Test-retest Reliability of Jump Execution Variables using Mechanography: A Comparison of Jump Protocols Jesse A. Stein1, LuAnn Johnson3, James N. Roemmich3, Grant Tomkinson1, Lucas Bloms1, Sam Johnson2 & John S. Fitzgerald1 1University of North Dakota, Grand Forks, ND, USA 2Concordia University, St. Paul, MN 3Grand Forks Human Nutrition Research Center, USDA, Grand Forks, ND Abstract PURPOSE Method Novel Variables The purpose of this study is to examine the test-retest reliability of jump execution variables assessed during the squat jump using mechanography using two different protocols. Mechanography during the vertical jump test allows for evaluation of force-time variables reflecting jump execution, which may enhance screening for functional deficits that reduce physical performance and determining mechanistic causes underlying performance changes. However, utility of jump mechanography for evaluation is limited by scant test-retest reliability data of force-time variables. Purpose: To examine test-retest reliability of jump execution variables assessed from mechanography using two different protocols. Methods: 32 women (mean ± SD: age = 20.8 ± 1.3 yr, height = 167.6 ± 6.3 cm, mass = 68.2 ± 12.7 kg) and 16 men (age = 22.1 ± 1.9 yr, height = 181.5 ± 5.0 cm, mass = 94.1 ± 24.6 kg) attended a familiarization session followed by two testing sessions, all one week apart, during which they performed the vertical jump test and had mechanography data recorded. Participants performed six squat jumps (SJ) per session, with squat depth self-selected for the first three jumps and controlled using a goniometer to 110º knee flexion for the remaining three jumps. Raw data were sampled at 1,000 Hz and filtered with a cutoff frequency of 90.9 Hz using Bertec Digital AcquireTM. Jump execution variables were calculated using a macro program in Microsoft Visual Basic. Eight force-time variables were assessed. Test-retest reliability was quantified as the systematic error (using %difference between jumps), random error (using coefficients of variation), and test-retest correlations (using intraclass correlation coefficients).Results: Jump execution variables demonstrated good reliability, evidenced by very small systematic errors (mean ±95%CI: –1.2 ±2.3%), small random errors (mean ±95%CI: 17.8 ±3.7%), and very strong test-retest correlations (range: 0.73-0.97). Differences in random errors between controlled and self-selected protocols were negligible (mean ±95%CI: 1.3 ±2.3%). Conclusion: Jump execution variables demonstrated good reliability, with no meaningful differences between the controlled and self-selected SJ depth protocols. To simplify testing, a self-selected SJ depth protocol can be used to assess force-time variables with negligible impact on measurement error. Healthy adults aged 18-50 years were recruited. Basic background information, self-reported total physical activity as evaluated using the International Physical Activity Questionnaire (IPAQ), along with jumping mechanography data were collected in session one. Mechanography data were collected during the squat jump (SJ), which was performed on a force platform using two different jump protocols. First, three SJs where performed in which the participant self-selected a squat depth, held this position for approximately three seconds, then jumped vertically for maximum height. At least one minute separated each jump. A three-minute rest was provided after three jumps. Next, three SJs were performed in a similar fashion except squat depth was standardized at 110° using a goniometer. Novel variables of interest were time to peak force, time to half peak force, force gradient, acceleration gradient, and time to peak power. Intersession reliability was assessed using intraclass correlational coefficient and coefficient of variation. Peak Force Highest vertical force before take off Time to Peak Force Time from jump initiation to peak force 1/2 Peak Force Peak Force/2 Time to ½ Peak Force Time needed to obtain ½ Peak Force Average Rate of Force Development Peak force/ Time to Peak Force Peak Rate of Force Development Peak time derivative of the vertical force trace Starting Gradient ½ Peak force/ time to ½ Peak Force Acceleration Gradient Half peak force// (Time to peak force – Time to ½ Peak Force Descriptive analysis Table 1: Group descriptive analysis. Men Women N 16 32 Age 22.1 ± 1.9 yr 20.8 ± 1.3 yr Weight (kg) 94.1 ± 24.6 kg 68.2 ± 12.7 kg Height (cm) 181.5 ± 5.0 167.6 ± 6.3 cm Results Table 2: Reliability analysis of controlled and self-selected jump execution variables. Graph 1: Force-time trace of participant during squat jump Background Conclusion Jump test performance provides information about the functioning of an individual’s motor system and their ability to perform activities of daily living and physical activity. Jump mechanography allows for the measurement of force-time variables that reflect the execution of the jump and may augment the ability to screen for functional deficits. Existing data on test-retest reliability of force-time variables are scant and generally report “fair” reliability. The countermovement jump has been used in a majority of jumping mechanography studies although this technique complicates interpretation of performance data since countermovement depth is typically not controlled for and may not have significant effects compared to the squat jump (SJ). Utilizing a jump without a countermovement, such as the SJ, may decrease the measurement error associated with the calculation of jump execution variables. 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