Computational fluid dynamics vs. inverse dynamics methods to determine passive drag in two breaststroke glide positions.

Computational fluid dynamics (CFD) plays an important role to quantify, understand and "observe" the water movements around the human body and its effects on drag (D). We aimed to investigate the flow effects around the swimmer and to compare the drag and drag coefficient (CD) values obtained from experiments (using cable velocimetry in a swimming pool) with those of CFD simulations for the two ventral gliding positions assumed during the breaststroke underwater cycle (with shoulders flexed and upper limbs extended above the head-GP1; with shoulders in neutral position and upper limbs extended along the trunk-GP2). Six well-trained breaststroke male swimmers (with reasonable homogeneity of body characteristics) participated in the experimental tests; afterwards a 3D swimmer model was created to fit within the limits of the sample body size profile. The standard k-ε turbulent model was used to simulate the fluid flow around the swimmer model. Velocity ranged from 1.30 to 1.70 m/s for GP1 and 1.10 to 1.50 m/s for GP2. Values found for GP1 and GP2 were lower for CFD than experimental ones. Nevertheless, both CFD and experimental drag/drag coefficient values displayed a tendency to jointly increase/decrease with velocity, except for GP2 CD where CFD and experimental values display opposite tendencies. Results suggest that CFD values obtained by single model approaches should be considered with caution due to small body shape and dimension differences to real swimmers. For better accuracy of CFD studies, realistic individual 3D models of swimmers are required, and specific kinematics respected.

The balance control effects on sitting posture induced by lumbosacral orthosis wear vary depending on the level of stability.

This study aimed to assess the differential impacts of lumbosacral orthosis (LO) wear in different sitting conditions through posturographic measurements. Twelve healthy subjects sat on a force platform with three variable stability levels (stable and on seesaws with a long and short radius, inferring slightly and highly unstable sitting, respectively) and three orthosis conditions (no LO, neutral LO, lordotic LO). Using fractional Brownian motion modelling of the centre of pressure (CoP) displacements, it appears that a stable sitting position did not highlight any particular differences between the LO models. With the lordotic LO, a slightly unstable sitting position decreased the mean time by 72% (p < 0.002) before postural corrective mechanisms took over. In contrast, in highly unstable sitting conditions, the lordotic LO induced larger CoP displacements (increasing variance by 162%, p < 0.038). Thus, depending on the amount of perturbation and the device design, wearing an LO may have a neutral, positive or negative impact on postural control in the sitting position.