Between-session reliability of performance and asymmetry variables during lower limb strength tests and sport-specific tasks in netball players

Abstract

Title: Between-session reliability of performance and asymmetry variables during lower limb strength tests and sport-specific tasks in netball players Inter-limb asymmetry, the ratio that represents the performance comparison between two limbs, has been quantified during lower body plyometric, ballistic and isometric strength assessments. Asymmetries vary across different tasks and performance metrics, making asymmetries very task specific and metric dependent. In addition, inertial measurement units (IMUs) have recently developed into a popular tool to quantify training load and inter-limb differences in a sport-specific environment. The reliability of a measure shows its reproducibility across repeated trials. Any assessment requires a high reliability to ensure low measurement error. Measurement studies assessing inter-limb differences have generally shown acceptable within- and between-session reliability for observed performance measures. However, recent research investigating the within- and between-session reliability of the actual derived asymmetry value found this to be highly variable and unreliable. This measurement study aimed to investigate the within-day and between-day reliability of force metrics and inter-limb force asymmetry during unilateral and bilateral variations of isometric, ballistic and plyometric laboratory-based strength assessments. A second aim was to investigate the between-day reliability of IMU-derived impact load and impact load asymmetry during sport-specific drills. During the netball pre-season, 25 healthy female university netball players (mean ± SD age: 20 ± 1.7, stature: 177.6 ± 7.0 cm, mass: 69.9 ± 8.3 kg) participated in this study. Testing consisted of four days. On Day 1 participants performed three trials of both unilateral and bilateral variations of the drop jump (DJ), countermovement jump (CMJ), and isometric squat (ISQ). On Day 2 participants performed six routine warm-up drills with IMUs attached to the shin to measure the frequency and intensity of ground contacts experienced. Day 1 and Day 2 was repeated on Day 3 and Day 4. Inter-limb asymmetries were quantified for peak force in the drop jump, countermovement jump and isometric squat. For the field-based assessments inter-limb asymmetry was quantified for impact load. The coefficient of variation (CV) and intraclass correlation coefficient (ICC) was determined for each participant for all performance and asymmetry metrics to define the within-and between-day reliability. Good to excellent within- and between-day reliability was seen for all force metrics for both variations of the strength assessments. Relative within- and between-day reliability for force asymmetry variables during the bilateral strength tests ranged from good to excellent (ICC: 0.89 – 0.94). For the unilateral strength assessments relative within- and between-day reliability were poor to moderate (ICC: 016 – 0.67). Absolute within- and between-day reliability for all force asymmetry variables were unacceptable (CV: 26.4 – 645.5%). During the sport-specific drills, moderate relative reliability (ICC: 0.50 – 0.60) and unacceptable agreement (CV: 13 – 19%) were seen for impact load in all the controlled drills. When considering all the drills together, impact load reliability was moderate (ICC: 0.58 – 0.60), with a CV of 11%. In all the sport-specific drills impact load asymmetry was inconsistent between days and showed very poor between-day reliability (CV: 44.3 – 422.6%; ICC: -0.21 – 0.15). Performance variables used to quantify inter-limb asymmetries are reliable within- and between sessions, however, high variability is seen when considering the reliability of asymmetry measures. When describing, comparing, or tracking lower limb asymmetries during unilateral and bilateral strength assessments, as well as during sport specific drills, practitioners should carefully consider test selection, and metric- and asymmetry reliability. Practitioners should not only look at the ecological validity of a specific test to create an asymmetry profile of an athlete, but also consider the reliability and variability of the test, test metrics and asymmetry measures.