Canine trachealis muscle will shorten by 70% of resting length when maximally stimulated in-vitro. In contrast, trachealis muscle will shorten by only 30-40% when stimulated in-vivo. To examine the possibility that an elastic load applied by the tracheal cartilage contributes the in-vivo limitation of shortening, 5 dogs had single pairs of sonomicrometry crystals inserted into the trachealis muscle at the level of the 5^th cartilage ring. The segment containing the crystals was then excised and mounted on a tension testing apparatus. Points on the active length tension curve, and the passive length tension relationship of the cartilage-only were determined. The preload applied to the muscle prior to contraction varied from 10-40 gms (mean 21 ±4 gms). The afterload applied by the cartilage during trachealis contraction ranged from 13-56 gms (30 ±6 gms). The calculated elastic afterloads were substantial and appeared to be sufficient to explain the degree of shortening observed in 4 of the 7 rings; in the remaining 3 rings, the limitation of shortening seen was greater than would be expected from the elastic load provided by the cartilage. Additional sources of loading and/or additional mechanisms may contribute to limited in situ shortening. In summary, tracheal cartilage applies both a preload and an elastic afterload to the trachealis which is substantial and contributes to the limitation of trachealis muscle shortening seen in-vivo.