Validity and Reliability of Surface Electromyography Measurements from a Wearable Athlete Performance System.

Author:Lynn, Scott K.
Position:Research article - Report


Surface electromyography (sEMG) provides access to the activation signal that causes the muscle to generate force, produce movement, and accomplish the essential functions of everyday life (DeLuca, 1997). The sEMG signal represents the sum of the motor unit action potentials recorded by the electrodes and provides crucial insight into the nervous system's activation of the muscle (Day and Hullinger, 2001; Keenan et al., 2005). sEMG is used in various applications including clinical, research and sport to explore the neuromuscular system and the relationship between muscle activation, movement and force. For example, sEMG provides clinicians with a robust biofeedback tool that has been demonstrated to improve muscle function in children with cerebral palsy (Bloom, 2010). Moreover, sEMG has been effective in diagnosing, treating and researching populations with various pathologies including hypo- and hypertonicity (Herrington, 1996); stroke (Park and Kim, 2017); lower back pain (Kaur and Kumar, 2016; Matheve et al., 2017); and patellofemoral pain syndrome (PFPS) (Kalytczak, 2016). The research applications of sEMG are also broad and include detecting differences in muscle activation patterns with changes in exercise or movement technique (Lynn and Noffal, 2012; Lynn and Costigan, 2009), recognizing abnormal activation strategies (Michener et al., 2016), developing methods for prosthetic control (Daley et al, 2012), as well as developing biomechanical models to predict the loading on joints (Callaghan et al., 1998).

Movement strategy is critical in sport and sEMG has been used to evaluate muscle activation in sport applications including recovery, performance and evaluating injury risk factors. As a biofeedback tool, sEMG has been demonstrated to increase quadriceps strength recovery post anterior cruciate ligament (ACL) reconstruction (Draper, 1990). Further, muscle activation based on sEMG has been used to evaluate the efficacy of different training techniques such as comparing the activation from different muscle groups based on exercise or equipment type (Krause, 2009) or evaluating the impact of training technique on specific physiological adaptation (Walker, 2012). Muscle activation data has also been used to research criteria that may relate to different injury risk in sport, for example, quadriceps dominance during single leg squats as a possible risk indicator of ACL injury (Zeller, 2003).

Although the clinical, research and sport applications of sEMG are extensive, there are many hurdles that make it difficult for wide ranging use. Measurement of sEMG typically requires significant setup cost including skin preparation and application of single use adhesive-based Ag/AgCl electrodes (SENIAM) (Merletti, 1997). Also, electrodes are generally tethered to a data acquisition system constraining the movement of the subject and context that can be studied. Further, the signal acquired often requires further processing and filtering by the user to report on metrics based on the data. The setup cost and complexity of the equipment as well as the extensive processing often required makes sEMG analysis and application difficult outside the laboratory or clinic.

With advancements including component miniaturization, material development and improved manufacturing methods, new technologies for measuring human physiology are emerging that may reduce the setup cost and complexity of measuring sEMG. The Athos[R] training system ( is an example of one of these new technologies. Athos has integrated sEMG measurement into the construction of athletic compression apparel. The sEMG signals are acquired by a portable device that clips into the apparel, processes, and sends wirelessly to a client device for presentation to the coach or athlete. Through the combination of a mobile and browser application, Athos provides athletic trainers, coaches and athletes with performance metrics derived from the sEMG measurements. The sEMG based metrics are used to evaluate activation and recruitment patterns between muscles and over time during training.

While Athos provides sEMG measurements integrated into the construction of compression athletic apparel, the validity and reliability of this system needs further testing. One study has compared the Athos sEMG signal to a research grade system (Aquino & Roper, 2018) and found it to be valid; however, the two sEMG systems were not worn concurrently, so data from the same contractions could not be compared. Therefore, the purpose of this study is to compare Athos sEMG measurements against an established research system and protocol (Finni et al., 2007) on the same contractions. Athos electrodes are integrated into the construction of the garment. The research system comprises traditional Ag/AgCl adhesive electrodes placed directly distal the Athos electrodes and following standard SENIAM protocol for skin preparation. There was no difference in filtering applied prior to sampling across the EMG spectrum of 10-500 Hz and the sampled signals were processed using the same processing steps.

