Surface eMG analysis or surface slectromyography records how the muscles activate when the nervous system sends a signal to the muscle to contract in order to move, stabilize or decelerate a joint. The small electrodes that are similar to ECG electrodes are placed on the skin directly above the targeted muscle in order to measure the electrical signal caused by the movement of ions across the muscle cell membranes. This voltage, which is often measured in millivolts, indicates the recruitment of motor units as a result of the contraction process. A motor unit is a grouping of muscle fibers that is connected with a common nerve. These very low voltage signals are detected by the electrodes, amplified and filtered down to a computer where they are converted into a readable signal.
These signals are then used to identify what and when muscles are active, how much they are active, and can provide information on whether they may be fatiguing. Knowing this, it is then useful in determining what muscles are used in performing a movement as well as hinting at how the muscle may be responding to forces at the joint itself or adapting to an injury or impairment. For example, an athlete may have suffered an injury such as an ACL (anterior cruciate ligament) tear and may be completing her rehab to return to play. Coming back from an injury can be daunting as not knowing if the muscles are ready to be subjected to the forces of intense movements could increase the risk of reinjury. Strength, flexibility, and reaction training all play a key role in preparing for the return but will the muscles respond properly when the time comes? By examining the muscle activity pattern through EMG, we can learn more about how quickly the thigh muscles can set the knee joint, or pre-activate, to protect the ACL from too much strain from excessive movement or muscle delay and weakness. This gives more information to the coach and the athlete if she is ready to return. If the muscles are too slow to react to the movement, the forces on the ACL can increase too quickly leading to injury. However, if the muscles are properly trained to respond quickly and anticipate before the forces become too great, the knee is protected and the risk of injury is decreased.
Efficiency of movement is another aspect that can be examined by looking at the recorded EMG signal. The muscles around the joints often function in pairs with one muscle or muscle group providing the force to create movement (agonist) while the opposing muscle group applies force to stabilize the joint and help in controlling execution (antagonist). Too much activity from an opposing muscle group or antagonist will cause the agonist or movement muscle to work harder against the added resistance, thus decreasing efficiency in movement. Monitoring technique and muscle activation thus becomes important to increase efficiency and with the help of EMG analysis you can easily see if a muscle is activating at the appropriate time. Interpretation can then be determined if the activation is necessary for joint stabilization or if it is an unnecessary motor pattern that needs more attention. Comparing the left side to right side gives more information on symmetry and the rhythm of muscle contraction as well.
Targeting certain muscles for a specific training effect for strength, stamina, or growth can all benefit from real time analysis. Often an individual’s recruitment pattern may be different from someone else due to learned habits or genetic factors such as muscle attachment and bone length. Are squats the best exercise to strengthen the quadriceps or is a lunge better? Are crunches on an exercise ball better for upper abdominal activation or does a decline crunch target that area better? Should I move faster or slower? These are questions that an EMG assessment excels at when looking at the individual instead of relying on group averages from a research study.
More advanced analysis can also be helpful in finding the point or threshold when muscles start to fatigue in order to develop training zones around these shifts in activation or to help decide if a training technique fatigued the muscle enough for a training effect. Rate of EMG development gives more information on power production and if the training routine is causing the necessary neurological changes as well as its role in recovery. Combining fatigue measurements (in regard to frequency shifts in the EMG spectrum) with amplitude could also give a window into helping decide if the muscle is fully recovered from a training bout or competition.
There are some limitations to EMG readings and interpretation that are also important to remember. The readings are not representative of the whole muscle, just the muscle fibers immediately under the electrodes and a certain depth into the muscle. Making definitive conclusions of recruitment of muscle fiber types as well as muscle force production is generally elusive and often the results are subject to general interpretation for specific situations. For example, although we may see that the EMG signal is stronger we can’t say for certain if it is producing proportionally more force as individual muscles can vary and often the strength to EMG signal is not always linear and can vary from muscle to muscle. However, quantifying muscle activation and timing is quite precise and reliable and is the basis of thousands of research studies examining muscle use during movement. EMG analysis helps take the guesswork out of movement and provides an accurate measurement of muscle activity that a visual, interpretive, or touch assessment just can’t do.