The viscosity of bovine bone marrow was measured using samples taken from proximal and distal sites of five radii. The viscosity was found to be independent of shear rate and temperature above 37 degrees C (distal samples) and 42 degrees C (proximal samples). The viscosity of all samples fell to a lower limit of 0.04 Pas above these temperatures, irrespective of the treatments used. Below them the measured viscosity of the proximal marrow was some ten times that of the distal marrow at 35 degrees C and some 15 times that of the distal marrow at 30 degrees C. Below 30 degrees C the proximal marrow solidified. Distal marrow remained liquid to below 20 degrees C.

In quadrupedal galloping, one foot of each pair (the ‘trailing’ foot) is set down shortly before the other (‘leading’) foot. Alexander, Jayes & Ker (1980) argued that the trailing and leading feet should exert different patterns of force on the ground, to make the most effective use of tendon elasticity to reduce the energy cost of locomotion. We have used two force plates mounted in tandem in a playing field to obtain records of the forces exerted by individual feet of galloping dogs. We find that the patterns of force exerted by trailing and leading feet are much more similar than had been predicted.

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Hydraulic strengthening, viscous interaction between marrow and trabecular bone, and viscous resistance by the marrow to shear, appear not to occur in long bones when subjected to non-destructive axially applied compressive loads. These conclusions are based on the following observations. Firstly, volumetric measurements suggest that long bones deform in a way that maintains a near constant volume. Hence the mechanism by which ‘hydraulic strengthening’ could operate seems not to exist. Secondly, with little or no volume change, no significant movement between the marrow and the adjacent trabecular bone is likely to occur. The only mechanism by which the marrow could function mechanically would be by its own viscous resistance to shear deformation. As marrow is near liquid at 37°C (with a measured viscosity of 67 × 10-3 Pa s) this ‘resistance’ is shown to be negligible, even when calculated using values most favourable to the production of large shear forces.

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