Reliability and Validity of Kinetic Variables Collected on a Knee Extension Isometric Contraction, Using Load Cell Technology in a Healthy Population
Objective, data-driven assessments, specifically assessments which measure an individual’s capacity or potential, are of increasing interest within the realms of physical medicine and performance. In physical medicine, this type of assessment typically includes aspects of tissue specific muscle force production (peak force) and various measures of muscular strength. These variables, while certainly valuable as a component of a tissue capacity examination, are limited when compared to domains of explosive-type force, or force produced in very short domains of time (<100-200 ms). While the concept of explosive force is commonly relegated to the world of sport and athletics, it is also extremely valuable in a variety of daily tasks, such as the maintenance of balance. Therefore, force variables, such as rate of force development (RFD) and impulse (IMP), within these time domains are being explored as influential factors of force profiling that could be useful in determining risk or recovery status after an injury. Unfortunately, the traditional process of collecting explosive force data requires specific technologies, most of which are expensive or impractical. The purpose of this thesis is to explore the use of a load cell device as an alternative to more traditional dynamometry for the purposes of quantifying explosive force. Acceptable outcomes of these studies would serve to support the use of the load cell device to enhance clinical testing capacities, ultimately improving the quality of diagnostic information provided to the clinician.
The overarching question that guided the direction of the thesis was: can a load cell device be used to collect kinetic data, as a clinical diagnostic tool, for the assessment of knee extension force output in a rehabilitation setting? This thesis explored variables of force production using the aforementioned load cell, with specific interest of explosive force, for the purposes of accuracy (reliability and validity). These studies involved the completion of three maximal, explosive, knee extension isometric contractions (at 60 and 90 degrees of knee flexion) within various unique protocols. These protocols, described as ‘constrained’ or ‘unconstrained’, were intended to proxy as versions of clinical use, and therefore were designed to incorporate a comparison of laboratory data collection and practical collection methodologies. The term ‘protocol’ was used to define five unique scenarios or layouts, incorporating varying degrees of fixation, constraint, and knee position.
The kinetic force variables included in this study were as follows: peak force (PF), peak rate of force development (PRFD), rate of force development (RFD2080), and impulse (IMP2080). The subscript denotation, “2080”, corresponds to measures across a window of time that exists from 20% to 80% of peak force.
The first study determined intrasession reliability of the load cell device. The study included 32 healthy subjects (14 males and 18 females: age 31.8±7.91 years) compared across three trials completed in a single session. ICC inferences of medium to very high were found for protocol 1 and 3, the 90° knee flexion position, for all kinetic variables. These were notably higher than the 60 degree knee position. However, while the ICC values were high throughout for protocols 1 and 3, larger variability (CV%) were also found for RFD and IMP: (PF ICC = 0.97 to 0.99; CV% = 3.20% -4.50%), (RFD2080 ICC = 0.86 to 0.97; CV% = 10.5% - 17.9%), (PRFD ICC = 0.82-0.94; CV% = 8.90% - 13.4%), and (IMP2080 ICC = 0.85 to 0.98; CV% = 11.4% -20.7%). The second study determined intersession reliability of the load cell device. The study included 12 healthy subjects (6 male and 6 females; age 31.0±6.4 years) compared across three time points with 7-10 days between each testing period. When compared across protocols, PF was the only variable to demonstrate small and acceptable variability (CV’s being less than 1.5%, with the plinth at 90° protocol having <10% CV and >0.90 ICC for both testing sessions). The constrained version (protocol 1) was associated with lower variability of RFD2080 (the lowest CV being 10.1%), however, the unconstrained version at 90°demonstrated lower variability with PRFD (lowest CV% being 3.77, and highest ICC 0.83). Finally, study 3 focused on evaluating the validity of the load cell device comparing 26 subjects (12 males and 14 females; age 32.0±8.9 years) across all protocols including protocols collected using the isokinetic dynamometer. The isokinetic device represented the gold standard (constrained) protocol by which each of the other protocols were compared. With respect to the kinetic variables, no significant differences in means were identified between devices (P =>0.05), across all three protocols. Slightly higher means were noted with the isokinetic constrained protocol; however, these were non-significant. Only the 90 degree knee flexion position was explored based on the findings from studies one and two, noting a significantly improved reliability for the 90 degrees position compared to the 60 degree.
The findings of this thesis support the use of a load cell device for the purposes of obtaining kinetic variables within sessions and across various types of constraints (protocols), however it appears the 90 degree knee position is superior to the 60 degree position in terms of withing and between session reliabilities. Caution should be used when exploring PRFD, RFD, and IMP as these metrics appear to demonstrate questionably large variabilities (large CV%), especially in the cases of between session testing. In reference to Study 2, the use of the described procedures and protocols, cannot be recommended for between session testing for PRFD, RFD, and IMP.
Although Study 3 highlighted the consistency of the values across devices and protocols, the differences across sessions (Study 2) were not acceptable in regards to RFD/IMP/PRFD, and thus should be used with caution. The findings in this thesis support the use of the load cell device, in knee extension kinetic variables, for clinical practice and comparative analyses in the context of intrasession methodology, notably when collected at 90 degrees of knee flexion. It is recommended that future research further explore the intersession characteristics in an attempt to better understand and capture these kinetic variables for outcome testing and normative data profiling.