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Hamstring strains are among the most common sports-related injuries, and account for a significant number of missed competitions for elite athletes in a variety of sporting codes.[1] There is a high rate of hamstring strain reoccurrences which impacts on the athlete’s return-to-play; and indicates the persistent nature of the injury.[1] This is not surprising as, from an anatomical and biomechanical perspective, the hamstring muscle complex is at an increased risk of injury due to its biarticular structure and its functions as a hip extensor and knee flexor.[2] It is particularly vulnerable during sporting activities that involve sudden acceleration/deceleration and jumping, such as field hockey and soccer.[3] Stretching has been advocated as a method of improving or maintaining flexibility and has been used prophylactically in many sporting codes to prevent muscle strains. This is despite the lack of clear evidence for the proposed benefits of improved flexibility in injury reduction.[4] Different stretching techniques, such as static, ballistic and proprioceptive neuromuscular facilitation (PNF), have been used.[5] The concept of PNF stretching is based on neurophysiological principles. The methods include reciprocal and autogenic inhibition through manual contact, diagonal and spiral movement of the limb, and normal timing. These methods facilitate sensory input, functional muscle contraction (including synergist overflow), sequential contraction and coordinated movement, respectively.[6] There are several PNF stretching techniques which may be utilised, including the contract-relax-agonist-contract method (CRAC) which combines different elements of a stretch.[7] In a CRAC stretch, the antagonists (e.g. hamstrings) are first passively stretched, followed by a six to 15 second isometric contraction against resistance at the point of limitation. This contraction is immediately followed by a six to 15 second concentric contraction of the agonists (e.g. quadriceps). This is followed by 20 seconds of rest before the cycle is repeated four to five times. The use of the isometric contraction of the antagonists (hamstrings) at the end of the range serves a dual purpose: the fatigue of fast-twitch fibres and sensory receptor stimulation. Firstly, fatigue of the fast-twitch fibres reduces their capacity for maximum force production when exposed to subsequent stretch resistance.[3] Isometric contraction induces post-isometric relaxation in the muscle, which results in reduced muscle tone. Post-isometric relaxation has been defined as the 15 second refractory period after isometric contraction during which the new point of resistance of a joint or muscle may be achieved more easily.[8] Secondly, sensory receptor stimulation occurs due to the effect of isometric contraction on the Golgi tendon organs (GTO) and muscle spindle fibres. The isometric contraction of a stretched muscle serves to pre-tension the GTO.[9] The increase in tension causes inhibition of the contracting antagonist, while there is simultaneous stimulation of the agonist muscle (quadriceps) by the process of autogenic inhibition.[9] During the transient period of decreased hamstring sensitivity and excitability caused by autogenic inhibition, post-isometric relaxation and H-reflex inhibition, the concentric quadriceps contraction is initiated. Although hamstring electromyographic (EMG) activity may increase, smooth knee joint motion should occur during the quadriceps contraction as the hamstrings are Background: Although stretching is done routinely to prevent injury during explosive sport activities, there is some concern that effective stretching might negatively impact on performance. Objective: This study’s main objective was to investigate the impact of a specific stretch, the contract-relax-agonist-contract (CRAC) stretch, in which the muscle to be stretched, namely, the hamstrings, is actively contracted and then relaxed. This is followed by the antagonist muscle (the quadriceps) contracting. Secondly, the impact of the stretch on performance was examined. Methods: A randomised control trial was used. Forty healthy active males between 21 and 35 years old were assigned to either receive three repetitions of CRAC or rest. Hamstring flexibility and the Illinois Agility Test were the primary outcome measures. Results: The intervention was effective in improving hamstring flexibility by 37% immediately post-application and was maintained for eight minutes thereafter. It had no significant effect on agility or sprint times. Conclusion: CRAC, when applied to stretch the hamstring muscles of active males, resulted in a large increase of active knee extension range of motion, without decreasing performance. Therefore, CRAC appears to be a safe and effective method of increasing the length of the hamstrings pre-sport activity and should be utilised by sports physiotherapists if deemed necessary. It was also shown to be beneficial following the initial assessment.

south african journal of sports medicine

The effect of contract-relax-agonist-contract (CRAC) stretch of hamstrings on range of motion, sprint and agility performance in moderately active males: A randomised control trial