Endurance Training Intensity Does Not Mediate Interference to Maximal Lower-Body Strength Gain during Short-Term Concurrent Training

We determined the effect of concurrent training incorporating either high-intensity interval training (HIT) or moderate-intensity continuous training (MICT) on maximal strength, counter-movement jump (CMJ) performance, and body composition adaptations, compared with single-mode resistance training (RT). Twenty-three recreationally-active males (mean +/- SD: age, 29.6 +/- 5.5 y; (V) over dotO(2peak), 44 +/- 11 mL kg(-1) .min(-1)) underwent 8 weeks (3 sessions.wk(-1)) of either: (1) HIT combined with RT (HIT-RT group, n = 8), (2) work-matched MICT combined with RT (MICT+RT group, n = 7), or (3) RT performed alone (RT group, n = 8). Measures of aerobic capacity, maximal (1-RM) strength, CMJ performance and body composition (DXA) were obtained before (PRE), mid-way (MID), and after (POST) training. Maximal (one-repetition maximum [1-RM]) leg press strength was improved from PRE to POST for RT (mean change +/- 90% confidence interval; 38.5 +/- 8.5%; effect size [ES] +/- 90% confidence interval; 1.26 +/- 0.24; P < 0.001), HIT+RT (28.7 +/- 5.3%; ES, 1.17 +/- 0.19; P < 0.001), and MICT+RT (27.5 +/- 4.6%, ES, 0.81 +/- 0.12; P < 0.001); however, the magnitude of this change was greater for RT vs. both HIT+RT (7.4 +/- 8.7%; ES, 0.40 +/- 0.40) and MICT+RT (8.2 +/- 9.9%; ES, 0.60 +/- 0.45). There were no substantial between-group differences in 1-RM bench press strength gain. RT induced greater changes in peak CMJ force vs. HIT+RT (6.8 +/- 4.5%; ES, 0.41 +/- 0.28) and MICT+RT (9.9 +/- 11.2%; ES, 0.54 +/- 0.65), and greater improvements in maximal CMJ rate of force development (RFD) vs. HIT+RT (24.1 +/- 26.1%; ES, 0.72 +/- 0.88). Lower-body lean mass was similarly increased for RT (4.1 +/- 2.0%; ES; 0.33 +/- 0.16; P = 0.023) and MICT+RT (3.6 +/- 2.4%; ES; 0.45 +/- 0.30, P = 0.052); however, this change was attenuated for HIT+RT (1.8 +/- 1.6%; ES; 0.13 +/- 0.12; P = 0.069). We conclude that concurrent training incorporating either HIT or work-matched MICT similarly attenuates improvements in maximal lower-body strength and indices of CMJ performance compared with RT performed alone. This suggests endurance training intensity is not a critical mediator of interference to maximal strength gain during short-term concurrent training.