Muscle Activity Adaptations to Spinal Tissue Creep in the Presence of Muscle Fatigue

Abstract

Aim: The aim of this study was to identify adaptations in muscle activity distribution to spinal tissue creep in presence of muscle fatigue.

Methods: Twenty-three healthy participants performed a fatigue task before and after 30 minutes of passive spinal tissue deformation in flexion. Right and left erector spinae activity was recorded using large-arrays surface electromyography (EMG). To characterize muscle activity distribution, dispersion was used. During the fatigue task, EMG amplitude root mean square (RMS), median frequency and dispersion in x- and y-axis were compared before and after spinal creep.

Results: Important fatigue-related changes in EMG median frequency were observed during muscle fatigue. Median frequency values showed a significant main creep effect, with lower median frequency values on the left side under the creep condition (p≤0.0001). A significant main creep effect on RMS values was also observed as RMS values were higher after creep deformation on the right side (p = 0.014); a similar tendency, although not significant, was observed on the left side (p = 0.06). A significant creep effects for x-axis dispersion values was observed, with higher dispersion values following the deformation protocol on the left side (p≤0.001). Regarding y-axis dispersion values, a significant creep x fatigue interaction effect was observed on the left side (p = 0.016); a similar tendency, although not significant, was observed on the right side (p = 0.08).

Conclusion: Combined muscle fatigue and creep deformation of spinal tissues led to changes in muscle activity amplitude, frequency domain and distribution.

Fig 1. Illustration of the low back creep deformation protocol.

Fig 2. Representation of two 64-electrode matrices used in the recording of erector spinae muscle activity (model ELSCH064; LISiN-OT Bioelettronica, Torino, Italy).

Fig 3

Mean RMS (A) and MDF (B) values over time on the right and left sides (RMS: Root Mean Square; MDF: Median Frequency). Error bars indicate standard deviations. ★ represents a main effect of fatigue. ✦ represents a main effect of creep. Post hoc results are illustrated by * = p ˂ 0.01 and ** = p ˂ 0.001.

Fig 4

Mean dispersion values in x-axis (A) and y-axis (B) values over time on the right and left sides (Disp X: Dispersion in x-axis; Disp Y: Dispersion in y-axis). Error bars indicate standard deviations. ★ represents a main effect of fatigue. ✦ represents a main effect of creep. Post hoc results are illustrated by * = p ˂ 0.01.

Fig 5. Typical representation of dispersion data from a participant on the right erector spinae.

The large-array EMG was enlarged to better observed dispersion data. Bold black lines illustrate the migration of the centroid during the early (upper part of the figure) and late (lower part of the figure) muscle fatigue. Note the shift in the distribution of EMG amplitude toward the caudal region of the lumbar erector spinae (grey lines).