Kinematic Changes During a Marathon for Fast and Slow Runners

The purpose of this study was to describe kinematic changes that occur during an actual marathon. We hypothesized that (1) certain running kinematic measures would change between kilometres 8 and 40 (miles 5 and 25) of a marathon and (2) fast runners would demonstrate smaller changes than slow runners. Subjects (n = 179) were selected according to finish time (Range = 2:20:47 to 5:30:10). Two high-speed cameras were used to measure sagittal-plane kinematics at kilometres 8 and 40 of the marathon. The dependent variables were stride length, contact time, peak knee flexion during support and swing, and peak hip flexion and extension during swing. Two-tailed paired t-tests were used to compare dependent variables between kilometres 8 and 40 for all subjects, and regression analyses were used to determine whether faster runners exhibited smaller changes (between miles 5 and 25) than slower runners. For all runners, every dependent variable changed significantly between kilometres 8 and 40 (p < 0.001). Stride length increased 1.3%, contact time increased 13.1%, peak knee flexion during support decreased 3.2%, and peak hip extension, knee flexion, and hip flexion during swing decreased 27.9%, increased 4.3%, and increased 7.4%, respectively (p < 0.001). Among these significant changes, all runners generally changed the same from kilometres 8 and 40 except that fast runners decreased peak knee flexion during support less than the slow runners (p < 0.002). We believe that these changes, for all runners (fast and slow), were due to fatigue. The fact that fast runners maintained knee flexion during support more consistently might be due to their condition on the race day. Strengthening of knee extensor muscles may facilitate increased knee flexion during support throughout a marathon. Key points Runners changed kinematics significantly from kilometres 8 to 40 (increased stride length, contact time, peak hip flexion during swing, and peak knee flexion during swing, and decreased running speed, stride frequency, peak knee flexion during support and peak hip extension during swing). Fast runners demonstrated more peak knee flexion during support throughout a marathon. Runners generally changed kinematics similarly (between kilometres 8 and 40) except that fast runners exhibited a more consistent peak knee flexion during support than slow runners. Resistance training that would increase both muscular strength and endurance of knee extensors may increase peak knee flexion during support and help maintain it similar to the fast runners throughout a marathon. Key words: Fatigue, endurance, run, biomechanics, race Introduction Marathon running is becoming an increasingly popular sport. In 2001, the five most well-known marathons in the world-Boston, Chicago, Berlin, London and New York City-had a total of 121,291 finishers. In 2010, the total finishers of these marathons increased by over 43% to 173,958. Marathon running involves a challenging distance (42 km 125 m) and produces physiological changes that may alter running biomechanics during the race (Hausswirth and Lehenaff, 2001). Kinematics and economy of prolonged running have been extensively studied. Most runners choose their stride length to optimize their running economy when running non-fatigued (Cavanagh and Williams, 1982). When running fatigued, however, decreased stride length does not affect running economy (Kyrolainen et al., 2000; (Elliott and Roberts, 1980), and stride frequency decreases while oxygen uptake increases (Hunter and Smith, 2007). Contact time increases slightly as fatigue occurs (Elliott and Roberts, 1980), due to increased peak knee flexion during support (Derrick et al., 2002; Kellis and Liassou, 2009; Nicol et al., 1991). Peak knee flexion during swing also increases during fatigue (Hausswirth et al., 1997). Although peak hip flexion during swing does not change, peak hip extension during swing decreases during prolonged running (Elliott and Roberts, 1980). However, kinematic alterations when running under fatigued conditions vary among individuals (Nicol et al., 1991; Siler and Martin, 1991) and between study designs. Although the kinematics of prolonged running has been previously studied, little is known regarding how kinematics may change during an actual marathon. To our knowledge, no one has evaluated kinematic changes between the early and late stage of an over-ground marathon. Additionally, only one group of researchers has compared kinematic changes between fast and slow runners during a prolonged run: Siler and Martin, 1991 reported that kinematics for fast and slow runners change similarly during a fatiguing 10-km treadmill run. However, treadmill running in a laboratory setting likely results in kinematics that differs from over-ground racing (McKenna and Riches, 2007; Morin et al., 2009; Nigg et al., 1995; Riley et al., 2008). The purpose of this study was to evaluate running kinematic characteristics during early and late stages of an actual marathon, for fast and slow runners. We asked two research questions: (1) Do running kinematic characteristics change over an actual marathon? and (2) Do potential kinematic changes differ between fast and slow runners? We hypothesized that certain running kinematics change from the early to late stage of an actual marathon, and that observed kinematic changes would be smaller for fast runners than slow runners.