High Density Surface Electromyography Activity of the Lumbar Erector Spinae Muscles and Comfort/Discomfort Assessment in Piano Players: Comparison of Two Chairs

Abstract

Introduction: At a professional level, pianists have a high prevalence of playing-related musculoskeletal disorders. This exploratory crossover study was carried out to assess and compare quantitatively [using high density surface electromyography (HDsEMG)], and qualitatively (using musculoskeletal questionnaires) the activity of the lumbar erector spinae muscles (ESM) and the comfort/discomfort in 16 pianists sitting on a standard piano stool (SS) and on an alternative chair (A-chair) with lumbar support and a trunk-thigh angle between 105° and 135°. Materials and Methods: The subjects played for 55 min and HDsEMG was recorded for 20 s every 5 min. For the quantitative assessment of the muscle activity, the spatial mean of the root mean square (RMS _ROA ) and the centroid of the region of activity (ROA) of the ESM were compared between the two chairs. For the qualitative assessment, musculoskeletal questionnaire-based scales were used: General Comfort Rating (GCR); Helander and Zhang's comfort (HZc) and discomfort (HZd); and Body Part Discomfort (BPD). Results: When using the A-chair, 14 out of 16 pianists (87.5%) showed a significantly lower RMS _ROA on the left and right side (p < 0.05). The mixed effects model revealed that both chairs (F = 28.21, p < 0.001) and sides (F = 204.01, p < 0.001) contributed to the mean RMS _ROA variation by subject (Z = 2.64, p = 0.004). GCR comfort indicated that participants found the A-Chair to be "quite comfortable," and the SS to be "uncomfortable." GCR discomfort indicated that the SS caused more numbness than the A-Chair (p = 0.05) and indicated the A-Chair to cause more feeling of cramps (p = 0.034). No difference was found on HZc (p = 0.091) or HZd (p = 0.31) between chairs. Female participants (n = 9) reported greater comfort when using the A-Chair than the SS (F = 7.09, p = 0.01) with respect to males. No differences between chairs were indicated by the BPD assessment. Conclusion: It is concluded that using a chair with lumbar support, such as the A-chair, will provide greater comfort, less exertion of the ESM and less discomfort than the standard piano stool.

Keywords: back muscles; high-density sEMG; musculoskeletal questionnaire; piano players; surface electromyography.

FIGURE 1

(A) Pianist playing on the SS (standard piano stool) keeping the trunk erect with a trunk-thigh angle of 90°; (B) Pianist playing on the A-Chair keeping the trunk erect, with a trunk-thigh angle between 105° and 135°. The pianist back is always in contact with the lumbar support which has a dimension of A = 25 cm by B = 40 cm; (C,D) An example of electrode grid positioned on the lumbar portion of the right and left ESM (between spinal processes T11 and L4). The grids have an inter-electrode distance = 10 mm and electrode diameter Ø = 3 mm. The first (C1) and last (C8) columns of the electrode grids are indicated.

FIGURE 2

Single differential signals from a pianist (column 8 of the left grid and column 1 of the right grid) on a time window of 4 s on the standard stool (A) and the A-Chair (B). These signals were recorded after 55 min of continuous playing. The RMS values of each channel over the entire length of the signal (20 s), are reported next to each trace. Note the different scales on the signal plots.

FIGURE 3

Single differential RMS maps relative to subject 15 for chair SS (left side panels) and A-chair (right side panels) at the beginning (0 min) and the end (55 min) of the test. Maps were computed on the entire 20 s length of the signals. An image interpolation by a factor of 15 was applied (1 pixel = 0.6 mm). The region of activity (ROA) is identified by means of map segmentation (Caselles et al., 1997). Above each map the mean, minimum and maximum values of surface electromyography (sEMG) over the entire map are reported (μV_RMS). The centroid of each ROA, the color scale (0–30 μV_RMS) and a schematic representation of the vertebrae (T11-L4) are indicated. The sign convention for centroid displacements is described at the bottom, a shift if X_CM to the left is in the lateral direction (away from the spine), for the left map, whereas it is in the medial direction (toward the spine) for the right side map.

FIGURE 4

Boxplots showing the Helander and Zhang’s statements of: (A) Comfort; and (B) Discomfort for the SS and A-Chair on the 16 pianist. For most Comfort questions (statements), the A-Chair had a better or equal score than the SS. The only exception was the restfulness of the chair: more pianists felt the SS was better than the A-Chair. The only statistically significant difference between chairs was “I like the chair,” with A-Chair being significantly more liked, than the SS. The sitting on the A-Chair was perceived as significantly “cramped” when compared to the SS (p = 0.034), whilst the SS was perceived to provide greater numbness feeling (p = 0.05) than the A-Chair. For the rest of the discomfort questions, no statistically significant difference was found, however, in five of the remaining seven statements, the pianists agreed the SS had a greater discomfort than with the A-Chair. Statistical outliers are indicated with * and **.