sEMG and its uses in golf
When muscles are active, they produce an electrical current. Surface electromyography (sEMG) is used to measure the activation of the muscles for a particular movement through these electrical currents. There are several other ways to measure muscle activation, such as using the fine wire and needle methods, however, these are invasive methods requiring the piercing of skin. sEMG is a non-invasive method where the electrodes are placed upon the surface of the skin.
When conducting sEMG experiments, the skin has to be prepared in order to obtain a strong, clean and balanced signal with the largest possible signal/noise ratio. The signal is the desired data when conducting an sEMG, however ‘noise’ is the unwanted interference, so if this can be minimised, then the amount of useful information can be increased. The higher the signal to noise ratio, the more reliable the EMG data is when accounting for the underlying baseline noise. There are other ways to reduce the amount of ‘noise’ within the data set; the size of the electrode should be no larger than 10mm to reduce the cross talk from several muscle fibres, the inter-electrode distance should be around 20mm and the electrodes should be placed parallel to the muscular fibres. It is important to note that the information given from the sEMG is the level of activation of the muscle, not the power or strength of the muscle.
When collecting data using an sEMG, Nyquist Theorem states that you must use a sampling frequency that is at least double that of the raw analogue signal produced by the muscles. This prevents the collection of too little data to recreate an accurate representation of the activation wave produced. This is known as aliasing, as shown by the black and dotted red lines in graph b (figure 1). By following Nyquist Theorem, we can produce a graph that more accurately represents the raw signal, as shown by the blue dotted signal in graph b (figure 1)
Graph A Graph B
Figure 1. Graph a represents the raw analogue activation wave produced by the muscle. Graph b represents signal collected when following and not following the guidelines of Nyquist Theorem. (DelSys.com)
Application to Golf
When playing golf, there are wide range of muscles that are used during the swing. These muscles can be split into different sections; Upper body (right and left), Trunk and lower body (left and right). This enables measurement of muscle activity to be collected easily and more information can be gained. The different sections allow for a more specific prognosis of weaknesses during a golf swing. The more sections the body can be split into the more it reduces the likelihood of information being missed.
According to Pollard and Mchardy (2005), there are 5 phases of the golf swing; back swing, forward swing, acceleration, early follow through and late follow through. At each of these phases, a level of activation for each muscle is different. For example, at the acceleration phase the level of active of the pectoralis major is 93%, whilst at the late follow through it is at 39%. The study conducted by Pollard and Mchardy enabled the explanation of why certain muscles are active during specific phases of the swing. This enables a baseline for what could be expected during the swing and a comparison can be made. These comparisons can then give information to coaches and trainers for S+C training intervention.
If you would like SSC services for sEMG testing or have any questions about this article, please contact the team.