Passive and active muscle stiffness in plantar flexors of long distance runners
Introduction
In the last decade, several studies have demonstrated that plyometric training improves running performance and economy in long distance runners (Paavolaine et al., 1999, Saunders et al., 2006, Spurrs et al., 2003). One of the proposed explanations for these improvements is the increase in stiffness in muscle–tendon complex of lower limb allow elastic energy to be stored and utilized efficiently during running (Barnes and Kilding, in press). Muscle can be modeled by three components, consisting of a contractile component and two elastic components, parallel elastic component and series elastic component (Zajac, 1989). Mechanical properties of parallel elastic component of human muscle were measured under passive condition during slow stretching (Gajdosik, 2001, Herbert et al., 2011, Hoang et al., 2007, Kubo et al., 2002, Morse et al., 2008, Muraoka et al., 2005). However, mechanical properties of muscle have not yet been assessed under passive conditions in long distance runners. On the other hand, passive muscle stiffness measured using this method has been partially linked to flexibility, e.g., range of motion (Magnusson et al., 1997, McHugh et al., 1998). For example, McHugh et al. (1998) showed that there was a significant correlation relationship between change in passive tension within a given range of motion and maximal joint range of motion (n=16, r=−0.81). Previous studies showed that long distance runners had tighter hamstrings and plantar flexors than untrained subjects (James et al., 1978, Wang et al., 1993). Considering these findings on flexibility of runners, passive muscle stiffness of long distance runners may be higher than that of untrained subjects.
Regarding series elastic component of muscle, mechanical properties of muscle under active conditions have been studied using several methods, such as sinusoidal pertubations (Petit et al., 1990), the quick release test (Goubel and Marini, 1987), and the short range stiffness experiment (Proske and Rack, 1976, Walmsley and Proske, 1981). Walmsley and Proske (1981) performed a short range stiffness experiment and showed that series elastic elements of cat muscle containing predominantly type I fibers were stiffer than those of muscle with higher proportion of type II fibers. On the other hand, trained long distance runners are known to have many slow twitch fibers in their lower limbs (Costill et al., 1976, Thorstenssor et al., 1977). Furthermore, Goubel and Marini (1987) reported that endurance training resulted in an increase in type I muscle fibers with an increase in muscle stiffness. Therefore, the mechanical properties of muscle under active conditions may be stiffer in trained long distance runners than in untrained men.
More recently, Kubo (2014) claimed that the stiffness of human muscle could be determined under active conditions in vivo through observations of lengthening of fascicle during fast stretching. The aim of the present study was to compare passive and active muscle stiffness and tendon stiffness in plantar flexors between long distance runners and untrained men. We hypothesized that long distance runners with higher percentage of slow-twitch fibers had stiffer plantar flexors under passive and active conditions than untrained men, and also that there was no difference in stiffness of tendon structures in plantar flexors between the two groups.
Section snippets
Subjects
The subjects for this study were 20 well-trained male long distance runners and 24 untrained men. All long distance runners had participated in competitive meets at the regional or intercollegiate level within the preceding year, and their mean best official record in a 5000 m race within 1 year prior to these tests was 14.43 (SD 0.16) min. All untrained men were either sedentary or mildly active, but none had been involved in any type of regular exercise program for at least 1 year prior to the
Results
No significant differences in absolute and relative muscle thickness of plantar flexors (p=0.469 for absolute, p=0.289 for relative) or MVC (p=0.172 for absolute, p=0.159 for relative) were found between long distance runners and untrained men (Table 2). The relationship between passive muscle force and elongation of fascicle during slow stretching is shown in Fig. 2. Passive muscle stiffness was significantly higher in long distance runners than in untrained men (p<0.001).
For increase in
Discussion
Regarding the mechanical properties of parallel elastic component of muscle, passive torque and fascicle length were measured during slow stretching in the present study. Needless to say, the measured passive toque was affected by other tissues (i.e., ligament, skin, and another connective tissues surrounding ankle joint) as well as muscle. Several researchers have suggested that the major factor contributing to passive tension is the extensibility of connective tissue elements of endomysium,
Conflict of interest statement
The authors have no conflicts of interest to disclose.
Acknowledgments
This study was supported by a Grant-in-Aid for Scientific Research (B) (26262173 to K. Kubo) from the Japan Society for the Promotion of Science. We thank Mr. Fusao Akasaki and Mr. Ryosuke Akasaki (Applied Office) for their technical support in the design and construction of the device.
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