Influence of a hamstring eccentric training on strength and flexibility: a randomized controlled trial

Introduction
Muscle imbalances and poor flexibility are frequently described as risk factors for hamstring injury. Preventive strategies should include eccentric exercises (2, 3), but the influence of multiple, on-field eccentric exercises on these risk factors remains unclear. The main purpose of this study was to investigate the efficacy of an on-field hamstring eccentric program on strength, passive and active flexibility of hamstring muscles.
Methods
Twenty-seven male amateur athletes (22.7±1.9 years, 182±9 cm, 75.1±10.4 kg) were randomly assigned to intervention (n = 13) or control group (n = 14). The intervention group was instructed to perform 15 sessions of four progressively increased eccentric exercises (single-leg Roman dead-lift T-drop, slide leg exercise, Askling’s “glider” and Nordic Hansmtring exercise) during a 6-week period. Both groups were assessed as follows: isokinetic (hamstring and quadriceps) strength and hamstrings-to-quadriceps ratios (Cybex Humac Norm® dynamometer (CSMI, Stoughton, USA)), ranges of motion during passive flexibility (straight leg raise - SLR) and active flexibility (Askling’s H test)(1) using an optoelectronic 3D system (Codamotion® CX1 units (Charnwood Dynamics, Rothley, UK)).
Results
In the intervention group, passive SLR was significantly increased by 11.4° (+12.7%, p<0.001, large effect size), but not active flexibility (+3.1%, p>0.05)(Figure 1). Hamstring eccentric peak torque (+7.1-8%), eccentric body weight normalized peak torque (+7.6-8%) and functional ratios (+8.9-9.3%) were significantly increased (p<0.05, small to medium effect size) in both dominant and non dominant legs of the intervention group participants. No modifications of strength or flexibility were observed in the control group (p>0.05).
Conclusions
A 6-week eccentric program, including four on-field exercises for hamstring muscles, is an effective method of improving several hamstring injury risk factors such as passive flexibility, eccentric strength and functional ratios. As this program may be easily implemented in a real-world context, an association of multiple eccentric exercises might be useful in an injury prevention strategy, especially in amateur athletes.
References
(1) Askling CM, Nilsson J, Thorstensson A. A new hamstring test to complement the common clinical examination before return to sport after injury. Knee Surg Sports Traumatol Arthrosc Off J ESSKA. 2010; 18(12):1798–803.
(2) Guex KJ, Lugrin V, Borloz S, Millet GP. Influence on Strength and Flexibility of a Swing Phase-Specific Hamstring Eccentric Program in Sprinters’ General Preparation. J Strength Cond Res. 2016; 30(2):525–32.
(3) van der Horst N, Smits D-W, Petersen J, Goedhart EA, Backx FJG. The preventive effect of the nordic hamstring exercise on hamstring injuries in amateur soccer players: a randomized controlled trial. Am J Sports Med. 2015; 43(6):1316–23.