In simplest terms, direct electrical stimulation of the muscle, via field or needle electrode, results in a very similar cascade of events as nerve stimulation. However, this process bypasses the motoneuron and depolarizes the sarcolemma directly to eliminate synaptic transmission as a factor in assessing muscle function. Using our earlier example of measuring TA (dorsiflexion) function in-vivo, needle electrodes are placed subcutaneously with the tips of the electrodes resting on the belly of the muscle or group of interest (Figure 2). Be mindful not to puncture the muscle tissue as this could result in damage to the fibers. In some cases, placing the electrodes at the proximal and distal end of the muscle will also work, but this will require higher amperage to fully stimulate the muscle and thus it is not recommended for dorsiflexion in-vivo, due to the likelihood of activating the antagonistic group. When measuring muscle function in-situ, electrode placement is quite similar to in-vivo. However, isolated muscle mechanics would make use of field stimulation using platinum-iridium electrodes in a tissue bath to generate a strong electric field which depolarizes the sarcolemma.
Direct muscle stimulation is incredibly useful when studying neuromuscular or neurodegenerative diseases where the quality or viability of the muscle is the primary detriment. In this instance, researchers may ask whether the muscle machinery still works or how it is affected in diseases such as Muscular Dystrophy, Aging or Cachexia. Or in other cases, injury to the muscle through volumetric muscle loss (VML) or ischemia/reperfusion (IR).
This is not to say that these disease or injury models lack a neurodegenerative component, in many cases they do (Sartori et al.)
But for many of these disease models, the researcher may be asking questions such as; does the muscle still contract and produce force or are there signs of atrophy? Are there disruptions in the myofilament that are hindering muscle function? etc.
Figure 2: Needle electrodes placed subcutaneously to directly stimulate dorsiflexor muscles. Courtesy Dr. Harun Noristani, Dr. Young-Jin Son Lab, Shriners Pediatrics Research Center.