Electromyography (EMG) is an experimental technique concerned with the development, recording and analysis of myoelectric signals. Myoelectric signals are formed by physiological variations in the state of muscle fiber membranes (Basmajian and DeLuca, Muscles Alive). An example set of data is shown in the figure below. While the muscle is inactive the signal is near zero, and when the muscle contracts much higher voltages are recorded. In our lab we use surface electrodes to detect muscle activation patterns in walking and standing. A good example of using EMG as a diagnostic tool can be seen in the following figures.
In the first figure the EMG signal and the ground reaction force of a healthy person are plotted against time. In the second figure the EMG signal and the ground reaction force are presented for a person with drop foot, a condition where the ankle dorsiflexion muscles are weakened or paralyzed resulting in an inability to actively move the foot toward the lower leg. A person with healthy ankle dorsiflexors will activate these muscles while the foot is in the air (when the ground reaction force is zero the foot is in the air). Notice in the top figure the EMG signal is coordinated with the time the foot is in the air, and in the bottom figure the EMG signal shows no recognizable pattern. With the aid of EMG, we are able to detect inconsistencies in the neuromuscular system.