The spinal reciprocal inhibition during co-contraction remains unclear. Reports on the reciprocal Ia and D1 inhibitions in the co-contraction are lacking, and a point about the muscle activity amount during co-contraction is unclear. This study aimed to clarify the influence of changes in the ratio of soleus (Sol) and tibialis anterior (TA) muscle activities in co-contraction on reciprocal Ia and D1 inhibitions. Twenty healthy adults were subjected to four stimulatory conditions: a conditioning stimulus-test stimulation interval (CTI) of - 2, 2, or 20 ms or a test stimulus without a conditioning stimulus (single). Co-contraction [change in (Sol)/(TA) activity] was examined at task A, 0%/0% maximal voluntary contraction (MVC); task B, 5%/5% MVC; task C, 15%/15% MVC; task D, 5%/15% MVC; and task E, 15%/5% MVC. At 2-ms CTI, the H-reflex amplitude value was significantly lower in tasks A, B, C, and D than in the single condition. Among the tasks, the H-reflex amplitude values were lower for A, B, C, and D than for E. At 20-ms CTI, the H-reflex amplitude was significantly lower in tasks A, B, C, D, and E. Among the tasks, the H-reflex amplitude was significantly lower from task A and B to task E. The change in the muscle activity ratio during co-contraction could modulate reciprocal Ia inhibition depending on the Sol/TA muscle activity ratio. D1 inhibition at rest did not differ significantly when the Sol/TA ratio was equal or when TA muscle activity was high. During co-contraction with high Sol muscle activity, D1 inhibition decreased from rest.