Background: Smoking is widely recognized as a contributor to numerous health problems; Furthermore, the knee extensors contribute greatly to the stability as well as proprioception of the knee joint. Purpose: This study was carried out to examine the correlation of serum cotinine level, serum serotonin level, peripheral fatigue of Knee extensors as well as proprioception of the Knee joint. Materials and methods: Sixty-six healthy males were divided into three groups of similar size based on their serum cotinine levels, 22 non-smokers Group (A) (control group) having a mean age (30.63 ± 8.43) years, mean body mass index (BMI) (21.64 ± 1.24) kg/m2 and mean cotinine level (6.36 ± 1.43) ng/mL, 22 moderate smokers Group (B) having a mean age (29.32 ± 7.65) years, mean BMI (21.97 ± 1.01) kg/m2 and mean cotinine level (32.86 ± 5.88) ng/mL, as well as 22 heavy smokers Group (C) having a mean age (32.77 ± 5.93) years, mean BMI (21.95 ± 1.29) kg/m2 in addition their mean cotinine level (364.13 ± 18) ng/mL. Each participant had a blood sample obtained to test for cotinine as well as serotonin levels in the lab, accuracy in repositioning was evaluated using an isokinetic dynamometer (joint reposition error) and compute the fatigue index as a means of quantifying a person's vulnerability to fatigue. Results: The fatigue index demonstrated statistically significant differences among the three groups, with the greatest mean value being reported in the heavy smokers' group, and the amount of serotonin in the blood varied significantly. There was a negative correlation between serum cotinine as well as serotonin levels, with the greatest mean value being reported in the nonsmokers group. Moreover, the joint reposition error was significantly larger in the smokers' group compared to the non-smokers' group Error rates were lower among non-smokers compared to smokers. The correlation between serum cotinine and serum serotonin was a strong negative significant correlation, The correlation between serum cotinine with fatigue index of right and left knee extensors and JPE of right and left knee at 70⁰ and 30⁰ knee flexion were strong positive significant correlation and The correlation between serum serotonin with fatigue index of right and left knee extensors and JPE of right and left knee at 70⁰ and 30⁰ knee flexion were strong negative significant correlation. Conclusion: Cotinine has substantial impacts on the peripheral fatigue, repositioning accuracy of knee extensors as well as on serum serotonin level.