Effect of hypertrophy training at moderate altitude on the response of metabolic stress markers and associated muscle growth mechanisms

Abstract

Resistance training (RT) is a well-established interventional strategy for increasing muscle strength and hypertrophy. However, the available systematic reviews display discrepancies among training methodologies used which makes it difficult to draw firm conclusions about the potential benefit of RT in hypoxia (RTH) compared to the equivalent training in normoxia (RTN). Seven separate research experiments were carried out to 1) determine the current status of knowledge of RTH on muscle hypertrophy (Section I); and to 2) analyze the effect of the RT exercise under the two types of acute (Section II and III) and chronic (Section IV) moderate hypoxia conditions (terrestrial vs. simulated) on muscle mass and strength performance markers. The first study, a systematic review and meta-analysis on the topic, found similar improvements in muscle cross-sectional area (CSA) (SMD [CIs]=0.17 [-0.07; 0.42]) and 1RM (SMD=0.13 [0.0; 0.27]) between RTH and RTN. However, sub-analyses indicated that hypertrophy appears to benefit from shorter (≤60s) inter-set rest intervals during RTH while greater gains in strength were achieved with longer rest intervals (≥120s). Moderate loads (60-80% 1RM) enhanced both hypertrophy and strength. The use of moderate hypoxia (14.3-16% FiO2) seemed to somewhat benefit hypertrophy but not strength. The two preliminary studies (study 2 and 3) examined the effects of a hypertrophic RT session at acute terrestrial hypoxia on serum biomarkers associated with muscular adaptations. In a counterbalanced fashion, 13 resistance trained males completed a RT session (6 exercises x 3 sets x 10 RM; 120 s min rest) at both moderate altitude (HH; 2320m asl) and normoxia conditions (N; <700m asl). Venous blood samples were taken before and throughout the 30 min post-exercise period for determination of metabolites (lactate), ions (inorganic phosphate [Pi], liquid carbon dioxide [CO2L], calcium [Ca2+]), cytokines (IL-6, IL-10, TNFα), hormones (growth hormone [GH], cortisol [C], testosterone [T]) and miR- 378. Session-related performance and perception of effort (RPE-30) were also monitored. The results showed no differences in performance and RPE- 30. All blood variables displayed statistically significant changes compared to basal levels (p<0.05), while miR-378, T and inflammatory responses remained near pre-exercise conditions. No altitude effect was observed in maximal blood lactate, Ca2+ and anabolic hormones (p>0.05), although the CO2L reduction in HH (p<0.001) seems compatible with an increase in buffering capacity. At HH, the RT session produced a moderate to large increase in the absolute peak values of the studied cytokines. miR-378 revealed a moderate association with GH and C in both N and HH (r>0.051; p<0.05). The studies 4, 5 and 6 aimed to analyze the combined effect of the type of acute hypoxia (terrestrial vs. simulated) and the inter-set rest configuration (60 vs. 120 s) during a hypertrophic RT session on physiological, perceptual, muscle performance and serum markers. Sixteen active men were randomized into two groups (HH: 2320 m asl; vs. normobaric hypoxia, NH: FiO2 = 15.9%) and completed four RT sessions: two under normoxia and two under the corresponding hypoxia condition at each prescribed inter-set rest period. Volume-load, muscle oxygenation (SmO2) of the vastus lateralis and heart rate (HR) were monitored during training and RPE-30 was determined at the end of the recovery period. Maximal blood lactate [max- Lac], circulating hormones, ions (Ca2+, Pi, and CO2L), cytokines (irisin and myostatin) and miRNAs (miR-378, miR-206 and miR-29c) were measured throughout the initial 30 min post exercise. Volume-load was similar in all environmental conditions and inter-set rest periods. Shorter inter-set rest periods displayed greater increases in maxLac, HR, RPE-30, CO2L, Pi, C and GH in all conditions (p<0.05). Compared to HH, NH showed a moderate reduction in the inter-set rest-HR (ES>0.80), maxLac (ES>1.01) and SmO2 (ES>0.79) at both rest intervals. Additionally, higher values of circulating Ca2+ and Pi, and lower CO2L, were observed after training in HH compared to NH. The exercise with 60 s rest revealed a large early decrement of irisin in HH with respect to N and NH (ES < −1.10; p = 0.048). Both hypoxias moderately reduced circulating myostatin after exercise by a similar proportion (ES < 0.23; p > 0.21). Moderate to large significant increments in miR- 378 and miR-29c were detected in N, HH and NH. Compared to HH, a moderate to large rise in miR-29c and miR-206 was found in NH (ES > 0.96; p < 0.08). The study 7 aimed to analyze the effect of a RT period at terrestrial and simulated hypoxia on both muscle hypertrophy and maximal strength development with respect to the same training in normoxia. Thirty-three strength-trained males were randomly assigned to N (FiO2 = 20.9%), HH (2,320 m asl.) or NH (FiO2 = 15.9%). Subjects completed an 8-week RT program comprised by 3 sessions/week (full body routine of 6 exercises; 3 sets x 6-12 repetitions, 65-80% 1RM and 90 s rest). Muscle thickness of the lower limbs and 1RM in back squat (1RMSQ) and bench press were assessed on weeks 1, 6 and 8 of the training program. Maximal blood lactate, circulating cytokines (IL-6, IL-10, TNFα), hormones (GH, IGF-1), % active mTOR and miRNAs (miR-206, miR-378 and miR-29c) were measured before and throughout the initial 30 min post-RT exercise after the first (S1) and last (S22) session. RT program increased 1RM in all groups (p>0.001). NH reached a large significant enhancement compared to N in 1RMSQ (ES=1.20). Muscle growth similarly improved in N and HH after the RT program (ES= -0.14; p=1.0), while NH remained near to the pre-training values (ES= 0.23; p= 0.160). Similar blood lactate increments were found after S1 and S22 in all groups (p=0.895; η2p=0.001). Compared to N, HH and NH groups increased IL-6 and TNFα in S1 (ES >1.12; p<0.022), returning near to resting values at the end of the training period. Post-exercise GH increased in all conditions, although no changes were detected in the serum IGF-1. HH group showed a moderate to large increment in % active mTOR after S1 with respect to N (ES=1.04; p=0.017) and NH (ES=1.34; p=0.002) with no differences among groups after S22. The NH group displayed the highest serum miR-206 and miR-29c values (p<0.020). Moderate to large nonsignificant increases in serum miR-206 above pre-training values (ES>0.76) and a slight reduction below resting values of miR-29c were depicted in N and HH groups (p<0.08). Results under acute hypoxia suggest that the acute metabolic and physiological responses of a hypertrophic RT exercise are mediated by rest intervals between sets and the type of hypoxia. Altered circulating ions, myokines and miRNA could indicate acute differences in the type of hypoxia on muscle signalling pathway activation after a RT. Overexpression of miR-206 in acute NH could indicate a muscle preservation tendency and interfere with muscle growth after longer training periods. However, the results obtained in this study do not support the expected added benefit of RTH compared to RTN on muscle mass development, although it seems to favour gains in strength. The greater muscle growth achieved in HH over NH confirms the impact of the type of hypoxia on the outcomes. This is supported by the acute and chronic response of some of the evaluated biomarkers. Future research should elucidate the impact of RT and the role of hypoxia on serum biomarkers associated with muscle growth and the adaptation of other non-structure factors related to muscle strength development.