Contractile properties of human motor units in health, aging, and disease

The primary function of skeletal muscle is to produce force for postural co ntrol and movement. Although the contractile properties of the whole muscle are useful functional indicators, they do not accurately reflect the heter ogeneity of the constituent motor units (MUs) and their changes in health a nd disease. However, data on the contractile properties of human MUs, in co mparison to other animal species, are relatively sparse. This, in part, is due to greater methodological challenges of in vivo studies of MUs in the h uman. The purpose of this review is to critically appraise the methods used in humans; to describe the normative data from different muscle groups; to discuss differences between data from healthy humans and other animal spec ies; and, last, to characterize changes of the MU contractile properties in aging, disease, and in response to intervention. Because the spike-trigger ed averaging technique can only be used to study the twitch properties, oth er methods were subsequently developed to measure a wider range of contract ile properties. Although there is general agreement between human data and those from other animal species, major differences do exist. Potential reas ons for these discrepancies include true biological differences, but differ ences in the techniques used may also be responsible. Although limited, mea surement of MU contractile properties in humans has provided insight into t he changes associated with aging and motoneuronal diseases and provides a m eans of gauging their adaptive capacity for training and immobilization. (C ) 2001 John Wiley & Sons, Inc.