The flow stress of many metals is known to vary more strongly with strain rate above a critical value [MATH] than below it. In this paper an analysis of previous authors' measurements on copper indicates that the value of [MATH] is highly dependent upon specimen size and so it may not be fully representative of inherent material properties. An estimate of inertial stress shows that inertia may be an important mechanism behind this observation. Qualitative arguments suggest that deformation velocity, friction and wave propagation could also contribute