During muscle contraction, myosin motors anchored on thick filaments bind to and slide actin thin filaments. These motors rely on  ATP, supplied at the limits of diffusion from the sarcoplasm to the interior of the lattice of actin and myosin . Classic sliding filament theory suggests that lattice spacing is constant . If so, then the lattice changes volume during contraction and could provide fluid motion and hence assist in transport of molecules between the contractile lattice and  surrounding intracellular space. If, however, the lattice is isovolumetric, it must expand when muscle shortens. Doing so would alter the binding dynamics of myosin which are sensitive to spacing. We first create a convective-diffusive flow model and show that  flow into and out of the sarcomere lattice would be significant in the absence of lattice expansion. Convective transport coupled to diffusion has the potential to substantially enhance metabolite exchange within the crowded sarcomere. Using time resolved x-ray diffraction of contracting muscle  we next show that the contractile lattice is neither isovolumetric , nor constant in spacing. Instead  lattice spacing is time-varying, depends on activation, and can manifest a negative (auxetic) Poisson ratio. This unusual material behavior arises from the multiscale interaction of muscle axial strain, lattice spacing and myosin binding. The resulting fluid flow in the sarcomere lattice  is even greater than would be expected from constant lattice spacing conditions. Akin to ``breathing,'' convective-diffusive transport in sarcomeres is sufficient to promote metabolite exchange , and may play a role in the regulation of contraction itself.
During muscle contraction, myosin motors anchored to thick filaments bind to and slide actin thin filaments. These motors rely on energy derived from ATP, supplied , in part, by diffusion from the sarcoplasm to the interior of the lattice of actin and myosin filaments. The radial spacing of filaments in this lattice may change or remain constant during during contraction. If the lattice is isovolumetric, it must expand when the muscle shortens. If, however, the spacing is constant or has a different pattern of axial and radial motion, then the lattice changes volume during contraction , driving fluid motion and assisting in the transport of molecules between the contractile lattice and the surrounding intracellular space. We first create an advective-diffusive-reaction flow model and show that the flow into and out of the sarcomere lattice would be significant in the absence of lattice expansion. Advective transport coupled to diffusion has the potential to substantially enhance metabolite exchange within the crowded sarcomere. Using time-resolved x-ray diffraction of contracting muscle , we next show that the contractile lattice is neither isovolumetric nor constant in spacing. Instead , lattice spacing is time-varying, depends on activation, and can manifest as an effective time-varying Poisson ratio. The resulting fluid flow in the sarcomere lattice of synchronous insect flight muscles is even greater than expected from constant lattice spacing conditions. Volume change and advective solute transport varies with the phase of muscle stimulation during periodic contraction but remains significant at all conditions. Nonetheless, advective transport varies will occur in all cases where the sarcomere is not isovolumetric. Akin to "breathing," advective-diffusive transport in sarcomeres is sufficient to promote metabolite exchange and may play a role in the regulation of contraction itself.
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on
to
A
energy derived from
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at the limits of
, in part, by
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. Classic sliding filament theory suggests that lattice spacing is constant . If so,
filaments. The radial spacing of filaments in this lattice may change or remain constant during during contraction. If the lattice is isovolumetric, it must expand when the muscle shortens. If, however, the spacing is constant or has a different pattern of axial and radial motion,
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and could provide
, driving
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hence assist in
assisting in the
A
the
D
If, however, the lattice is isovolumetric, it must expand when muscle shortens. Doing so would alter the binding dynamics of myosin which are sensitive to spacing.
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a convective-diffusive
an advective-diffusive-reaction
A
the
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Convective
Advective
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time resolved
time-resolved
A
,
D
,
A
,
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a negative (auxetic)
as an effective time-varying
D
This unusual material behavior arises from the multiscale interaction of muscle axial strain, lattice spacing and myosin binding.
A
of synchronous insect flight muscles
D
would be
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Akin to ``breathing,'' convective-diffusive
Volume change and advective solute transport varies with the phase of muscle stimulation during periodic contraction but remains significant at all conditions. Nonetheless, advective transport varies will occur in all cases where the sarcomere is not isovolumetric. Akin to "breathing," advective-diffusive
D
,