Muscle contraction has been the focus of scientific investigation for
more than two centuries, and major discoveries have changed the field
over the years. Early in the twentieth century, Fenn (1924, 1923) showed
that the total energy liberated during a contraction (heat + work) was
increased when the muscle was allowed to shorten and perform work. The
result implied that chemical reactions during contractions were
load-dependent. The observation underlying the "Fenn effect" was taken
to a greater extent when Hill (1938) published a pivotal study showing
in details the relation between heat production and the amount of muscle
shortening, providing investigators with the force-velocity relation for
skeletal muscles. Subsequently, two papers paved the way for the current
paradigm in the field of muscle contraction. Huxley and Niedergerke
(1954), and Huxley and Hanson (1954) showed that the width of the
A-bands did not change during muscle stretch or activation. Contraction,
previously believed to be caused by shortening of muscle filaments, was
associated with sliding of the thick and thin filaments. These studies
were followed by the classic paper by Huxley (1957), in which he
conceptualized for the first time the cross-bridge theory; filament
sliding was driven by the cyclical interactions of myosin heads
(cross-bridges) with actin. The original cross-bridge theory has been
revised over the years but the basic features have remained mostly
intact. It now influences studies performed with molecular motors
responsible for tasks as diverse as muscle contraction, cell division
and vesicle transport.