The effect of preload on variability in dynamometric measurements of knee extension

Abstract

The unwanted variability in dynamometric measurements limits accurate interpretation of these measurements. Although unpredictable variability in measurements has been identified repeatedly, few studies have investigated strategies for reducing measurement error. This study investigated the effect of preload on variability in dynamometry. Preload is the force that must be overcome before the lever arm begins to move at a preset speed. On the basis of related research, it was hypothesised that under higher preloads, subjects would perform more consistently. Twenty subjects (ten male and ten female), aged 19-24 years, performed repeated right concentric knee extension tests. Using the Kin-Com dynamometer and a lever arm speed of 60 degrees x s(-1), extension torques were recorded for each subject under four different preload conditions. Preloads were 5%, 25%, 45% and 65% of the torque achieved during each subject's maximal voluntary contraction (MVC). Each subject performed six test repetitions under each preload. Variability in peak torque was significantly smaller under the 65% preload compared to the 5% preload. In addition, the regression of preload against group mean variability indicated that as preload increased, the variability in peak torque decreased in a highly predictable way. The results from this study suggest that subjects perform more consistently under conditions of higher preload than under lower preloads. By adopting higher preloads for dynamometry testing, measurement accuracy and the potential clinical utility of measurements is improved.

Cortical and Spinal Excitability during and after Lengthening Contractions of the Human Plantar Flexor Muscles Performed with Maximal Voluntary Effort

“…The appearance of such a twitch implies that voluntary activation was incomplete, even during the purely isometric contraction where no ‘lengthening’ inhibition would be expected to occur. To ensure highest possible activation during stretch, eccentric contractions should be preceded by a maximal isometric contraction [41], [42]. In several studies where subjects failed to produce eccentric torques higher than their isometric maximum, lengthening contractions either started from low pre activation levels [6], [7], [11], [13] or were performed as combined shortening-stretch cycles [12] although it is well known that force production following shortening is depressed [43].…”

Eccentric torque-velocity relationship of the elbow flexors

“…It is important to note that body positioning has been shown to influence reliability, and the seated preacher curl position used in this protocol appears superior to a supine position [9]. Since the level of isometric loading prior to eccentric contraction has been demonstrated to affect isokinetic testing reliability [18], this research highlighted the need for a preload of at least 65% of the subjects' maximal voluntary contraction to gain reproducible results. During the present study, subjects were instructed to contract maximally at the beginning of the range of movement (60 • elbow flexion), which was consistent with the instructions in other studies [15,19].…”

Voluntary activation level and muscle fiber recruitment of human quadriceps during lengthening contractions

“…As this preload was very low, there was no isometric phase before the movement. It has been shown that preload influences the torque magnitude such that, with increasing preload, the torque increases (10). It might be hypothesized that preload could affect the amount of voluntary activation, as it has been found that motor unit activation patterns are influenced by preactivation (25).…”