AUTHOR=Smajla Darjan , Žitnik Jure , Šarabon Nejc 

TITLE=Advancements in the Protocol for Rate of Force Development/Relaxation Scaling Factor Evaluation

JOURNAL=Frontiers in Human Neuroscience

VOLUME=15

YEAR=2021

URL=https://www.frontiersin.org/journals/human-neuroscience/articles/10.3389/fnhum.2021.654443

DOI=10.3389/fnhum.2021.654443

ISSN=1662-5161

ABSTRACT=<p>Brief submaximal actions are important for wide range of functional movements. Until now, rate of force development and relaxation scaling factor (RFD-SF and RFR-SF) have been used for neuromuscular assessment using 100–120 isometric pulses which requires a high level of attention from the participant and may be influenced by physiological and/or psychological fatigue. All previous studies have been conducted on a smaller number of participants which calls into question the eligibility of some of the outcome measures reported to date. Our aims were: (1) to find the smallest number of rapid isometric force pulses at different force amplitudes is still valid and reliable for RFD-SF slope (k<sub><italic>R</italic></sub><sub><italic>F</italic></sub><sub><italic>D</italic></sub><sub>–SF</sub>) and RFR-SF slope (k<sub><italic>RFR–SF</italic></sub>) calculation, (2) to introduce a new outcome measure – theoretical peak of rate of force development/relaxation (TP<sub><italic>RFD</italic></sub> and TP<sub><italic>RFR</italic></sub>) and (3) to investigate differences and associations between k<sub><italic>RFD–SF</italic></sub> and k<sub><italic>RFR–SF</italic></sub>. A cross-sectional study was conducted on a group of young healthy participants; 40 in the reliability study and 336 in the comparison/association study. We investigated the smallest number of rapid isometric pulses for knee extensors that still provides excellent reliability of the calculated k<sub><italic>RFD–SF</italic></sub> and k<sub><italic>RFR–SF</italic></sub> (ICC<sub>2</sub>,<sub>1</sub> ≥ 0.95, CV &lt; 5%). Our results showed excellent reliability of the reduced protocol when 36 pulses (nine for each of the four intensity ranges) were used for the calculations of k<sub><italic>RFD–SF</italic></sub> and k<sub><italic>RFR–SF</italic></sub>. We confirmed the negligibility of the y-intercepts and confirmed the reliability of the newly introduced TP<sub><italic>RFD</italic></sub> and TP<sub><italic>RFR</italic></sub>. Large negative associations were found between k<sub><italic>RFD–SF</italic></sub> and k<sub><italic>RFR–SF</italic></sub> (<italic>r</italic> = 0.502, <italic>p</italic> &lt; 0.001), while comparison of the absolute values showed a significantly higher k<sub><italic>RFD–SF</italic></sub> (8.86 ± 1.0/s) compared to k<sub><italic>RFR–SF</italic></sub> (8.03 ± 1.3/s) (<italic>p</italic> &lt; 0.001). The advantage of the reduced protocol (4 intensities × 9 pulses = 36 pulses) is the shorter assessment time and the reduction of possible influence of fatigue. In addition, the introduction of TP<sub><italic>RFD</italic></sub> and TP<sub><italic>RFR</italic></sub> as an outcome measure provides valuable information about the participant’s maximal theoretical RFD/RFR capacity. This can be useful for the assessment of maximal capacity in people with various impairments or pain problems.</p>