Optimizing electrical field stimulation parameters reveals the maximum contractile function of human skeletal muscle microtissues

Skeletal muscle microtissues are engineered to develop therapies for restoring muscle function in patients. However, optimal electrical field stimulation (EFS) parameters to evaluate the function of muscle microtissues remain unestablished. This study reports a protocol to optimize EFS parameters for eliciting contractile force of muscle microtissues cultured in micropost platforms. Muscle microtissues were produced across an opposing pair of microposts in polydimethylsiloxane and polymethyl methacrylate culture platforms using primary, immortalized, and induced pluripotent stem cell-derived myoblasts. In response to EFS between needle electrodes, contraction deflects microposts proportional to developed force. At 5 V, pulse durations used for native muscle (0.1-1 ms) failed to elicit contraction of microtissues; durations reported for engineered muscle (5-10 ms) failed to elicit peak force. Instead, pulse durations of 20-80 ms were required to elicit peak twitch force across microtissues derived from five myoblast lines. Similarly, although peak tetanic force occurs at 20-50 Hz for native human muscles, it varied across microtissues depending on the cell line type, ranging from 7 to 60 Hz. A new parameter, the dynamic oscillation of force, captured trends during rhythmic contractions, whereas quantifying the duration-at-peak force provides an extended kinetics parameter. Our findings indicate that muscle microtissues have cell line type-specific contractile properties, yet all contract and relax more slowly than native muscle, implicating underdeveloped excitation-contraction coupling. Failure to optimize EFS parameters can mask the functional potential of muscle microtissues by underestimating force production. Optimizing and reporting EFS parameters and metrics is necessary to leverage muscle microtissues for advancing skeletal muscle therapies.