A realistic model for approaching the activation question of neuromuscular magnetic stimulation

Neuromuscular magnetic stimulation is an auspicious tool in rehabilitation. It causes low distress and allows a more homogeneous activation without the problematic current concentration at electrodes as known from electrical stimulation. However, the performance of available technology is poor. For improvements of the coil technology, important mechanistic knowledge about the activation process is missing. In contrast to brain stimulation, where focality of the induced field is usually the key aim, a corresponding physical objective for neuromuscular magnetic stimulation is entirely unknown. We want to close the gap between physics - i.e., the field characteristics - and physiology - i.e., the force response that has to be increased. A comparison of five stimulation coils which notably differ in their physical properties and experimental performance enables an identification of the important physical metric. Instead of the gradient of the electric field, we perceive the strength of the induced electric field itself as the property that has to be increased for improving muscle activation.