Ergonomic observation of actual work shows that in most cases even a task that is rated as taking place at 35% VO2max involves a combination of bursts of work interspersed with periods of rest rather than a constant rate (Müller 1953; Kilbom 1995). The health correlation of different combinations of work and rest are not clear, but frequent pauses are considered to relieve fatigue and enhance productivity more than the equivalent amount of time in a single longer rest; this implies that carrying a load at slow speed with pauses consumes less total energy (Jackson and Jones 1998).