The validity of the Athos system was evaluated by first comparing characteristics of the sEMG signal from both systems and second by comparing the relationship between sEMG from both systems and the resulting torque produced by those contractions. We evaluated the reliability of sEMG measures from the two systems across days where the electrodes are re-applied. We hypothesized that there would be no significant differences in sEMG output or the relation between EMG and torque for the two systems. Moreover, we hypothesized that the test-retest reliability of the sEMG signal from Athos would be comparable to the research grade system.



Twelve healthy subjects (6 males, 6 females, see Table 1) were recruited for this study. Subjects were screened through a pre-research questionnaire to determine level of training and ensure full commitment to the completion of data collection. Level of training was defined as untrained ( 3 years training; 1 male, 1 female). Testing was performed at the same time for each testing trial, and subsequent trials were separated by a minimum of 48 hours. Each subject was required to participate in a total of seven testing trials over a three-week period. All subjects were notified of potential risks and provided written informed consent approved by the University Institutional Review Board prior to data collection.


For each subject, anthropometrics (hip and waist measurements) were recorded to determine the appropriate Athos gear size. Each subject used the same gear throughout the whole study, and gear was washed following the last trial of each week.

SEMG measurements from the vastus lateralis, vastus medialis and bicep femoris were collected with both Athos and the Biopac electrodes (Biopac Systems, Inc., Goleta, California) simultaneously. The Athos compression garments were fit to each subject to ensure the electrodes embedded in the garments were directly over the muscle bellies of vastus lateralis, vastus medialis, and biceps femoris. Athos electrodes are designed to provide a bipolar differential EMG measurement with an interelectrode distance of 2.1 cm (Figure 1). Athos electrodes are comprised of a conductive polymer and no skin or electrode preparation was performed at the site corresponding to each electrode. No skin or electrode preparation was performed at the site corresponding to each Athos electrode as in a practical setting, skin preparation is not performed when wearing Athos. For each muscle, the Athos shorts were cut just below the Athos bipolar electrodes to place the Biopac bipolar electrodes (Biopac EL500, Ag/AgCl electrodes, Bio-Pac systems Inc., Goleta, CA, USA) as close to the Athos electrodes as possible and directly distal on the same muscle. The bipolar Biopac electrodes provided a differential EMG measurement and an interelectrode distance of 2.1 cm was used to match the interelectrode distance of the Athos electrodes. When applying Biopac electrodes, the area of skin was shaved and cleaned with an alcohol wipe. Biopac electrodes were marked on the skin and the electrode location was re-marked following testing to prevent fading and keep the placement consistent for each trial. The Biopac reference electrode was placed on the right wrist at the styloid process of the ulna as has been done previously (Cochrane et al., 2014).

Experimental procedure

The study protocol consisted of 1 baseline testing session and 6 repeated testing sessions (Figure 2). A HUMAC Norm (CSMi, Inc., Stoughton, MA, USA) isokinetic dynamometer was used to control the knee extension and flexion sets and to measure angular displacement and torque output. The dynamometer was used to reduce variability in the performance of the movement by controlling for speed and movement position. Torque output measurements were taken to control for repeatable torque across trials and to relate the output torque to the resulting sEMG response for each muscle.

Day 1: Familiarization and Baseline Testing: Prior to the first data collection trial, height and mass were recorded. Subjects were instructed to cycle for 10-minutes on a stationary bike at a self-selected pace followed by a dynamic warm-up. Subjects were then seated on the HUMAC Norm dynamometer and were positioned according to the HUMAC testing and rehabilitation user's manual with the padded arm of the dynamometer positioned 3 cm proximally to the lateral malleolus and the axis of rotation of the knee aligned with the axis of rotation of the dynamometer. Isometric 1 repetition maximum (RM) strength testing for knee extensors and knee flexors was performed with the knee positioned at 90[degrees] of flexion and the hip at 85[degrees] as was previously described (Luc et al., 2016; Roberts et al., 2012). All tests included familiarization comprising warm-up repetitions to...

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