Effects of Contraction Intensity on the Variability and Complexity of Force and Acceleration During an Isometric Squat 

Miguel López-Fernández1, Fernando García-Aguilar1, Pablo Asencio1, Alejandro Oliver-López1, 2 , Daniel Cerdán-Guzmán1 and Rafael Sabido1 

¹ Department of Sports Sciences, Miguel Hernández University, Spain 

2 Faculty of Medicine, Health and Sports. Department of Sports Sciences, European University of Madrid, Spain 

Introduction 

Force variability and complexity are emerging indicators of neuromuscular performance and motor control. In isometric contractions—particularly those involving single-joint or less complex tasks—variability has been shown to increase with contraction intensity, while complexity tends to peak at low-to-moderate intensities and declines at higher intensities (1). In contrast, previous research has indicated that higher intensities during resistance exercise may result in reduced variability and increased complexity, especially in dynamic tasks (2). However, there is limited evidence on how these parameters respond during sustained isometric contractions involving multijoint resistance exercises such as the squat. This study aimed to analyse how different intensities of isometric squat influence the variability and complexity of force and acceleration output. 

Keywords 

Complexity; Variability; Isometric Squat 

Methods 

Nine healthy participants (age: 24 ± 3.9 years; mass: 77 ± 9.4 kg; height: 177.5 ± 5.1 cm) performed an isometric half-squat, by exerting maximal upward force against a fixed bar to determine maximum voluntary contraction (MVC). After performing three 6-second MVC trials, participants completed five 30-second isometric trials at 10%, 30%, 50%, 70%, and 90% of MVC, with 5-minute rest intervals between sets. Force was measured at 1000 Hz using a MyoQuality dynamometer, and trunk acceleration was recorded using an inertial measurement unit (IMU) placed at L5–S1. Variability was assessed using standard deviation (SD) and complexity using Sample Entropy (SampEn), for both force and acceleration signals. A repeated-measures ANOVA and Bonferroni-corrected post hoc tests were used to analyse differences between intensities (α = 0.05). 

Results 

Repeated measures ANOVA revealed significant differences in force variability (SD) between intensities (p = 0.009). Pairwise comparisons showed a significant increase in SD between 50% and 90% MVC (p = 0.004), indicating greater variability in force output under maximal effort. Similarly, signal complexity (SampEn) differed significantly across intensities (p = 0.008). An increase was found between 30% and 50% MVC (p = 0.005), suggesting a non-linear modulation of complexity, with a peak at moderate intensities and a reduction at higher levels. 

For acceleration signal, no significant effect was observed in SampEn across intensities (p = 0.534). However, pairwise comparisons revealed a significant increase from 70% to 90% MVC (p = 0.037), indicating greater signal irregularity at the highest contraction level. In contrast, SD showed a significant main effect (p = 0.012), although post-hoc pairwise comparisons did not reach statistical significance. A significant linear trend (p = 0.042) reflected a progressive increase in acceleration variability with higher force levels. 

Conclusion 

Increasing isometric load in a squat position leads to higher variability in both force and acceleration, but complexity behaves differently across variables. While force complexity peaks at moderate intensities and decreases under high loads, acceleration complexity remains relatively stable. These results highlight the differentiated motor control strategies involved in force production and postural control in resistance exercise. This suggests that training at moderate intensities may promote more adaptable motor behavior, whereas submaximal efforts lead to more constrained force patterns. These results can help coaches and practitioners tailor isometric protocols according to specific neuromuscular adaptation goals. 

References: 

López-Fernández M, García-Aguilar F, Asencio P, et al. Force variability and complexity during isometric contractions at different intensities. PLoS One. 2024;19(9):e0307949. https://doi.org/10.1371/journal.pone.0307949  

Slifkin AB, Newell KM. Noise, information transmission, and force variability. J Exp Psychol Hum Percept Perform. 1999;25(3):837–51. doi:10.1037//0096-1523.25.3.83 

Socurces of Funding 

This research is related to the national project funded by the Ministerio de Ciencia e Innovación [PID2022-139600NB-I00]. The contribution of Miguel López-Fernández was funded by the Generalitat Valenciana, Spain [grant number: CIACIF/2021/452].