Let $n$ be an integer with $3 \leq n \leq 7$, let $M$ be a compact manifold of dimension $n$ with boundary $\partial M$, and let $g$ be a Riemannian metric on $M$ with scalar curvature at least $−n(n−1)$. Under a topological assumption on $M$, we establish an inequality relating the infimum of the boundary mean curvature to the systole of the boundary $\partial M$. As a consequence, we obtain a new positive energy theorem, with equality being attained by the Horowitz-Myers metrics. This is joint work with Pei-Ken Hung.