The Independent Effects And Interaction Of Gravitational And Inertial Forces On Gait Transition Speed.

Abstract

Prior research suggests the relation between gravitational and inertial forces plays an important role in the determination of the preferred transition speed (PTS) for both walk to run transitions (WRT) and run to walk transitions (RWT), and that humans prefer to transition gait when the ratio of inertial force (IF) to gravitational force (GF) (i.e. Froude #) is ~0.5. PURPOSE: In this study, we investigated the effect of gravitational and inertial forces on PTS and Froude # for both WRT and RWT. We hypothesized that decreasing gravitational force (-GF) would decrease PTS and increase the Froude # at PTS, but that increases in inertial forces (+IF) or increases in both gravitational force and inertial forces (+GF+IF) would not affect PTS. METHODS: Twelve healthy adults (9 M, 3 F) performed WRT and RWT trials on a motorized treadmill across seven combinations of altered body weight (BW) and body mass (BM). Subjects performed the PTS at 1.0BM/1.0BW, 0.70 and 0.85 BW /1.0 BM (-GF), 1.15 and 1.30 BM / 1.0 BW (+IF), and 1.15 and 1.30 of both BW and BM (+GF+IF). For each condition, we determined PTS (m/s and Froude #) by increasing speed (WRT) or decreasing speed (RWT) 0.09 m/s every 30 seconds through two speeds beyond the PTS. RESULTS: -GF decreased the PTS of WRT (p=.02). At 0.70 BW, the PTS was 93% (1.83 m/s) that of the control (1.97 m/s). Neither +IF (p=.156) nor +GF+IF (p=.149) affected the PTS of WRT. -GF increased the Froude # at WRT (p=.01). At 0.70 BW, PTS Froude # was 120% (Fr=.53) of the control (Fr=.44). +IF also increased the WRT Froude # (p<.001). At 1.30 BM, the Froude # was 132% (Fr=.58) of the control. +GF+IF did not affect the Froude # at WRT (p=.135). There was no change in PTS for RWT due to -GF (p=.263), +IF (p=.658) or +GF+IF (p=.202). Yet, -GF increased the Froude # of RWT (p<.001). At 0.70 BW, RWT Froude # was 131% (Fr=.55) of the control (Fr=.42). Moreover, +IF increased RWT Froude # (p<.001). At 1.30 BM, RWT Froude # was 140% (Fr=.59) of the control. +GF+IF did not affect the Froude # of RWT (p=.426). CONCLUSION: This study suggests WRT speed and Froude # are influenced by changes in gravitational forces but are unaffected by changes in inertial forces or proportional changes in both gravitational and inertial forces. In contrast, the relation between gravitational and inertial forces did not appear to influence RWT speed but did affect the RWT Froude #.