Relationships between multiple mechanical stiffness assessments and performance in middle-distance runners
Rogers, Simon Anthony
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Background: The ability to sustain high running speeds during competitive races is a key determinant of success in elite middle-distance (MD) runners. Rapid and economical movements are, therefore, highly valued characteristics in their ability to determine performance. It is suggested lower-body stiffness may serve as a valuable measure of a runner’s compressibility and energy utilization during their ground contact interactions at both maximal and submaximal speeds. Unfortunately, evidence concerning the relationship between stiffness and running performance is convoluted, and a lack of consensus regarding optimal stiffness assessment methods leave a degree of confusion for practitioners and researchers alike. Aim: To quantify the levels of lower-body stiffness in highly-trained male MD runners and determine whether stiffness is a determinant of running economy (RE) and maximal velocity (vmax). Further, the study aimed to determine the relationships between multiple levels of stiffness measures calculated from both laboratory and field settings. Method: Using a cross-sectional analysis, eleven highly-trained male MD runners (Mean ± SD: age: 20.0 ± 2.9 yr; body mass: 68.2 ± 6.7 kg; V̇O2peak: 67.6 ± 3.8 ml·kgˉ1·minˉ1; 1500 m personal best 4:02 ± 0:06 min:s) performed maximal overground sprints and a submaximal treadmill test to determine RE followed by a ramp protocol to elicit V̇O2peak. Multiple assessments of stiffness were conducted based on the spring-mass model (SMM), including leg (kleg) and vertical stiffness (kvert) during running and hopping respectively. Additionally, the Achilles tendon stiffness (kAT) was estimated using ultrasound during maximal isometric ankle plantarflexion. Results: Stiffness values were comparable with previous literature on trained runners. There was a very large negative correlation (r = -0.70) between RE and kAT. RE had large and moderate negative relationships (r = -0.60, -0.46) with kvert during the maximal hopping tasks on the right and left limbs respectively. When kleg was calculated during submaximal running, the association with RE was unclear. In addition, large positive correlations were detected between kAT and vmax (r = 0.52) and between kleg and vmax during sprinting (r = 0.59). Finally, examining the association between stiffness methods, kvert during maximal unilateral hopping (right leg) and kleg during sprinting has an extremely large (r = 0.92) relationship. Measures of kleg during sprinting also held very large (r = 0.73) relationships to unilateral kvert and kAT. Conclusion: This study demonstrated a clear association between both global and component stiffness measures and a faster and more economical running performance in trained runners. These findings in the context of MD runners of this calibre are unique. Although kAT had the strongest relationship with RE, the use of more practical methods of measuring stiffness is recommended. Namely, stiffness expressions utilizing the SMM in maximum velocity sprinting (primarily) and maximal unilateral repeated hops (secondly) appear to be useful surrogates for field-based assessment of stiffness related to MD running performance. Agreement between the SMM assessments and kAT, highlighted the contribution of the muscle-tendon structures of the lower-limb to models of human running. Researchers and practitioners should therefore consider SMM stiffness assessments from a range of tasks to profile MD running performance.