Mechanical and neural factors underlying muscle performance during submaximal voluntary shortening contractions

• Applicant: Dr. Brent James Raiteri, Ph.D.
• Subject Area: Orthopaedics, Traumatology, Reconstructive Surgery
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 447345165

Everyday tasks like walking are powered by active force production during muscle shortening, yet we have a poor understanding of motor control and muscle-tendon unit function under these conditions in vivo. This is likely because we neglect that a muscle’s contractile history can affect its mechanical function, force-producing capacity, and underlying control (i.e. neuromechanics). Specifically, during and after active muscle shortening, force output is depressed compared with what is expected based on the generally-accepted muscle contraction theories. This force depression during and after active muscle shortening are termed dynamic and residual force depression (dFD and rFD), respectively. While rFD is relatively well researched, our current understanding of which neuromechanical factors affect dFD and its relation with rFD, as well as how dFD affects motor control and muscle performance, is severely limited. This is because rFD is assessed during steady-state force production, whereas human movement involves non-steady-state conditions, and no human in vivo study has yet systematically researched and quantified dFD. As dFD can be induced whenever a muscle actively shortens, it is crucial that we better understand dFD and its neuromechanical determinants in vivo. It is also crucial that we improve our understanding of healthy motor control and muscle-tendon unit function during non-steady-state conditions relevant for walking to improve the treatment and rehabilitation of individuals with neurological disease, and to inform the control of wearable devices. This project therefore aims to 1) uncover how preload and higher muscle stresses affect dFD and motor control during active muscle shortening, and 2) understand how the muscle’s contractile history affects motor control and muscle performance during movement in simulated walking conditions. These aims will be addressed across three experimental studies on the human tibialis anterior (TA), a major dorsiflexor muscle important for walking and postural control, using cutting-edge techniques from biomechanics and neurophysiology (e.g. simultaneous ultrasound imaging and high-density surface electromyography). In the first experiment, the dorsiflexion preload will be systematically manipulated while TA’s starting and final lengths and activity level are controlled, and TA’s active force output, length changes, and motor unit behavior (i.e. neural drive), as well as dFD and rFD, will be determined. In the second and third experiments, TA’s neural drive and muscle performance will be assessed following walking during individualized simulated walking conditions that assess dFD. This research aims to provide a wealth of knowledge about the neuromechanical determinants of dFD and the compensatory motor control adjustments that occur during movements relevant for everyday life, which is sorely needed to better interpret changes to motor control and muscle-tendon unit function due to aging or disease.

• DFG Programme: Research Grants
• International Connection: Italy
• Co-Investigator: Professor Dr. Daniel Hahn
• Cooperation Partner: Professor Dr. Alberto Botter