Activity in both humans and even animals depends largely on functioning, active muscles. As a consequence, sensing and monitoring of muscle activity is important for many reasons. For instance, in sports medicine, information about the fatigue level of a muscle is useful for preventing over-exertion of muscles, which might lead to muscle-based injuries. In addition, sensing muscle activity could be leveraged for precision training, allowing athletes to monitor progress on targeted muscle groups as needed. Finally, other ubiquitous applications such as muscle-controlled prosthetics could also be enabled by muscle activity data.
There are two main methods for detecting and monitoring muscle activity. The first is electromyography (EMG), which accurately measures the electrical potential of active muscle fibers. By amplifying and processing these electrical signals, information about muscle activation and activity can be inferred. The second method is mechanomyography (MMG). Mechanomyographic methods use vibration transducers to detect skeletal muscle activity. A brief discussion of the physiology of skeletal muscle vibrations that enable mechanomyography based systems is provided herein.
FIG. 1 illustrates an example skeletal muscle 100 and in particular the quadriceps and the tendons 106a, 106b and muscle belly 102 (including muscle fibers 104) are shown. Skeletal muscles 100 generally consist of three parts: a muscle belly 102, composed of muscle fibers 104, and two tendon ends 106a, 106b. Activating skeletal muscles during physical activity causes the muscle fibers 104 to contract, resulting in mechanical vibrations due to: tremors of the human motor system, muscle fibers 104 sliding against each other and artifacts detected when a muscle belly's 102 circumference increases during muscle contractions and decreases during relaxation. These skeletal muscle vibrations are carried through to and detected at the skin level. The vibrations vary depending on the muscle fatigue status, muscle group size, and location in the body e.g., skeletal muscle proximity to bones of different mass and size.
As a method for inferring muscle activity, EMG while accurate, has the major setback that it is difficult, invasive and even painful to use. It requires perfect contact with the skin, necessitating the use of gels in the case of surface EMG, or the insertion of small needles into the muscle, in the case of fine-wire EMG. Mechanomyography methods on the other hand, work by externally detecting the mechanical vibrations occurring in the muscle belly of skeletal muscles when the muscle is activated using vibration transducers. As such they are less invasive and more convenient for muscle monitoring use in a dynamic environment. Current vibration transducer technologies include fabric stretch sensors, force sensitive resistors (FSR) and a combination of any of or all of available inertial sensors including accelerometers, gyroscopes and magnetometers.