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

Research examining the reliability of stiffness measures during hopping has shown strong consistency in leg-spring stiffness (k(leg)), but high variability in joint stiffness (k(joint)) 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 k(leg) and k(joint) during single-leg hopping within an SBSKinematic and kinetic data were collected on four occasions (Day_1, Day_2, Day_3 and Day_3(Offset)). 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 (k(leg)) and torsional (k(joint)) stiffness. Statistical analysis identified absolute and relative measures of reliability. Results showed moderate reliability for k(leg) 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 k(leg) at 1.5 and 2.2 Hz, but moderate agreement at 3.0 Hz. Limits in k(leg) reliability were accompanied by weak-to-moderate agreement in k(joint) 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 k(leg) and k(joint) 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 k(leg) and k(joint) reliability. Researchers should consider these findings when employing inclined-SBS of a similar design to examine SSC function.