The ornamental design for an apparatus for developingmuscles, as shown and described.
FIG. 1 is a top perspective view of an apparatus for developing muscles showing my new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a left side elevational view thereof;
FIG. 5 is a right side elevational view thereof;
FIG. 6 is a front elevational view thereof;
FIG. 7 is a rear elevational view thereof; and,
FIG. 8 is an enlarged view of the portion shown in FIG. 1.
The broken line circles in the drawings are for the purposes of illustrating specific portions of the apparatus for developing muscles. The circles form no part of the claimed design.