The hindlimb muscles of frogs and toads have served as an exemplary model for understanding muscle?s role in powering propulsive movements like jumping. Anything that jumps must ultimately land, but unlike during jumping, where muscles produce the energy to accelerate the body, controlled landing requires muscles to dissipate energy to decelerate the body. The role of muscles during energy dissipation remains poorly understood. This research will reveal important principles associated with this locomotor activity. Terrestrial toads are outstanding at landing, using their forelimbs exclusively to decelerate their bodies. The proposed work will use toad landing as a model system for understanding how muscles control forelimb joint movements (shoulder, elbow and wrist) during and after impact. During human jumps, pre-landing activity in leg muscles is used to stiffen joints in preparation for landing, and is tuned to the expected time and magnitude of impact. Recent work demonstrates similarly prescient activity in muscles acting at the elbow joints of hopping toads in mid-air. The proposed work will address the generality of such tuned, pre-landing activity at other forelimb joints, and will specifically test the hypothesis that its utility lies in preventing muscles involved in energy dissipation from stretching to overly long lengths during landing, where injuries are most likely to occur. We will also examine the importance of visual and proprioceptive feedback in helping animals tune muscle activity to coordinate landing after hops of variable height and distance. By integrating biomechanics, muscle physiology and sensory biology this work will highlight fundamental principles governing controlled deceleration, an action common to most locomotor systems. Undergraduate students will be essential to the execution and presentation of this work, and by involving students and faculty from local community colleges, we will broaden direct participation in cutting-edge research.
Project outcomes Intellectual Merit- In most man-made vehicles the motors that produce mechanical energy and the brakes that dissipate mechanical energy are separate structures each with their own unique design. However, in many biological systems, muscles are tasked with performing both mechanical functions. The results from the work supported by this award provide the fundamental rules that govern the function of muscles as brakes. We have revealed several mechanisms that serve to protect muscles from damage when they act to dissipate mechanical energy. First, we show that activation of muscles in anticipation of an impact (e.g. landing from a jump) functions to keep a muscle safe by keeping it from reaching dangerously long lengths. Second, we show that this anticipatory activation allows muscles to effectively use the elastic properties of tendons in order to attenuate power and limit direct energy input to the muscles. Finally, we show that muscles used as brakes tend to be stiffer than muscles used as motors, thereby placing a physical limit on the lengths muscles can reach during movement. While we have focused on using landing in toads as a model system, our findings have broad importance to muscle acting to dissipate energy in all vertebrate organisms including humans. Since energy dissipation is common to activities such as downhill walking/running, decelerating, or landing after a jump our work has broad implications for how muscles are used, protected and damaged during variety of locomotor tasks. Broader Impacts- The work under the award has provide significant training opportunities for graduate and undergraduate students. Twelve undergraduate students and three graduates students have directly participated in the research activities associated with the award. In addition, two community college students have participated as interns on the research projects. Of all of the participants receiving training under the award, eleven were females and four were from traditionally underrepresented minority groups. In addition, we have established a working relationship with a local elementary school during the period of the award. We have designed in class exercises that integrate the inherent curiosity of students about the natural world with basic physical concepts that are part of the science curriculum. The outreach efforts associated with this award have improved training and mentoring opportunities, broadened participation of underrepresented groups, enhanced integration of university science and K-12 education and enhanced infrastructure for science.
