Fascicle length is an important parameter of skeletal muscle architecture, which overall is indicative of a muscle’s ability to produce force1,2. Specifically, a muscle’s fascicle length provides insight into the absolute range of lengths over which a muscle can generate active force3,4. For example, given two muscles with identical values for all isometric force-generating parameters (i.e., average sarcomere length, pennation angle, physiological cross sectional area, contraction state, etc.) except for fascicle length, the muscle with the longer fascicles would produce its peak force at a longer length and would produce force over a wider range of lengths than the muscle with shorter fascicles3. Quantification of muscle fascicle length is important for understanding both healthy muscle function and changes in a muscle’s force-generating capacity, which can occur as a result of altered muscle use (e.g., immobilization5,6, exercise intervention7,8,9, high heel wearing10) or a change in the muscle’s environment (e.g., tendon transfer surgery11, limb distraction12). Measurements of muscle fascicle length were originally obtained through ex vivo cadaveric experiments that allow for direct measurement of dissected fascicles13,14,15,16. The valuable information provided by these ex vivo experiments led to an interest in implementing in vivo methods17,18,19 to address questions that could not be answered in cadavers; in vivo methods allow for quantification of muscle parameters in a native state as well as at different joint postures, different muscle contraction states, different loading or unloading states, and across populations with differing conditions (i.e. healthy/injured, young/old, etc.). Most frequently, ultrasound is the method employed to obtain in vivo muscle fascicle lengths18,19,20; it is quicker, less expensive, and easier to implement than other imaging techniques, such as diffusion tensor imaging (DTI)18,21.
Extended field-of-view ultrasound (EFOV-US) has been demonstrated to be a valid and reliable method for measuring muscle fascicle length in vivo. While commonly implemented, traditional ultrasound (T-US) has a field-of-view which is limited by the ultrasound transducer’s array length (typically between 4 and 6 cm, although there are probes that extend to 10 cm10)18,20. To overcome this limitation, Weng et al. developed an EFOV-US technology that automatically acquires a composite, two-dimensional “panoramic” image (up to 60 cm long) from a dynamic, extended distance scan22. The image is created by fitting together, in real-time, a sequence of traditional, B-mode ultrasound images as the transducer dynamically scans the object of interest. Because sequential T-US images have large overlapping regions, the small differences from one image to the next can be used to calculate the probe motion without the use of external motion sensors. Once the probe motion between two consecutive images is calculated, the “current” image is merged successively with the preceding images. The EFOV-US method allows direct measurement of long, curved muscle fascicles and has been demonstrated to be reliable across muscles, trials, and sonographers23,24,25 and valid for both flat and curved surfaces23,26.
Implementing ultrasound to measure muscle fascicle length in vivo is not trivial. Unlike other imaging techniques that involve more automated protocols (i.e., MRI, CT), ultrasound is dependent on sonographer skill and anatomical knowledge27,28. There is concern that probe misalignment with the fascicle plane may cause substantial error in fascicle measures. One study demonstrates little difference (on average < 3 mm) in measures of fascicle length taken using ultrasound and DTI MRI but also shows that measurement precision is low (standard deviation of difference ~12 mm)29. Still, it has been shown that a novice sonographer, with practice and guidance from an experienced sonographer, can obtain valid meaures using EFOV-US23. Thus, efforts should be made to demonstrate appropriate protocols to reduce human error and improve accuracy of measurements obtained using EFOV-US. Ultimately, developing and sharing appropriate protocols may expand the number of experimenters and laboratories that can reproduce fascicle length data from the literature or obtain novel data in muscles which have not yet been studied in vivo.
In this protocol, we demonstrate how to implement the EFOV-US method to obtain high quality musculoskeletal images that can be used to quantify muscle fascicle length. Specifically, we address (a) collecting EFOV-US images of a single upper limb and a single lower limb muscle (b) determining, in real-time, the “quality” of the EFOV-US image, and (c) quantifying muscle architecture parameters offline. We provide this detailed guide to encourage the adoption of the EFOV-US method for obtaining muscle fascicle length data in muscles that have gone unstudied in vivo due to their long fascicles.