Importance of the Double Knee Bend Phase: The vertical ground reaction forces commonly observed during a pulling movement can be noted in Figure 15. As previously discussed most lifters of reasonable standard use a stretch shortening cycle called the "double knee bend" phase. This consists of an un-weighting period in conjunction with eccentric and concentric muscle actions. This double knee bend phase is important because: 1) it reduces the tension on the back (Enoka 1979) and the sudden forceful stretch in some manner enhances the concentric portion of the pull. The exact mechanism whereby a stretch reflex enhances concentric action is not fully understood, but may involve: increased elastic energy use, a myototic or stretch reflex, optimising muscle length, imparting additional energy into the contractile apparatus or optimising muscle activation patterns (Enoka 1979, Bobbert et al. 1996, Newton et al.1997). The Importance of Using Weightlifting Movements in Sports Training: Weightlifting pulling movements include the snatch, clean and their derivatives. As previously noted, these movements are currently used on a world-wide basis as a part of the training for a number of different sports. However, the selection of any exercise for use in training must meet a number of criteria in order to enhance the effectiveness of the training protocol. These criteria are directly related to the concept of "specificity of exercise". Concept of specificity: Specificity deals with the degree of metabolic and mechanical similarity between different movements. The most important aspect of specificity is "transfer of training effect". Transfer of training effect is the degree to which a training exercise affects alterations in long-term performance. From a very practical standpoint this means that: - Adaptations will occur in accordance with imposed overload and - if the overload/stress contains the appropriate characteristics then the adaptations are more likely to improve performance. Characteristics for successful application of an exercise There are a number of characteristics that an exercise must meet for successful transfer of training effect. These characteristics include an appropriate movement pattern, force production, velocity and power considerations. There also must be an overload application for successful performance adaptation. If there is not continued overload then sport performance will not improve beyond adaptation to simple practice of the sport. Movement pattern characteristics include (Siff and Verkoshansky 1998, Stone et al. 2002): - the type of muscle action, - accentuated regions of force production, - the complexity, amplitude and direction of movement, and - ballistic versus non-ballistic movements Factors to be overloaded include: - force production, - rate of force production and - power output