Measuring the maximum power of an ex vivo engineered muscle tissue with isovelocity shortening technique

The final aim of muscle tissue engineering (TE) is to create a new tissue able to restore the functionality of impaired muscles once transplanted in the site of injury. Therefore, functional contractile properties close to that of healthy muscles are desirable to allow for a good compatibility and a proper functional contribution. Since skeletal muscles deal with locomotion during their normal activity, an accurate measurement of ex vivo muscle engineered tissues' isotonic properties is crucial. In this paper, we devised an experimental system to measure the mechanical power generated by an ex vivo muscle engineered tissue, the X-MET, based on the isovelocity contraction technique. The X-MET is developed without the use of any scaffolds, so that its mechanical properties are not affected by endogenous components. Our experiments allowed for delimiting the ranges of shortening and shortening velocity for which the tissue is able to generate and maintain power for the entire stimulation, which is the condition that better reproduces muscle physiological activity. Then, we measured the power generated by the X-MET and fit the experimental results to the Hill's equation usually employed for modeling the force-velocity relationship of skeletal muscles. The use of this model yielded to the measurement of maximum power and maximum shortening velocity. Results revealed that most of the isotonic properties were consistent with that proposed in the literature for slow-twitch muscles; in particular, the X-METs were able to generate a maximum power of 2.08± 0.78 W/kg and had a maximum shortening velocity of 1.84 ± 0.57 L₀/s, on average.