An analysis of data obtained in an experiment investigating the influence of stability training of the lumbo-pelvo-hip complex (LPHC) on stiffness of the hamstrings is presented. Randomized controlled trial. The study included 30 subjects (aged 18-42 years) with increased stiffness of the hamstrings at baseline. Over a period of 4 weeks, stability training aiming to activate the deep stabilizing muscle subsystem and to integrate its action with the superficial subsystem was introduced in the experimental group. The control group remained unaffected. Three series of measurements were applied (baseline, after 2 weeks, and after 4 weeks). A digital inclinometer was used to measure outcomes of passive knee extension in the supine test. In the experimental group, a tendency to decrease stiffness of the hamstrings was observed. It was the opposite in the control group. Significant intra-group differences in the experimental group between series 1 and 3 measurements for both the right and left lower extremities were revealed. Stability training of the LPHC showed a tendency to be effective in reducing stiffness of the hamstrings.