The ornamental design for an exercise apparatus, as shown and described.
FIG. 1 is a front perspective view of an exercise apparatus, showing my design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a front view thereof;
FIG. 5 is a back view thereof;
FIG. 6 is a left side view thereof; and,
FIG. 7 is a right side view thereof.
The broken lines are for illustrative purposes only and represent unclaimed portions that form no part of the claimed design. The shading represents contour of the claimed design.