Asynchronous muscles
[1] Unlike their synchronous counterparts that contract once per neural signal, mechanical oscillations trigger force production in asynchronous muscles.
Typically, the rate of mechanical contraction is an order of magnitude greater than electrical signals.
[1] Although they achieve greater force output and higher efficiency at high frequencies, they have limited applications because of their dependence on mechanical stretch.
[2] This finding helps explain how asynchronous muscles independently evolved across insect taxa.
[1] More recent work using similar X-ray diffraction techniques in Lethocerus discovered that troponin bridges may play a critical role in stretch activation.
As the muscle is stretched, these bridges move tropomyosin to reveal myosin-actin binding sites.
In comparison, synchronous muscles in Schistocerca americana are composed of 65% myofibril, 23.5% mitochondria and 9.6% sarcoplasmic reticulum.
Instead of directly controlling force generation, neural signals maintain [Ca2+] above a threshold for stretch-activation to occur.
[1] Asynchronous muscles produce work when they undergo mechanical oscillations provided there is sufficient Ca2+.
Asynchronous muscles sacrifice neural control and flexibility in exchange for high force production and efficiency.
Given the long twitch duration of asynchronous muscle, neural control is too slow to power flight.
For instance, the asynchronous muscles in Cotinus mutabilis contract ten times faster than expected given their twitch duration.
[10] Because of their high force production and efficiency, asynchronous muscles are used to power insect flight in 75% of species.
These insects possess two pairs of antagonistic asynchronous muscles that produce the majority of the power required for flight.
By utilizing the elastic thorax to store and return energy during wing deceleration and subsequent acceleration, Drosophila is able to reduce energetic costs by 10%.
[9] From the equations in the Resonant properties section, it is clear that the natural frequency of the system increases with stiffness.
[12] This property benefits heart function by maintaining papillary muscle tension during the entire systolic cycle well after the electrical wave has passed.
Because of challenges arising from miniaturization such as poor scaling of electric motors, researchers have turned towards insects to develop centimeter-scale flying robots.
