Muscle strains are the most common lower body injuries in professional soccer. Particularly, the biceps femoris and rectus femoris muscles are thought to be at great risk during fast movements (i.e. sprinting, accelerating, change of direction, kicking, landing, etc.) due to their bi-articulate design. PURPOSES: The primary objective of this training study was to determine the effects of eccentric exercise on the optimum length of both the knee flexors and extensors, and to monitor hamstring and quadriceps injuries over the pre-season (i.e. 4 weeks) in professional soccer. Twenty-eight athletes from a professional Spanish soccer team (Division II) were randomly assigned to an eccentric exercise intervention group (EG) or a control group (CG). Over the intervention two athletes from the control group suffered quadriceps injuries and two athletes were contracted by other clubs. After these exclusions, both groups (EG, n = 13; and CG, n = 11) performed regular soccer training during the four week study, which was conducted during the clubs pre-season. The EG performed an additional 10-15 minutes of eccentric exercise, three times per week for the four weeks. Isokinetic dynamometry was used to quantify the optimum length of the knee flexors and extensors, as well as the ratio of peak torque between quadriceps and hamstrings. After the four week intervention, the optimum length of the knee flexors were significantly (p < 0.05) increased by 2.3° in the CG and by 4.0° in the EG. The change in the EG was significantly greater than that of the CG. The optimum lengths of the knee extensors were significantly increased only in the EG by 6.5°. Peak torque levels and ratios of quadriceps to hamstring were not significantly altered throughout the study for either group. A relatively short duration eccentric pre-season programme can significantly increase the optimum length of both the knee extensors and flexors, which may have positive benefits in reducing the risk of injury. Given the strength and conditioning coach's role in prevention of injury and improvement of performance, it would seem good practice to at the very least include bouts of eccentric exercise in the athlete conditioning programme.