The training variables most often identified as risk factors for overuse running injuries include running distance, training intensity, rapid increases in weekly running distance or intensity, and stretching habits (James, Bates et al. 1978; Jacobs and Berson 1986; Marti et al. 1988; Messier and Pittala 1988; Paty 1994; James 1998; Plastarasv et al. 2005). Examining how these variables affect the stress–frequency relationship reveals how some of these training variables may lead to overuse injuries (figure 1.1). Increasing running distance increases the number of repetitions of the applied stress since the number of steps taken increases. Provided that running speed remained unchanged, the magnitude of the forces and moments produced at various musculoskeletal structures during each step remain unchanged also (neglecting fatigue effects). Thus, running a greater distance places the involved musculoskeletal structures further to the right on the graph by increasing the number of stress applications. Since this portion of the curve has a slight negative slope, locations further to the right on the curve require slightly lower stresses for a structure to enter the injury zone of the curve. Thus, the possibility that one or more structures will enter the injury zone of the graph increases with increasing running distance.
In running, training intensity relates to running speed. Faster running speeds generally produce greater forces and torsional stress to the involved musculoskeletal structures (Hamill et al. 1982; Nigg 1986; Derrick et al. 2000; Mercer et al. 2002). When training intensity increases, the stress level applied to all of these structures occurs higher on the stress-frequency graph (figure 1.1). Locations higher on this graph require fewer repetitions for a structure to enter the injury zone. In this way, when training intensity increases without a decrease in running distance or frequency, the likelihood of injury also increases.
The stress–frequency relationship can also explain how rapid changes in distance or intensity increase the risk of injury. When a musculoskeletal structure is subjected to a stress-frequency combination that is close to the stress–frequency curve yet below or to the left of the curve, positive remodeling of the structure may occur, shifting the curve upward and to the right as long as detraining does not occur. When these increases in running distance and intensity are gradual, it is possible to shift the stress–frequency curve to outpace the shifting of the structure’s location on the graph. However, rapid increases in running distance or intensity may cause the structure to cross the curve from the non-injury region to the injury region even when some positive remodeling and shifting of the curve has occurred.
Performing stretching exercises before running is a training-related variable that has been examined as a possible risk factor for running injuries. Unfortunately, there have been conflicting conclusions drawn regarding the association of this factor with overuse running injuries. A number of researchers have reported that people who stretch regularly before running experience a higher rate of injury than those who do not stretch regularly (Jacobs and Berson 1986; Hart et al. 1989; Rochcongar et al. 1995). On the other hand, others have not found an association between stretching before running and injuries (Blair et al.1987; Macera et al. 1989; Hreljac et al. 2000). No empirical studies have reported that regular stretching before running reduces the number of running injuries, even though this practice has been advocated as a means of preventing running injuries (van Mechelen et al. 1993). However, data related to the stretching and warm-up habits of runners generally rely on surveys or self-reporting, so these results must be considered cautiously. Indeed, it is very possible that stretching before running is important for some runners, while it may not be necessary for others. A systematic review and meta-analysis by Yeung and Yeung (2001) reported that research investigating protocols of stretching before exercise and stretching outside the training sessions did not produce a clinically useful or statistically significant reduction in the risk of soft tissue running-related injuries. Without conclusive evidence, other factors, such as training errors, should be considered first as potential contributors to injury.
Several clinical studies have estimated that over 60% of overuse running injuries are a result of variables related to training (Clement et al. 1981; Lysholm and Wiklander 1987; Kibler 1990; Macintyre et al. 1991). From a practical standpoint, it could be stated that all overuse running injuries are attributable to training variables. To sustain an overuse injury, a runner must have subjected some musculoskeletal structure to a stress–frequency combination that crossed over to the injury zone of the current stress–frequency curve for the injured structure. This can only be accomplished when an individual exceeds the current limit of running distance or intensity in such a way that the negative remodeling of the injured structure predominates over the repair process. The exact location of this limit would vary from structure to structure and from individual to individual, but there is no doubt that runners can prevent these injuries by training differently based on individual limitations or in some cases by not training at all.
One of the most appealing aspects of assigning the causes of all overuse running injuries to training variables is that all of these injuries could then be considered preventable, since runners have control over training variables. However, rarely do runners know that they are about to commit a training error that will place them outside of their injury threshold. Therefore, to prevent overuse running injuries and assess and understand the etiology of a current injury, knowledge of the current limits of all of the involved musculoskeletal structures is required. These limits primarily are determined by anatomical and biomechanical variables in addition to the current state of training, strength and flexibility of specific tissues and muscles, and the integrity and injury status of various structures. Of course, it is not possible to know these limits exactly, but it is possible to minimize the risk of injury by thoroughly understanding the key clinical and biomechanical risk factors.
This book provides clinicians with a thorough and scientifically grounded basis so that they can make evidence-based decisions about these clinical and biomechanical factors. With this in mind, we discuss the most common running injuries and their multifactorial nature based on the current research.