Limits in reliability of leg-spring and joint stiffness measures during single-leg hopping within a sled-based system.

David Diggin,Andrew J Harrison,Ross Anderson

doi:10.1371/journal.pone.0225664

Abstract

Research examining the reliability of stiffness measures during hopping has shown strong consistency in leg-spring stiffness (kleg), but high variability in joint stiffness (kjoint) measures. Sled-based systems (SBS) reduce movement degrees-of-freedom and are used to examine stretch-shortening cycle (SSC) function under controlled conditions. The aim of this study was to examine the reliability of kleg and kjoint during single-leg hopping within an SBSKinematic and kinetic data were collected on four occasions (Day_1, Day_2, Day_3 and Day_3Offset). Participants completed two trials of single-leg hopping at different frequencies (1.5, 2.2 and 3.0 Hz) while attached to an inclined-SBS. Stiffness was determined using models of leg-spring (kleg) and torsional (kjoint) stiffness. Statistical analysis identified absolute and relative measures of reliability. Results showed moderate reliability for kleg at 1.5 Hz between inter-day testing bouts, and weak consistency at 2.2 and 3.0 Hz. Examination of intra-day comparisons showed weak agreement for repeated measures of kleg at 1.5 and 2.2 Hz, but moderate agreement at 3.0 Hz. Limits in kleg reliability were accompanied by weak-to-moderate agreement in kjoint measures across inter- and intra-day testing bouts. Results showed limits in the reliability of stiffness measures relative to previous reports on overground hopping. Lack of consistency in kleg and kjoint may be due to the novelty of hopping within the current inclined-SBS. Constraints imposed on the hopping task resulting from SBS design (e.g. additional chair mass, restricting upper body movement) may have also influenced limits in kleg and kjoint reliability. Researchers should consider these findings when employing inclined-SBS of a similar design to examine SSC function.

Highlights

Stretch-shortening cycle (SSC) tasks are characterised by eccentric lengthening immediately followed by concentric shortening [1]
Across intra-day testing bouts, analysis revealed no significant difference in contact time (CT) (F (2, 18) = 0.10; p > 0.05), flight time (FT) (F (2, 18) = 0.29; p > 0.05) and hopping frequency (HopFreq) (F (1.6, 43.6) = 1.24; p > 0.05) when measured six-hours apart
Examination of kinematic and kinetic variables (Table 3) shows limits in kleg measurement agreement originated from limits in Fz max and ΔLegL measurement consistency

Summary

Introduction

Stretch-shortening cycle (SSC) tasks are characterised by eccentric lengthening immediately followed by concentric shortening [1]. It is understood that the efficiency of SSC task performance is regulated by a combination of elastic and neural mechanisms [1]. Of importance to elastic mechanisms of SSC performance is the property stiffness (k). When examining stiffness characteristics during SSC tasks, researchers often model the elastic nature of the entire leg as a linear-spring (i.e. kleg) [9,19,20,21,22]. Torsional stiffness models are used to clarify the contribution of the stiffness of each joint (kjoint) to kleg [7]

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