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Fowler VM. Thin-filament length correlates with fiber type in human skeletal muscle. Am J Physiol Cell Physiol 302: C555-C565, 2012. First published November 9, 2011; doi:10.1152/ajpcell.00299.2011.-Force production in skeletal muscle is proportional to the amount of overlap between the thin and thick filaments, which, in turn, depends on their lengths. Both thin-and thick-filament lengths are precisely regulated and uniform within a myofibril. While thick-filament lengths are essentially constant across muscles and species (ϳ1.65 m), thin-filament lengths are highly variable both across species and across muscles of a single species. Here, we used a high-resolution immunofluorescence and image analysis technique (distributed deconvolution) to directly test the hypothesis that thin-filament lengths vary across human muscles. Using deltoid and pectoralis major muscle biopsies, we identified thin-filament lengths that ranged from 1.19 Ϯ 0.08 to 1.37 Ϯ 0.04 m, based on tropomodulin localization with respect to the Z-line. Tropomodulin localized from 0.28 to 0.47 m further from the Z-line than the NH2-terminus of nebulin in the various biopsies, indicating that human thin filaments have nebulinfree, pointed-end extensions that comprise up to 34% of total thinfilament length. Furthermore, thin-filament length was negatively correlated with the percentage of type 2X myosin heavy chain within the biopsy and shorter in type 2X myosin heavy chain-positive fibers, establishing the existence of a relationship between thin-filament lengths and fiber types in human muscle. Together, these data challenge the widely held assumption that human thin-filament lengths are constant. Our results also have broad relevance to musculoskeletal modeling, surgical reattachment of muscles, and orthopedic rehabilitation.actin; length-tension curve; myosin heavy chain; nebulin; tropomodulin FORCE GENERATION IN SKELETAL muscle arises from cross-bridge interactions between myosin (thick) filaments and actin (thin) filaments in the sarcomeres of the myofibrillar lattice. A sarcomere's force-generating capacity is described by the sliding filament model, which states that the degree of myofilament overlap determines the amount of force that can be actively produced (25). Therefore, myofilament lengths are among the chief determinants of the shape of the length-tension curve (13,19,46,56), which quantitatively describes a sarcomere's force output as a function of myofilament overlap and establishes a sarcomere's operating length range (9,17,18,24,56). Myofilaments are polymeric, but their lengths are strictly controlled during sarcomere assembly and maintenance and are highly uniform within a myofibril (38). Given that whole muscle contractile performance can be accurately predicted from sarcomere-level myofilament geometry (58), obtaining high-resolution measurements of myofilament lengths in human muscles to construct muscle-specific, length-tension relationships has profound clinical significance. For example, accurate length-tension propertie...

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Background: Recent studies evaluating nonoperative treatment of elbow ulnar collateral ligament (UCL) injuries augmented with platelet-rich plasma (PRP) have shown promising results. To date, no comparative studies have been performed on professional baseball players who have undergone nonoperative treatment with or without PRP injections for UCL injuries. Hypothesis: Players who received PRP injections would have better outcomes than those who did not receive PRP. Study Design: Cohort study; Level of evidence, 3. Methods: The Major League Baseball (MLB) Health and Injury Tracking System identified 544 professional baseball players who were treated nonoperatively for elbow UCL injuries between 2011 and 2015. Of these, 133 received PRP injections (PRP group) before starting their nonoperative treatment program, and 411 did not (no-PRP group). Player outcomes and a Kaplan-Meier survival analysis were compared between groups. In addition, to reduce selection bias, a 1:1 matched comparison of the PRP group versus the no-PRP group was performed. Players were matched by age, position, throwing side, and league status: major (MLB) and minor (Minor League Baseball [MiLB]). A single radiologist with extensive experience in magnetic resonance imaging (MRI) interpretation of elbow injuries in elite athletes analyzed 243 MRI scans for which images were accessible for tear location and grade interpretation. Results: Nonoperative treatment of UCL injuries resulted in an overall 54% rate of return to play (RTP). Players who received PRP had a significantly longer delay in return to throwing ( P < .001) and RTP ( P = .012). The matched cohort analysis showed that MLB and MiLB pitchers in the no-PRP group had a significantly faster return to throwing ( P < .05) and the MiLB pitchers in the no-PRP group had a significantly faster RTP ( P = .045). The survival analysis did not reveal significant differences between groups over time. The use of PRP, MRI grade, and tear location were not statistically significant predictors for RTP or progression to surgery. Conclusion: In this retrospective matched comparison of MLB and MiLB pitchers and position players treated nonoperatively for a UCL tear, PRP did not improve RTP outcomes or ligament survivorship, although there was variability with respect to PRP preparations, injection protocols, time from injury to injection, and rehabilitation programs. MRI grade and tear location also did not significantly affect RTP outcomes or progression to surgery.