Many traditional lower body strength exercises prescribed by coaches primarily train vertical force production. Olympic lifts, squats, deadlifts and their respective variations are perfect examples of movements that develop vertical forces. A common criticism of only utilizing these types of exercise is that it neglects the development of horizontal force production. There is great debate over which direction of force (horizontal or vertical) is the limiting factor in sprinting. Evidence exists supporting both sides of this argument. It is generally accepted that the acceleration phase of a sprint requires a higher level of horizontal force production while maximum velocity sprinting depends more on vertical force production.
Just recently, a new study was published in the International Journal of Sports Physiology and Performance that investigated this topic. Los Arcos et al (2013) divided a team of 22 elite adult male soccer players into two separate training groups over an 8 week period. One group trained with vertical force producing exercises while the other group trained performing both vertical and horizontal force producing exercise. The researchers assessed performance pre and post-training to determine the effects of the program. Performance tests included double and single leg counter-movement jumps, half squat peak power, 5 and 15m sprint performance and blood lactate concentrations at selected running periods. Both groups showed significant improvements in half squat peak power and small improvements in 5 and 15m speed and aerobic fitness. Only the combined training group saw improvements in counter-movement jump.
Further research into this matter is certainly needed, however it appears that including exercises that train force production in a horizontal direction may offer additional benefits verses training vertical force production alone. Some effective exercises for developing horizontal force production include supine hip thrusts, glute-ham raises, prone hip extensions (on bench) reverse hyper-extensions, reverse leg-press and of course, resisted sled towing. A complete training program will incorporate a variety of exercises that train force development in multiple directions.
Los Arcos, A., Yanci, J., Mendiguchia, J., Salinero, J. J., Brughelli, M., & Castagna, C. (2013). Short-Term Training Effects of Vertically and Horizontally Oriented Exercises on Neuromuscular Performance in Professional Soccer Players. International Journal of Sports Physiology and Performance, Ahead of Print