Reflex circuit in the spinal cord has been extensively studied in the anesthetized animals, and several type of interneuron has been identified as a key neurons that define unique input-output relations for each reflex. However, there are no direct evidence showing their function in the generation and control of muscle activity during voluntary movement. To address this issue, we identified spinal interneurons (INs) mediating segmental reflex from proprioceptors and examined their activities during voluntary movements. Three macaque monkeys were trained to perform a wrist flexion and extension task with an instructed delay period. An oval spinal chamber was implanted to vertebrae (C4-T1) and a glass-coated elgiloy microelectrode was used to record the activity of INs. Electromyographic activities (EMGs) were recorded from wrist flexor and extensor muscles (n=8-12) by chronically implanted wire electrodes. In addition, nerve cuff electrode was implanted to the deep radial (DR) nerve that innervates most of wrist extensor muscles. Surgeries for these implants were performed while the animals were anesthetized with isoflurane (1.0% – 2.0% in 2:1 O2:N2O) or sevoflurane (1.5% – 3.0% in 2:1 O2:N2O) under aseptic conditions. Experiments in this study were performed in accordance with the National Institutes of Health Guidelines for the Care and Use of Laboratory Animals and were approved by the Animal Research Committee at the National Institute for Physiological Sciences, Japan. In the monkeys performing wrist movement, we identified the peripheral input to INs by their responses to the electrical stimulation to the DR nerve. The INs that responded within a segmental latency of 1ms were identified as the cells with direct projection (first-order INs) from DR afferent nerve. Output of these INs to muscles were examined by the spike-triggered averaging of EMGs. In the 81 INs that showed monosynaptic response to the DR stimuli, 32 INs (39%) showed postspike effects to one or more muscles (22 post-spike facilitations (PspF, 69%), 7 post-spike suppressions (PspS, 22%), or 3 PspF & PspS (9%)). Among 22 INs with PspF, 18 INs (82%) showed their PspF exclusively in the extensor muscles, and number of muscle that showed PspF from each INs (“muscle field”) was 2.0±1.0. These results suggest that the spinal INs that mediating proprioceptive input from extensor muscles preferentially facilitated the extensor muscles (autogenetic facilitation). Next, we analyzed the modulation of firing rate of the first-order INs as a function of task sequence. First-order INs with PspF to extensor muscles (n=18) showed sustained activity throughout the extension torque. In contrast, the first-order INs without PspF in any tested muscles (n=49) transiently increased their firing during torque onset, but it decreased significantly while monkey sustained extension torque (p< 0.01). These results suggest that disynaptic, excitatory reflex pathway that mediating autogenetic facilitation of extensor muscle is involved in the maintenance of static muscle force during voluntary movements. We propose that voluntary muscle activity could be automatically augmented by this reflex system.