Microscopic examination reveals one of the most characteristic features of vertebrate skeletal muscles: they are striated. Suitable optical techniques or staining methods show that bands of light and dark material alternate along the length of the myofibrils, and that these bands are aligned across the breadth of the fibre.

Detailed descriptions of these striation patterns, and sometimes observations on how they altered with changes in fibre length, were made by a number of nineteenthcentury microscopists. But, as Andrew Huxley (1980) has pointed out, this knowledge was disregarded and further structural studies were largely neglected in the first half of the twentieth century. The advances in understanding of muscle during this time arose largely from biochemical and physiological studies, and the nature of the striation pattern seemed to have no relevance to these approaches.

All this changed with the advent of the sliding filament theory in 1954. Quite suddenly muscle fine structure made sense in terms of function. The search for structural detail as the means of interpreting physiological and biochemical observations began afresh, with new and increasingly sophisticated methods. As a result we now have some exciting glimpses of the molecular activity that underlies muscular contraction. But let us first put the sliding filament theory in its context by taking a look at the biochemical and structural background from which it emerged.

The myofibril in 1953

The contractile machinery of striated muscle cells consists of a small number of different proteins which are aggregated together in filaments.