Terrestrial animals coordinate the elastic properties of their limb joints during locomotion by activating specific muscles at specific times. During bipedal hopping by kangaroo rats, the metatarsal phalangeal (MTP) joint in the foot must be compliant during the landing phase to accommodate surface variations. In the process of transitioning from landing to takeoff, the MTP joint must become stiff to produce the ground reaction forces needed for propulsion. The objective of this research is to elucidate the mechanisms by which the MTP can transition from compliant to stiff in kangaroo rats. We hypothesize that this transition can be produced by the activation of the digital flexor (DF) muscle and of the plantaris (PL) muscle, which is often considered solely as an ankle plantarflexor. Furthermore, we hypothesize that limb posture will affect how quickly muscles are able to change the MTP stiffness upon activation due to slack in the muscle tendons in more plantarflexed positions. To test these hypotheses, we estimated MTP stiffness of anesthetized kangaroo rats by rotating their MTP joint and measuring MTP moment while activating the DF and PL separately. Preliminary analysis shows ~50% of maximal activation of the PL and DF resulted in an MTP stiffness of 0.056 N-cm/deg and 0.057 N-cm/deg, respectively, which indicates that both muscles can contribute substantially to MTP stiffness. In the more plantarflexed limb posture tested, isometric activation of the PL (starting from a passive state) took 25% more time to generate 60% of the maximal joint moment compared to the more dorsiflexed posture tested, a result that indicates more tendon slack in the plantarflexed posture. Both results are important for understanding how different muscles and postures can modulate MTP stiffness during locomotion.