We claim the ornamental design for a muscle arm float, as shown and described.
FIG. 1 is a perspective view of a muscle arm float, showing our new design;
FIG. 2 is an enlarged front elevational view thereof;
FIG. 3 is a left-side elevational view thereof, the right-side elevational view being a mirror image thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken lines showing a child and an additional arm float in FIG. 1, and an inflation port in the figures depict environmental matter and form no part of the claimed design.