# Precision and Reliability of a Dynamometer for Trunk Extension Strength and Steadiness Assessment
## 1 Introduction  
Low back pain (LBP) remains one of the leading causes of disability worldwide, with approximately 85% of cases lacking a specific peripheral etiology [1]. Despite the uncertainty surrounding its origin, recurrence rates of 60% to 80% are observed after the initial episode [2]. It has been hypothesized that weakness and poor endurance of the paravertebral muscles lead to increased loading on passive spinal structures, such as intervertebral disks and ligaments, thus contributing to the persistence and chronicity of pain [3, 4, 5, 6, 7]. Among the muscles involved, trunk extensors play a particularly critical role in both locomotion and postural control. Dysfunction in these muscles not only impairsmotor capacity but also significantly impacts quality of life, with profound physical and psychological implications [8; 9; 10]. Motor function impairment, characterized by reduced strength and muscle performance, can substantially limit autonomy in activities of daily living [11; 12; 13; 14]. As such, the accurate assessment of trunk extensor strength becomes vital across various populations, including older adults [15; 16], individuals with LBP [14; 17], and healthy subjects [18; 19], as a means of preventing further musculoskeletal dysfunctions [20; 21]. Understanding key variables such as maximal force production capacity, mean force, and steadiness during muscle contraction is essential for effectively monitoring therapeutic responses and ensuring accurate interpretation of observable functional changes [22; 23]. These assessments provide invaluable insight into the factors driving the persistence of LBP and the functional limitations that accompany it [24; 25]. Different methods have been employed to assess trunk extensor strength, though their reliability remains variable [26; 27; 28; 29; 30; 31]. The isometric handheld dynamometer (HHD) is widely regarded as the gold standard for isometric muscle strength evaluation [14; 32; 33]. However, previous studies have reported intraclass correlation coefficients (ICC) for different HHD models ranging from 0.67 to 0.93, with considerable variability attributed to factors such as participant posture and differing measurement protocols [14; 34; 35; 36]. Despite its widespread use, for example, the Lafayette HHD has notable limitations, including high cost, the requirement for specialized personnel, and stringent control conditions, which can hinder its broader clinical application [15]. Moreover, the HHD's inability to synchronize with other data acquisition systems limits its utility for real-time feedback during assessments, particularly when evaluating submaximal isometric voluntary contractions (SMVC). In such cases, sustaining a steady force is essential for evaluating variables like steadiness. In contrast, load cell systems offer higher sampling frequencies, which are essential for detecting immediate variations in force output and improving the precision of strength assessments. However, questions remain about the test-retest reliability of these methods, as results may be influenced by factors like subject posture during evaluation (prone, seated, or standing), the type of equipment used, and the fixation methodology of the dynamometer, which can complicate cross-study comparisons [14; 34; 36; 37; 38]. The lack of standardization in measurement protocols, along with the influence of gravitational force, can also introduce biases in the obtained values, further complicating result interpretation. Despite recent advancements in the reliability of portable dynamometers, uncertainties remain regarding the impact of posture and testing conditions on the assessment of maximal and submaximal isometric strength, as well as muscle steadiness during trunk extension [14; 28; 32; 34; 35; 39]. Given these challenges, the implementation of fixation structures designed to minimize inertial effects could significantly enhance the accuracy and reliability of measurements, addressing a critical gap in current strength assessment practices.