Academic journal article Research Quarterly for Exercise and Sport

Effects of Stretch Shortening Cycle Exercise Fatigue on Stress Fracture Injury Risk during Landing

Academic journal article Research Quarterly for Exercise and Sport

Effects of Stretch Shortening Cycle Exercise Fatigue on Stress Fracture Injury Risk during Landing

Article excerpt

The purpose of this study was to examine changes in landing performance during fatigue that could result in increased stress fracture injury risk. Five participants performed nonfatigued and fatigued drop landings (0. 60 m), while ground reaction force (GRF), electromyographic (EMG) activity, and kinematics were recorded. Fatigue was defined as a 5-20% reduction in vertical jumping performance. Single-subject analyses revealed that all participants were affected (p [less than or equal to]. 05) by fatigue. Post hoc comparisons revealed a group effect (p [less than or equal to]. 05) for selected variables. Participants landed with (a) less joint flexion at contact and used a greater range of motion, (b) greater GRF peaks and loading rates, and (c) less EMG activity. These changes were consistent with greater risk of stress fracture.

Key words: EMG, GRF, kinematics, single-subject design

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Lower extremity stress fracture is a common overuse injury that occurs in physically active individuals who participate in repetitive impact activities such as running, dancing, basketball, and volleyball (Burr, 1997; Hame, LaFemina, McAllister, Schaadt, & Dorey, 2004; Iwmoto & Takeda, 2003). In physically active individuals, the most common sites for lower extremity stress fracture are the metatarsals, calcaneus, and tibia (Burr, 1997), but the specific site varies by activity. Participants involved in jump-landing activities (basketball, volleyball) tend to experience stress fractures of the tibial shaft, medial malleolus and metatarsals (Iwamoto & Takeda, 2003). The incidence of stress fracture varies also by activity but has been ranked between the second and eighth most common injury in runners, with an incidence of 4 to 14.4%, and may be even higher in military recruits (Burr, 1997). A retrospective study of more than 10,000 athletes who visited a sports medicine clinic (Iwamoto & Takeda, 2003) reported 1.8% incidence of stress fracture across a broad spectrum of athletes and anatomic sites. Stress fracture is a common injury but does not occur in everyone, as indicated by the reported incidence values. Detecting individuals who have a greater risk of stress fracture requires a detailed assessment and intensive study at the individual level (i.e., single-subject analysis). Information gained from an aggregate group analysis is less beneficial for this purpose, because important individual behaviors are often masked (Baloff & Becker, 1967; Estes, 1956), especially when individuals use different strategies resulting in heterogeneous responses (Bates, 1996).

The etiology of stress fracture is multifactorial and involves a complex interaction of biological and mechanical processes that occur as a result of repetitive submaximal loading (Grimston & Zernicke, 1993). Repetitive loading initiates the bone adaptation (remodeling) response. When the adaptation is outpaced by the frequency of the loading stimulus, weakened tissue is exposed to additional loading cycles, resulting in a fatigue (stress) fracture. In this process, greater load magnitudes require fewer repetitions to produce failure, thus accelerating the time-course to injury (Nordin & Frankel, 2001).

Mechanically, the interaction of bone strength and bone load can precipitate stress fracture, which can be described by a simple deterministic model (see Figure 1). The model is consistent with other models of overuse injury (Williams, 1992) and stress fracture (Grimston & Zernicke, 1993); it is not intended to be exhaustive but is meant to emphasize the mechanical factors related to stress fracture injury risk. In the model, bone strength is depicted as a function of bone geometry and tissue composition, which are both mediated by remodeling. Bone load at a specific site is determined by the magnitude and other characteristics (rate, direction, point of application) of forces that originate external to the bone but exert force on it, such as muscle contraction and ground reaction force (GRF), among others. …

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