Hill running can provide many health benefits to runners, but there are also potential associated risks (Ferley et al., 2013; Jamurtas et al., 2013; Koji et al., 2012). Among benefits are improved cardiovascular conditioning and increased strength (Telhan et al., 2010), favorable blood lipid changes (Jamurtas et al., 2013), reduced risk of injury due to an adaptive effect of training (Proske and Morgan, 2001), and improved performance with specificity of exercise. Understanding physical changes to body tissues during hill running is important for training and competition. Of particular relevance is how hill running affects the Achilles tendon (AT), which is a common location for injuries in runners. An estimated 52% of recreational and competitive distance runners have an AT injury at some point (Kujala et al., 2005). The progressive lengthening of an elastic structure when it is cyclically loaded (creep), as in running, may be indicative of micro-damage to the AT (Ker et al., 2000) and consequently, indicative of injury risk (Farris et al., 2012; Kongsgaard et al., 2005; Pang et al., 2006; Ying et al., 2003). One method of monitoring tendon size changes is to measure the cross-sectional area (CSA) using ultrasound imaging (Farris et al., 2011).
The properties of tendons allow them to stretch withstand tension, and change length and thickness, thus allowing for economical locomotion (Wilson and Lichtwark, 2011). Tendons stretch up to 10% of their resting length, so this gives the tendon a large stretch ability relative to the length of the muscle (Wilson and Lichtwark, 2011). Recent in vivo studies that used ultrasound have shown that the human AT rapidly lengthens and shortens during the stance phase of running, which stores and returns elastic energy (Ker et al., 2000; Magnusson and Kjaer, 2003; Wilson and Lichtwark, 2011). The reutilization of this elastic energy in tendons greatly reduces energy demands in running (Cavagna et al., 1964). Researchers claim that the stretch-recoil of the tendon reduces the work that the muscle must perform and thus reduces energetic cost (Arampatzis et al., 2006; Lichtwark and Wilson, 2006; Wilson and Lichtwark, 2011).
Tendons have been shown to be adaptable (Magnusson et al., 2008), but whether the tendon adapts to physical activity by changing the CSA remains unknown (Magnusson and Kjaer, 2003). Conflicting study results report that during running the tendon cross-sectional area increased (Sommer, 1987; Woo et al., 1980; 1982), remained unchanged (Buchanan and Marsh, 2001), or decreased (Tardioli, 2011; Woo et al., 1982). There remains uncertainty as to how AT CSA responds to exercise. Of particular interest is grade running, that is uphill and downhill running, and how it relates to injury risk. Gottschall and Kram (2005) compared peak impact and active ground reaction forces during uphill and downhill running. Their impact force data suggest that downhill running substantially increases the likelihood of an overuse running injury, but no active force data was affected by grade running. The Boston, Chicago, New York City, and Los Angeles marathons include hill grades ranging from 0.2% to 6%. In selecting a test grade for research, 6% would realistically be seen in racing and has also been shown to produce optimal running economy (Minetti et al., 2002). We wanted to study downhill running compared to uphill running, taking into account Achilles tendon CSA and active peak vertical forces.
The primary purpose of this study was to determine the relationship between the grade of the running surface and the change in AT CSA in a population of female runners. We hypothesized: (1) the percent change of AT CSA would have the greatest decrease after running on the incline grade, (2) the percent change of AT CSA would be the least after downhill running, and (3) the percent change of AT CSA would decrease after running on the level grade.
This study was a crossover design. Each individual was measured three times on different days and in different running grade conditions. We used Latin-square randomization, with participants drawing their treatment order from a container. Intra-individual comparisons were made with the independent variable being grade of the running surface (-6%, 0%, +6%). The dependent variables were change in Achilles tendon cross-sectional area from prerun to post-run measurement, peak vertical force and plantar-flexion velocity.
Twenty female runners (age = 20.7 [+ or -] 1.8y, height = 1.65 [+ or -] 0.06m, mass = 60.5 [+ or -] 7.2kg, weekly mileage run = 26.8 [+ or -] 12.1km, fastest 5000m time = 18:31.59 [+ or -] 2:11.32 min, mean [+ or -] standard deviation) volunteered for this study. They ranged from recreational to collegiate Division I runners. None of the participants had suffered an Achilles tendon injury within six months or a lower extremity injury within three months of data collection, or were pregnant. Inclusion criterion was current capability to run 5000m in under 24:00 min. All procedures were granted approval by the Institutional Review Board. Power assessment indicated that with a SD of 0.04, 16 participants were needed to detect a significant difference of 4 [mm.sup.2] between pre- and post-run AT CSA measurements (p = 0.05, [beta] = 0.8). We collected data on 20 participants, assuming a 20% dropout rate. All participants completed the study.
Each approved participant came to the Human Performance Research Center Biomechanics lab on the Brigham Young University campus on three separate days. The time between visits was at least 48 hours, which time period was found to be adequate for a return to baseline pre-run measurements of AT CSA during pilot work. Participants came at approximately the same time of day for each visit. Participants completed a pre-participation questionnaire to determine eligibility before any data collection. Prior to arrival, each participant received instructions to maintain normal eating and hydration habits, refrain from eating a meal within two hours of testing, follow normal...