FIG. 1 is a lateral side view of a spring element for an article of footwear embodying my new design and shown for a left foot;

FIG. 2 is a medial side view thereof;

FIG. 3 is a bottom plan view thereof;

FIG. 4 is a rear elevational view thereof;

FIG. 5 is a front elevational view thereof;

FIG. 6 is a lateral side view of a modified form showing a non-tapered spring element for an article of footwear and shown for a left foot;

FIG. 7 is a medial side view of FIG. 6;

FIG. 8 is a bottom plan view of FIG. 6;

FIG. 9 is a rear view of FIG. 6;

FIG. 10 is a front view of FIG. 6;

FIG. 11 is a lateral side view of a modified spring element for an article of footwear having asymmetrical curvature on the medial and lateral sides, shown for a left foot;

FIG. 12 is a medial side view of FIG. 11;

FIG. 13 is a bottom plan view of FIG. 11;

FIG. 14 is a rear view of FIG. 11;

FIG. 15 is a front view of FIG. 11;

FIG. 16 is a side view of a tapered inferior spring element for an article of footwear; and,

FIG. 17 is a side view of a non-tapered spring element for an article of footwear; the dotted line representations of the Figures being for illustrative purposes only and form no part of the claimed design.