Eccentric resistance training of the knee extensor muscle: Training programs and neuromuscular adaptations

PURPOSE: This review is aimed at describing the methods used in knee extensor eccentric resistance training in healthy subjects and at evaluating the adaptations in strength, activation and structure of this muscle group. METHODS: Seventy-five studies were carefully analyzed and 30 are considered in this review. RESULTS: Training programs comprised of 1-4 sessions per week for a period ranging from four to 20 weeks with isokinetic dynamometers or conventional strength training machines were considered. Isokinetic eccentric training programs included 1-6 sets of 6-12 repetitions, while isotonic eccentric training programs consisted of 3-7 sets of 5-10 repetitions. Eccentric strength gains per training session (0.45-3.42%) were typically found to be greater compared to isometric (0.08-1.30%) and concentric (0.23-1.44%) strength gains. Quadriceps activation was improved in tests performed eccentrically and isometrically, but there is poor evidence of increased concentric activation and reduced co-activation of antagonistic muscles. Regarding muscle structure, significant hypertrophic responses have been demonstrated through increases in anatomical/physiological cross-sectional area, muscle thickness and fiber diameter. Most studies measuring muscle architectural changes reported increased fascicle lengths without changes in pennation angle. Adaptations in fiber type distribution were inconsistent. CONCLUSIONS: Strength gains following knee extensor eccentric training are caused by neural and structural adaptations, and may contribute to physical fitness in healthy populations and health improvement in patients.