Grant: Discovery Grants Program - Individual

A whole muscle may contain several hundreds of thousands of individual muscle fibres. These fibres are grouped into functional units (10 -1000 fibres) activated by a single motor neuron. A single motor neuron and all the muscle fibres that it activates is called a motor unit (MU). The motor unit is the basic building block of force production. Activation of one MU will cause its muscle fibres to contract and relax in unison (or, "twitch") producing one unit of force. Force can be increased by recruiting additional motor units (recruitment), or by increasing the number of times per second (rate-coding) that each motor unit is activated. Activating motor units more frequently allows the forces generated by the motor units to overlap in time to a greater degree and sum up more efficiently. When a motor unit is recruited by the nervous system, it generates electrical activity that can be monitored by inserting a needle electrode into the muscle, termed indwelling electromyography (EMG). Unfortunately, indwelling electrodes record from a limited volume of tissue, which is not representative of the whole muscle. Cutting-edge technology now allows the electrical activity from many more motor units to be detected non-invasively from the skin surface, termed surface EMG. This is like the use of electrocardiography (EKG) to record the electrical activity of heart muscle from the skin surface. Unfortunately, we do not know how deep into the muscle we can record MUs from the skin surface. The proposed series of studies takes a unified approach involving both indwelling and surface EMG to investigate strategies that the nervous system uses to organize the activity of motor units to generate force within a single muscle and how groups of muscles are coordinated to produce force at the joint.  As part of our ongoing research program, we discovered that males and females have different motor unit activity patterns while performing the same types of muscle contractions.  As part of the proposed studies, we will also explore the possibility that sex-differences in the neural control of motor units can be explained by musculoskeletal biomechanics.