Studies on skinned fibers and single motor units have indicated that slow twitch fibers are stiffer than fast twitch fibers. This suggests that skeletal muscles with different motor unit compositions may have different short-range stiffness (SRS) properties. Furthermore, the natural recruitment of slow before fast motor units may result in a different SRS-force profile compared to electrical stimulation. However, muscle architecture and the mechanical properties of surrounding tissues also contribute to the net SRS of a muscle and it remains unclear how these structural features each contribute to the SRS of a muscle. In this study, the SRS-force characteristics of cat medial gastrocnemius (MG) muscle were measured during natural activation using the crossed-extension reflex, which activates slow motor units before fast motor units, and during electrical activation, in which all motor units were activated synchronously. Short, rapid, isovelocity stretches were applied using a linear puller to measure SRS across the range of muscle forces. Data were collected from 8 animals. Although there was a trend towards higher stiffness during natural activation, this trend was small and not statistically significant across the population of animals tested. A simple model, in which the slow fibers were assumed to be 30% stiffer than fast fibers, was used to simulate the experimental results. Both experimental and simulated results show that motor unit composition or firing rate has little effect on the SRS property of the cat MG muscle, suggesting architectural features may be the primary determinate of SRS.