Changes in Acceleration Variability During an Isometric Squat as an Indicator of Fatigue and Its Neuromuscular Implications 

García-Aguilar, Fernando¹; López-Fernández, Miguel¹; Asencio, Pablo¹; CerdáGuzmán, Daniel¹; Oliver-López, Alejandro¹²; and Sabido, Solana¹. ¹Research Center for Sport Sciences, Miguel Hernández University of Elche, Valencian Community, Spain. ²Department of Sport Sciences, Faculty of Medicine, Health and Sport, European University of Madrid, Madrid, Spain. 

Introduction 

Fatigue monitoring is a crucial component in the planning and tracking of strength training programs. Fatigue induces alterations in motor behavior and can modify motor variability, an inherent feature of human movement. Motor variability has been analyzed from two main perspectives: the amount of variability (linear perspective) and the temporal structure of variability (non-linear perspective). From the non-linear approach emerges the “loss of complexity theory,” which posits that when complexity decreases— whether due to shifts toward more predictable or random behavioral patterns—the organism enters a less functional and adaptive state (1,2). Previous research has shown that fatigue tends to reduce complexity in isometric, single-joint tasks, leading to more predictable behaviors (3). However, no studies to date have explored these effects in multi-joint isometric actions. Therefore, the aim of this study was to analyze the effects of fatigue on motor variability during an isometric squat, as well as to elucidate the potential neuromuscular changes that may occur. 

Method 

Twelve trained men participated in the study (≥2 years of strength training experience, 1RM squat ≥ 1.4 times body weight). Following a familiarization session, participants completed a fatigue protocol consisting of 10-repetition sets to failure at 70% of 1RM, with 2 minutes of rest between sets. Before and after the fatigue protocol, they performed an isometric squat at 40% of 1RM for 30 seconds. Lumbosacral acceleration was recorded using an inertial measurement unit (IMU), and electromyographic (EMG) activity was recorded from five muscles: rectus femoris, vastus medialis, vastus lateralis, semitendinosus, and longissimus. The indicators analyzed were standard deviation (SD) and sample entropy (SampEn) for the acceleration signal; and for EMG: root mean square (RMS), median power frequency, and SampEn. Results No significant changes were observed in the SD of acceleration, but there was a significant post-fatigue decrease in SampEn (p = 0.02; d = 0.68). Regarding EMG, there was a significant increase in RMS in all muscles except the vastus medialis (p < 0.05). Median power frequency significantly decreased only in the longissimus. For SampEn, significant reductions post-fatigue were observed in the semitendinosus and longissimus. Conclusion 

The results suggest that fatigue alters the structure of motor variability, particularly in trunk stabilizing muscles, and that non-linear analysis of lumbosacral acceleration may be a sensitive tool for detecting fatigue. The increase in muscle activation reflects greater motor unit recruitment to compensate for fatigue, with no widespread changes in conduction velocity, likely due to the submaximal nature of the task. This approach offers an accessible, cost-effective methodology applicable in real-world training contexts, allowing fatigue assessment without requiring maximal efforts. Thus, it presents a promising avenue for load monitoring and injury prevention in strength training programs.  

References 

Lipsitz LA, Goldberger AL. Loss of ‘complexity’ and aging: potential applications of fractals and chaos theory to senescence. JAMA. 1992;267(13):1806–9. 

Stergiou N, Harbourne RT, Cavanaugh JT. Optimal movement variability: a new theoretical perspective for neurologic physical therapy. J Neurol Phys Ther. 2006;30(3):120–9. 

García-Aguilar F, Caballero C, Sabido R, Moreno FJ. The use of non-linear tools to analyze the variability of force production as an index of fatigue: A systematic review. Front Physiol. 2022;13:1074652. 

Funding 

This study is related to the national project funded by the Spanish Ministry of Science and Innovation [PID2022-139600NB-I00]. The contribution of Fernando García Aguilar was funded by the Generalitat Valenciana, Spain [grant number: ACIF/2020/159].