---

Background and Objectives: Measuring the H-reflex latency (Tp) is an electrophysiologic test to evaluate the S1 root lesions This reflex traverses a relatively long course and its sensitivity to diagnose mild S1 root lesions is low, so investigators have tried to shorten this pathway Evaluation of the central loop of the H-reflex (Tc) is one of these efforts and may be more sensitive in diagnosing S1 radiculopathy (Compared to standard H-reflex latency). We have tried to determine its influencing factors and find the related regression equation. Materials and Methods: Forty volunteers were investigated with their informed consent. Their history, physical examination and a brief electrophysiologic study were normal. H-reflex latencies (Peripheral- central) of right gastrocnemius-soleus muscle as described in literature, right leg length and age were recorded. Results: This group consisted of 26 males (65%) and 14 female (35%). Mean latencies (And SDs) of peripheral and central H-reflexes were 28.8 (SD: 1.66) and 6.78 (SD: 0.3) milliseconds respectively. The correlation between Tc and leg length was significant (P<0.05). There was no significant correlation between Tc and age (P<0.05). Tc and Tp were correlated. These regression equations were found: Tc=0.04L+5.28, Tc=0.05Tp+4.56, Tp=0.04A+0.3L+16.13 (L: leg length in centimeters, A: age in years, Tc and Tp in milliseconds). Discussion: The results of this study, regarding mean and SD of Tc are in line with the previous studies, and regarding the influence of age on Tc are in contrast with only one study and in line with another study. Conclusion: The correlation between Tc and leg length was significant (P<0.05). There was no significant correlation between Tc and age (P<0.05). These regression equations were found. Tc=0.04L+5.28, Tcx=0.07Tp+4.56, Tp=0.04A+0.3L+16.13 (L: Leg length in centimeters, A: Age in years, Tc and Tp in milliseconds). We concluded that it is wise to use leg length for calculating normal Tc and limiting the normal values. On the other hand, the authors believe that more research on specificity and sensitivity of this test (To diagnose S1 radiculopathy) and comparing it with other diagnostic tools is needed.