Are high-intensity interval training and low load resistance training two sides of the same physiological coin: a...

Resistance and endurance training are typically associated with mode specific physiological and functional adaptations with minimal overlap. However, low load resistance training (LL) and interval training (IT) demand maximal effort, induce fatigue over similar durations, and are characterized by comparable peak and average forces. Therefore, a high potential for overlapping functional and physiological adaptations exists following IT and LL. Untrained healthy adults (age 19-30) were recruited (N = 19). Participants' legs were randomly assigned to the LL (3 sets, 30% 1-RM completed to failure, 2-3 sessions/week) or IT (a combination of 7-10 1 min intervals at 90% of W_max and 4-5, 30 s maximal intervals, 2-3 sessions/week) condition over a 10-week period. An overall effect of time (p < 0.05) was observed for 1-RM knee extension strength, isometric knee extension (90°) strength, anaerobic peak power output, W_max, vastus lateralis muscle thickness, type I and II capillary contacts and capillary density, and mitochondrial protein content. Local muscle endurance (LME, repetitions completed at 30% 1-RM) increased from baseline in both interventions but was greater in the LL condition relative to the IT condition (p = 0.007). Vastus lateralis cross-sectional area (p = 0.038) and anaerobic mean power (p = 0.008) were greater in the IT intervention leg following training. Both LL and IT demand comparable mechanical force and effort and induce similar physiological and functional adaptations in untrained young adults. Although, mode-specific adaptations exist for performance tests that closely resemble training interventions. Study registration-NCT05945641.