Intensity specific responses to single-limb leg extension exercise to failure

The intensity-duration relationship is hyperbolic across modalities. It can also be represented linearly as the relationship between total work performed and duration. The asymptote of the intensity-duration relationship is synonymous with the slope of the work-duration relationship. This value is termed the critical intensity and separates the heavy and severe intensity domains. A separate supra-severe intensity domain, termed the extreme domain, has been established where task failure occurs before VO2max can be obtained. These domains and relationships have been tested during dynamic constant external resistance exercise; however, the critical intensity has been predicted to cause task failure ~2 minutes compared to critical intensity in other modalities occurring between 15-30 minutes. The purpose of this study was to examine the critical intensity (critical load) estimates using traditionally used intensities for deriving critical load (CL) and loads that elicit task failure around the same time as the intensities used to predict the critical intensity of other modalities. The secondary purpose of this study was to evaluate the physiological responses to resistance exercise within the severe and extreme domains. 24 resistance-trained subjects (12M, 21.7±1.4 years, 12F, 22.5±2.0 years) participated in this study where a one-repetition maximum (1RM) was tested on a single-limb leg extension machine followed by 6 subsequent visits with randomized intensities for a repetitions to failure test (3 loads eliciting time to task failure between 2-15 minutes, termed S1, S2, and S3 as well as 70%1RM, 80%1RM, and 90%1RM). Maximal voluntary isometric contractions (MVCs) were tested before and after each task failure trial. Electromyography and Near-Infrared Spectroscopy was measured on the belly of the vastus lateralis. CL was estimated to be 3±2%1RM for severe exercise (S1-S3) compared to 48±16%1RM for extreme exercise (70%, 80%, and 90%; p<0.001). Severe intensity exercise had higher performance fatigability (pre:post MVCs) compared to extreme intensity exercise (p<0.001). These findings indicate that the intensity domains derive different CL values. The CL value derived via traditionally used intensities is not synonymous with the critical intensity of other modalities; however, the severe intensity predicted CL is similar, and possibly synonymous with the critical intensity of other modalities.