A critical orientation on muscle marker placement: Multiple angles to consider

The muscles that drive animal and human movement are spatially rich in three-dimensional deformation, yet capturing this complexity in vivo is extremely challenging. As a result, much of our research relies on a single metric of muscle geometry: fascicle length. A variety of muscle marker technologies, such as sonomicrometry, fluoromicrometry, and magnetomicrometry, have enabled us to monitor muscle fascicle lengths over time. Due to the complex nature of muscle deformation, the three-dimensional placement of these muscle markers is important. We aimed to determine just how important that muscle marker placement is. To this end, we simultaneously monitored muscle long-axis distances and fascicle lengths under a variety of muscle actions in an avian gastrocnemius. We found that muscle long-axis distances did not reliably correspond to fascicle lengths, even occasionally showing inverted length changes, and we found that the muscle long-axis distances lacked a consistent relationship with fascicle lengths and even with each other. In summary, we showed that fascicle length cannot in general be predicted from arbitrary muscle tissue length. Our results suggest that improper placement of muscle markers for tracking fascicle lengths cannot be corrected for in post-processing, and we strongly encourage the biomechanist to take special care in placing each muscle marker pair along a fascicle when studying or otherwise monitoring muscle fascicle lengths.