Thoracic spine surgery carries a high risk of postoperative motor deficits. Transcranial motor evoked potential (TcMEP) is one of the most sensitive monitoring modalities for postoperative motor deficits. The purpose of this study was to clarify the inter-muscular difference in the TcMEP response to find which muscle is suitable for cortico-spinal tract monitoring in thoracic spine surgery. 109 consecutive thoracic spine surgeries monitored with TcMEP were included in this study. MEP response were recorded from bilateral Adductor Longus (AL), Quadricesps (Q), Hamstrings (H), Tibialis Anterior (TA), Gastrocnemius (G), Abductor Hallucus (AH). Pre- and postoperative motor status in each of these muscles were evaluated. AL, Q and H were grouped as proximal muscles, and TA, G and AH were grouped as distal muscles. Successful derivation rates were calculated, and Cut-off amplitude, sensitivity and specificity for postoperative motor deficits were evaluated using ROC analysis. From 1001 muscles in 100 patients, MEP were successfully recorded. Derivation rates from proximal and distal muscles were 77.8%(461/593) and 91.4%(540/591) respectively. Especially in cases with derivation rate below 80%, the difference in derivation rate was remarkable, 25% in proximal and 74% in distal muscles. The cut-off amplitude for postoperative motor deficits in proximal and distal muscles were 14% and 9%, and their sensitivity were 69%, 76%, and 74%,76%, and Area Under the Curve (AUC) were 0.765, 0.812 respectively. AH had the best profile as a monitoring muscle with derivation rate of 94.3%(181/192), cut-off amplitude 7%, sensitivity 78%, specificity 81%, and AUC of 0.841. In summary, distal muscles had higher derivation rate and higher sensitivity, specificity and AUC compared to proximal muscles. Monitoring based on distal muscles is realistic and effective in cases with difficulty in deriving baseline TcMEP responsein thoracic spine surgery.