The aim of the research is to investigate the relationship between chemical energy utilisation and mechanical output from skeletal muscle. The principle areas in which research has been pursued are: - efficiency of skeletal muscle, in which studies are aimed at understanding the determinants of the ratio between chemical energy turnover in the muscle and the mechanical output in locomotor type activities. - the metabolic (biochemical) factors underlying muscle fatigue, and the consequences of fatigue upon muscle functioning during different types of exercise. - the significance of the organised variability of the contractile and metabolic properties of motor units in skeletal muscles, and the level and pattern of activation of the different types of motor units during (natural) movement patterns of varying intensity and duration. Our work based on both animal and human work has made an important contribution to the understanding of the significance of muscle fibre and unit variability in relation to muscle energetics. In human studies application of our recently developed technique for microdissection of single fibre fragments from needle biopsy enabled us to demonstrate how metabolic properties are related to the continuum of contractile protein isoform expression. In complementary animal experiments we have been able to describe how contractile and metabolic properties of motor units vary between physiological compartments of the same muscle and how this compartmentalization is related to recruitment patterns during different locomotor speeds. In parallel studies work has continued on elucidating the role of high energy phosphate availability and utilization in high intensity exercise. An important aspect of this work is the development of our understanding of the exercise induced "acute" plasticity of muscle fibre contractile and metabolic properties. Building on our earlier work, including the fatigue related changes in muscle contractile properties, we have recently demonstrated that these changes are not necessarily associated with changes in the rate of energy turnover for a given mechanical output. That is mechanical efficiency may, paradoxically, remain relatively constant despite "slowing" of the muscle consequent upon fatigue. Taken together these studies are fundamental for understanding the neuromuscular strategies - constraints and limitations - which determine the ability to sustain human movement.