Previous studies reported that concurrent endurance and resistance training, compared to resistance training alone, leads to a reduction in muscular strength (Dolezal et al., 1998; Hickson, 1980; Kraemer et al., 1995), hypertrophy (Hickson, 1980; Kraemer et al., 1995), and power (Kraemer, Patton et al. 1995). The effects of concurrent training have been explained as the activity of selected negative regulators of protein synthesis, such as adenosine monophosphate-activated protein kinase (AMPK), which is increased by endurance exercise in an intensitydependent manner (Bodine et al., 2001; Bolster et al., 2002; Rose et al., 2009). Based on molecular mechanisms, the effects of concurrent training should be observed when the same muscle is subjected to both resistance and endurance exercise. A meta-analysis reported that local interference with lower-body strength or muscle gain occurs when whole-body or lower-body resistance training and lower-body endurance training, such as running and cycling, are performed concurrently (Wilson et al., 2012).
Contemporaneous whole-body resistance training and lower-body endurance training have also been examined (Dolezal and Potteiger, 1998; Kraemer et al., 1995; Robineau et al., 2016). Dolezal et al. (1998) examined 8 weeks of concurrent whole-body resistance training and cycling endurance training and showed that change in bench press strength gain (12%) was less than that with resistance training alone (24%). Although significant differences were not observed between the groups, there is a possibility that concurrent interference could have occurred even with different muscle groups exercised in strength versus endurance training. Robineau et al. (2016) evaluated whether the duration (0, 6, or 24 hours) of recovery between strength and endurance training influences the response to concurrent training. They suggested that 0- and 6-hour recovery periods in concurrent training produces lower adaptation of upper-and lower-body strength and muscle hypertrophy compared to strength training only over a 7-week period. This evidence suggests that systemic interference might occur if different muscles are exercised in strength and endurance training.
In our previous study, several possible mechanisms were discussed concerning systemic interference with upper-body strength and muscle gain after concurrent upper-body resistance training and lower-body endurance exercise. One possible mechanism involves blood redistribution due to aerobic leg exercise (Kagaya et al., 1997). Previous studies suggested that permanently high levels of creatine (Cr) in trained muscle increase AMPactivated protein kinase (AMPK) activity (Ponticos et al., 1998). If the blood flow is concentrated in the exercising leg muscles due to blood redistribution, slow recovery of increased Cr after arm resistance training might activate AMPK in the targeted arm muscle. AMPK signaling is activated during medium (60% maximal oxygen consumption [[VO.sub.2max]]) and high (80% [VO.sub.2max]) intensity endurance training, but not during low (40% [VO.sub.2max]) intensity exercise (Chen et al., 2003). Since blood redistribution also occurs in long-lasting exercise, moderate intensity (55% [VO.sub.2max]), long duration (30 min) cycling subsequent to arm strength training might also interfere with arm muscle hypertrophy and strength gain.
The purpose of this study was to examine whether moderate intensity (55% [VO.sub.2max], 30 min) cycling exercise subsequent to upper-body strength training influences the training response of muscle hypertrophy and strength. We hypothesized that moderate intensity endurance exercise subsequent to strength training systemically interferes with muscle hypertrophy and strength.
Fourteen Japanese men (age: 22.0 [+ or -] 0.7 years, height: 1.72 [+ or -] 0.05 m, weight: 62.1 [+ or -] 5.8 kg, arm-curl 1RM: 22.3 [+ or -] 3.0 kg) volunteered to participate in this study. Subjects were randomly assigned to two groups. One group performed moderate intensity endurance training immediately after resistance training as concurrent training group (CT group, n = 7) and the second group performed moderate intensity endurance training and resistance training on separate days as control group (SEP group, n = 7) (Figure 1). Muscle CSA, 75% one repetition maximum (1RM), and [VO.sub.2max] were measured pre- and post-training. All participants were informed of the potential risks of the experiment and provided written consent to participate. The study was approved by the ethics committee of Nippon Sport Science University and was performed in accordance with the Declaration of Helsinki for Human Research.
A supervised progressive resistance training program was designed to induce muscular hypertrophy (week 1-2: 3 sets of 10 repetitions (reps), week 3-4: 4 sets of 10 reps, and week 5-8: 5 sets of 10 reps at 75% 1RM of bilateral arm-curl exercise with 2-min rest intervals). This program was performed using an arm-curl machine (Hammer strength plate-loaded seated biceps, Life fitness, Chicago, USA) for 8 weeks, with training carried out twice per week at least 24 hours apart (Figure 1). A warm-up set of 8-10 repetitions was performed at 50% of the individual's measured 1RM. Each session was completed to the set and repetition prescribed for that week, however, each final set was performed to failure. The training intensity was increased by 5% of the subject's baseline 1RM if they completed two...