  Myosin II is the muscle molecular motor that works in two bipolar arrays in
each thick filament of the striated (skeletal and cardiac) muscle, converting
the chemical energy into steady force and shortening by cyclic ATP--driven
interactions with the nearby actin filaments. Different isoforms of the myosin
motor in the skeletal muscles account for the different functional requirements
of the slow muscles (primarily responsible for the posture) and fast muscles
(responsible for voluntary movements). To clarify the molecular basis of the
differences, here the isoform--dependent mechanokinetic parameters underpinning
the force of slow and fast muscles are defined with a unidimensional synthetic
nanomachine powered by pure myosin isoforms from either slow or fast rabbit
skeletal muscle. Data fitting with a stochastic model provides a
self--consistent estimate of all the mechanokinetic properties of the motor
ensemble including the motor force, the fraction of actin--attached motors and
the rate of transition through the attachment--detachment cycle. The
achievements in this paper set the stage for any future study on the emergent
mechanokinetic properties of an ensemble of myosin molecules either engineered
or purified from mutant animal models or human biopsies.
