  We study the high-frequency micro-mechanical response of suspensions composed
by cardiac and skeletal muscle myosin by optical trapping interferometry. We
observe that in low ionic strength solutions upon the addition of magnesium
adenosine triphosphate (MgATP$^{2-}$), myosin suspensions radically change
their micro-mechanics properties, generating a viscoelastic fluid characterized
by a complex modulus analogous to a suspension of worm-like micelles. This
transduction of energy, from chemical to mechanical, may be related to the
relaxed states of myosin, which regulate muscle contractility and can be
involved in the etiology of many myopathies. Within a similar generic
mechanical response, cardiac and skeletal myosin suspensions provide different
stress relaxation times, elastic modulus values, and characteristic lengths.
These discrepancies probably rely on the dissimilar physiological functions of
cardiac and skeletal muscle, on the different MgATPase hydrolysis rates of
cardiac and skeletal myosins, and on the observed distinct cooperative behavior
of their myosin heads in the super-relaxed state. \textit{In vitro} studies
like these allow to understand the foundations of muscle cells mechanics on the
micro-scale, and may contribute to the engineering of biological materials
whose micro-mechanics can be activated by energy regulators.
