The Goldilocks running shoe—not too big, not too small—is gaining ground.

For four decades, the shoes worn by the vast majority of recreational runners shared the same silhouette: a chunky, cushioned heel tapering to a low, narrow toe. Five years ago, the best-selling book "Born to Run" jolted the running world by questioning whether runners should be wearing shoes at all. That spurred a wave of barefoot and barely-shod runners.

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