Measuring Engine Performance

Andrew Roberts

A considerable amount of research has concentrated on trying to equate the mechanics of movement with the originating energy expenditure.

The main problem with measuring performance is that the total work performed is expended both externally and internally:

Work(tot) = Work(ext) + Work(int)

Whilst measurement of the external work is relatively straightforward with Newtonian physics the internal work can only be guessed. This matters more when pathology is present because it often leads to an increase in internal work for example where co-spasticity is present in cerebral palsy or where muscle action is deficient because of pain or joints are abnormally stiff and resistant to movement.

The contribution of total body mechanical power to explain energy cost differences in normal children is limited whereas a large proportion of the variability in energy consumption in disabled children can be explained by this measure (2.4% vs 87% explanation of variability. Unnithan, 1999)

In the normal subject Duff et al showed that increasing walking speed led to an almost linear increase in the proportion of Work(tot) that could be explained by center of mass displacements:

In addition to a simplistic assessment of changes in potential energy, inertial forces and angular momentum need to be accounted for. The difficulties of understanding internal work mean that it is probable that assessments of external work are confined to research into normal and athletic gait rather than being useful in measuring pathological gait or the effects of treatment.

References

Unnithan VB, Dowling JJ, Frost G, Bar-Or O. Role of mechanical power estimates in the O2 cost of walking in children with cerebral palsy. Med Sci Sport Exerc 1999;31(12):1703-8.

Duff RM, Kerrigan DC, Corcoran PJ, Saini M. The proportional work of lifting the center of mass during walking. Am J Phys Med Rehabil 1996;75(5):375-9.