Running is a sport that, due to its diverse nature, presents a significant risk of injury to an athlete. The typical injuries encountered by a 100-m sprint specialist, where the focus of the athlete is the development of explosive power, may differ from those of the cross-country specialist or the marathoner. The sprinter, with a greater preponderance of fast-twitch fibers and likely greater muscular development, will undertake training regimes involving resistance training or other cross training that will likely differ significantly from those of the distance runner, thus exposing each group to different injury risks.
All running disciplines have injuries that result from the presence of one or more common factors. In almost every case of a running injury, one or more of these factors is present as a primary cause or as a contributor to the injury. Overtraining is the increase of training intensity or training volume, which often includes a sudden increase in the use of a particular training technique, such as interval running or hill training. Another contributor to injury is the nature of the training surface; as a very general proposition, hard artificial surfaces contribute to running injuries more often than do softer or natural surfaces. A switch to an unfamiliar surface may also contribute.
Important to preventing injury to the athlete is the quality of the running shoes used, either in terms of the condition of the shoes or with respect to the suitability of the footwear relative to the size and the physical characteristics of the athlete. Approximately 80% of all runners' feet strike the ground with "pronation," where the ankle and foot roll inward on impact; the remaining 20% of runners exhibit "supination," where the foot and ankle move outward. The shoe worn by the athlete must be one designed to accommodate the appropriate motion, or the athlete risks an unhealthy distribution of forces from the foot into the rest of the body on impact.
Structural misalignment, which directs unequal forces into a particular joint or bone, can cause a significant number of injuries. The most common misalignment is unequal leg length. Also, injuries can result from musculoskeletal imbalance, particularly between opposed muscle groups such as the quadriceps and the hamstrings. This imbalance is typically created through poor stretching and flexibility training.
In sprint running, including the hurdles events, the focus of the athlete in both the training that takes place on the track as well resistance training, is the development of explosive power, which the runner seeks to harness as speed. The tremendous forces generated by a sprinter out of the starting blocks place significant stress upon the knee joint, the hamstrings, and the quadriceps. From the crouched position of the start, where the hamstrings hold the knee in the flexed position, the quadriceps will extend the knee explosively at the start and drive the legs forward. It is a well-accepted principle of biomechanics that the ideal proportion in the relative strength of the quadriceps to the hamstring will be approximately 3:2. When this ratio is not observed (generally the hamstring will be weaker of the two structures), the risk of a serious hamstring pull or tear is significant.
Other possible impacts on the sprinter that arise due to the power of the start are injuries to the Achilles tendon. Repeated movements from the starting blocks when the athlete has a structural imbalance due to muscle power or inflexibility will often irritate the tendon fibers, causing the painful condition of tendonitis, or may, in a worst case, rupture the tendon. The muscles of the lower leg, the gastrocnemius and soleus, are also at risk due to the movement delivered at the instant of the push off from the blocks, but in most cases the Achilles tendon bears the greater brunt of stress that may be magnified through a muscular imbalance. The groin muscles also have exposure to similar injury due to the function that they perform in sprint starts, the stabilizing of the body as it bursts forward.
Middle distance runners who train and compete primarily on hard surfaces as opposed to the newer cushioned track surfaces often develop plantar fasciitis, the connective tissue injury often diagnosed in conjunction with the formation of heel spurs on the bone of the affected foot. Research has determined that this condition is often a function of the hard training surface, inadequate attention to footwear (often lightweight racing shoes with less than optimal support through the arch of the foot, the location of the plantar tissue), and poor foot-specific stretching practices.
The popularity of distance running and the marathon that began in the later 1970s has directed corresponding attention to the specific injuries caused in distance running. Repetitive strain injuries, to all aspects of the musculoskeletal structure of the hips, legs, ankles, and feet of runners, are the most common running injuries. All five of the noted common causes of running injuries play a role in the formation of distance runner repetitive strain injuries. Of particular significance to distance runners are the issues of structural imbalance and stretching and flexibility training. What may present as a minor physical problem for the runner who accumulates a total weekly training mileage of 20 miles (35 km) will almost certainly become a full-blown and potentially chronic issue when the training reaches 50 miles (80 km).
The best way to avoid a significant running injury is to develop a comprehensive training plan. Planning requires preparation, which leads to a comprehensive assessment of physical needs. The identification of structural problems to be addressed by footwear selection, possible orthotic use to correct any imbalance, and the devotion to stretching and flexibility as a prevenatative measure are the most important parts of the runner's planning/injury prevention process.