Let X = G/K be a symmetric space of noncompact type and rank k ≥ 2. We prove that horospheres in X are Lipschitz (k − 2)-connected if their centers are not contained in a proper join factor of the spherical building of X at infinity. As a consequence, the distortion dimension of an irreducible Q-rank-1 lattice Γ in a linear, semisimple Lie group G of R-rank k is k − 1. That is, given m < k − 1, a Lipschitz m-sphere S in (a polyhedral complex quasi-isometric to) Γ, and a (m + 1)-ball B in X (or G) filling S , there is a (m + 1)-ball B in Γ filling S such that vol B ∼ vol B. In particular, such arithmetic lattices satisfy Euclidean isoperimetric inequalities up to dimension k − 1.Consider now a group G of the form G = m i=1 G i (k i ), where the k i are locally compact, non-discrete fields and the G i are connected, absolutely almost simple algebraic groups defined over k i . Let Γ be an irreducible lattice in G. In a remarkable paper [15], Lubotzky, Mozes and Raghunathan proved that if the total rank k ofIn [4], Bux and Wortman conjectured a far reaching generalization of this result. In order to formulate it, let X be the product of irreducible symmetric spaces and Euclidean buildings on which Γ acts. The total rank of G is then equal to the maximal dimension of an isometrically embedded Euclidean space in X, which we call the geometric rank of X and denote by geo-rank(X). For some point x ∈ X and a real number r define the the following thickening of the orbit Γ · x in X X(r) := {y ∈ X | d(y, Γ · x) ≤ r}.Note that by the Milnor-Švarc lemma, the induced inner metric on X(r) is quasi-isometric to (Γ, d Γ ). Following [4] we define Γ as being undistorted up to dimension m if: given any r ≥ 0, there exist real numbers r ≥ r, λ ≥ 1, and C ≥ 0 such that for any k < m and any Lipschitz k-sphere S ⊂ X(r), there is a Lipschitz (k + 1)-ball B Γ ⊂ X(r ) with ∂B Γ = S and volume(B Γ ) ≤ λ volume(B X ) + C for all Lipschitz (k + 1)-balls B X ⊂ X with ∂B X = S . The distortion dimension of Γ is then defined as dis-dim(Γ) = max{m | Γ is undistorted up to dimension m}.The conjecture of Bux and Wortman posits that dis-dim(Γ) = geo-rank(X) − 1. See [5], [20] for recent progress on this conjecture. The chief goal of the present paper is to prove the Bux-Wortman conjecture for Q-rank 1 arithmetic groups in linear, semisimple groups defined over number fields, i.e. finite extensions of Q. For such lattices the space X above is a symmetric space of noncompact type; that is, there are no building factors. In our proof, it will be convenient to replace the subset X(r) by the complement of a countable union of horoballs in X (see [13], Thm. 3.6). Like X(r), this is quasi-isometric to (Γ, d Γ ). A crucial fact is that for Q-rank 1 lattices these horospheres are disjoint.Theorem A (Distortion dimension). The distortion dimension of an irreducible Q-rank 1 lattice in a linear, semisimple Lie group of R-rank k is k − 1. If k ≥ 2, then such an arithmetic 1. E(∆) contains a copy ∆ of ∆ at its center.