximus (hip thrust and deadlifts) and eccentric strength of the hamstrings particularly during knee flexion (Nordics).