Shortening deactivation: quantifying a critical component of cyclical muscle contraction.

A muscle undergoing cyclical contractions requires fast and efficient muscle activation and relaxation to generate high power with relatively low energetic cost. To enhance activation and increase force levels during shortening, some muscle types have evolved stretch activation (SA), a delayed increased in force following rapid muscle lengthening. SA's complementary phenomenon is shortening deactivation (SD), a delayed decrease in force following muscle shortening. SD increases muscle relaxation, which decreases resistance to subsequent muscle lengthening. Although it might be just as important to cyclical power output, SD has received less investigation than SA. To enable mechanistic investigations into SD and quantitatively compare it to SA, we developed a protocol to elicit SA and SD from Drosophila and Lethocerus indirect flight muscles (IFM) and Drosophila jump muscle. When normalized to isometric tension, Drosophila IFM exhibited a 118% SD tension decrease, Lethocerus IFM dropped by 97%, and Drosophila jump muscle decreased by 37%. The same order was found for normalized SA tension: Drosophila IFM increased by 233%, Lethocerus IFM by 76%, and Drosophila jump muscle by only 11%. SD occurred slightly earlier than SA, relative to the respective length change, for both IFMs; but SD was exceedingly earlier than SA for jump muscle. Our results suggest SA and SD evolved to enable highly efficient IFM cyclical power generation and may be caused by the same mechanism. However, jump muscle SA and SD mechanisms are likely different, and may have evolved for a role other than to increase the power output of cyclical contractions.

Force-velocity and tension transient measurements from Drosophila jump muscle reveal the necessity of both weakly-bound cross-bridges and series elasticity in models of muscle contraction.

Muscle contraction is a fundamental biological process where molecular interactions between the myosin molecular motor and actin filaments result in contraction of a whole muscle, a process spanning size scales differing in eight orders of magnitude. Since unique behavior is observed at every scale in between these two extremes, to fully understand muscle function it is vital to develop multi-scale models. Based on simulations of classic measurements of muscle heat generation as a function of work, and shortening rate as a function of applied force, we hypothesize that a model based on molecular measurements must be modified to include a weakly-bound interaction between myosin and actin in order to fit measurements at the muscle fiber or whole muscle scales. This hypothesis is further supported by the model's need for a weakly-bound state in order to qualitatively reproduce the force response that occurs when a muscle fiber is rapidly stretched a small distance. We tested this hypothesis by measuring steady-state force as a function of shortening velocity, and the force transient caused by a rapid length step in Drosophila jump muscle fibers. Then, by performing global parameter optimization, we quantitatively compared the predictions of two mathematical models, one lacking a weakly-bound state and one with a weakly-bound state, to these measurements. Both models could reproduce our force-velocity measurements, but only the model with a weakly-bound state could reproduce our force transient measurements. However, neither model could concurrently fit both measurements. We find that only a model that includes weakly-bound cross-bridges with force-dependent detachment and an elastic element in series with the cross-bridges is able to fit both of our measurements. This result suggests that the force response after stretch is not a reflection of distinct steps in the cross-bridge cycle, but rather arises from the interaction of cross-bridges with a series elastic element. Additionally, the model suggests that the curvature of the force-velocity relationship arises from a combination of the force-dependence of weakly- and strongly-bound cross-bridges. Overall, this work presents a minimal cross-bridge model that has predictive power at the fiber level.

Can biomechanics turn youth sports into a venue for informal STEM engagement?

A common pitfall of existing Science, Technology, Engineering, and Math (STEM) outreach programs is that they preferentially engage youth with a preexisting interest in STEM. Biomechanics has the unique potential to broaden access to STEM enrichment due to its direct applicability to sports and human performance. In this study we examine whether biomechanics within youth sports can be used as a venue for STEM outreach, and whether recruiting participants through youth sports programs could broaden access to the STEM pipeline. We created a four-hour sports science clinic that was performed as part of National Biomechanics Day and invited two groups of student participants: youth recruited through local high school sports programs ("Sports Cohort", N = 80) and youth recruited through existing STEM enrichment programs ("STEM Cohort", N = 31). We evaluated interest in STEM, Sports Science, and Sports using a pre-post survey. Somewhat expectedly, youth recruited through sports programs (Sports Cohort) had a lower baseline interest in STEM and a higher baseline interest in sports, compared to those recruited through STEM programs (STEM Cohort). The Sports Cohort exhibited a statistically significant increase in STEM interest following participation in the clinic, while youth in the STEM Cohort maintained their high baseline of STEM interest. These findings provide evidence that youth sports programs can serve as an attractive partner for biomechanists engaged in STEM outreach, and that situating STEM within sports through biomechanical analysis has potential to introduce STEM interest to a wider audience and to broaden access to the STEM fields among diverse youth.