Among the desired outcomes of regular exercise training, strength and endurance are the most prominent physical abilities considered (Reilly et al., 2009). Strength training improves skeletal muscle contractile capacity (Costill et al., 1979), whereas aerobic training improves oxygen delivery to muscle and oxygen extraction from the blood (Holloszy and Coyle, 1984). Thus, both strength and aerobic training programs are commonly employed to improve cardiovascular fitness and force production (Garber et al., 2011).
The combination of aerobic exercise and strength training, known as concurrent training (CT), has received particular focus in the scientific literature due to the potential for antagonistic adaptations (Bell et al., 2000; Hakkinen et al., 2003; Kraemer et al., 1995). Some investigations have demonstrated that maximum strength was reduced after a period of concurrent training when compared with isolated strength training (Bell et al., 2000; Hickson, 1980; Kraemer et al., 1995; Fyfe et al., 2016), while others have failed to replicate this type of interference effect (Eklund et al., 2015; McCarthy et al., 2002; Gentil et al., 2017). Thus, this topic remains the aim of recent investigations (Fyfe et al., 2016; Gentil et al., 2017).
The causes of impairments in long-term strength gains (interference effect) are not well-established; however, acute training adaptations may be partially responsible (Leveritt et al., 1999; Craig et al., 1991). The acute hypothesis suggests that acute decrements in force production during resistance training when preceded by aerobic activity are potentially due to insufficient recovery between training sessions and residual multifactorial fatigue (i.e., pH reduction and decrease of Ca+ sensitivity) (Fitts, 2016). The acute force reductions and subsequent impairments in total work observed during CT (Sale et al., 1990) may partially explain the long-term impairment of strength development which may be dependent on the volume of work performed (Rhea et al., 2003).
In fact, many studies have shown performance decrements in strength tasks (maximum number of repetitions) when aerobic exercise is performed prior to strength exercise (Inoue et al., 2016; Panissa et al., 2015; 2016). The intensity of exercise is an important variable to consider in relation to the interference effect (Docherty and Sporer, 2000) with larger decrements in strength-endurance when aerobic exercise is performed at higher intensities (de Souza et al., 2007). This topic has increased relevance due to the recent popularity of interval training as an efficient strategy to increase aerobic fitness (Milanovic et al., 2015) and decrease fat mass (Panissa et al., 2016).
A recent meta-analysis (Murlasits et al., 2018) supports acute volume maintenance as a strategy to minimize interference effects since strength training followed by aerobic exercise results in superior gains in maximum strength (3.96 kg) compared to the reverse order. This finding comes with the assumption that volume is decreased when aerobic exercise precedes strength exercise. However, few studies have tested if the CT-related decrements in acute strength training volume interfere with long-term adaptations while controlling for volume-load (Craig et al., 1991; Sale et al., 1990).
Studies from Eklund et al. (2015) and Schumann et al. (2014) comparing opposite orders of execution showed no difference in maximum strength or hypertrophy after 12 and 24 months of training; however, these studies utilized moderate intensity aerobic training, which favors the maintenance of training volume (de Souza et al., 2007). More recently, Fyfe et al. (2016) showed an attenuation of maximal strength, independent of aerobic intensity, after 8-weeks of training sessions consisting of aerobic exercise (high or moderate intensities) following by strength exercise and reported no effect of acute impairment (although the strength training volume was not reported). Furthermore, a classic study from Hickson (1980) utilized a high-intensity exercise protocol for 10 weeks and a CT-related interference (reduction in strength gains) occurred only after the eighth week of training, indicating that long-term interventions must be considered.
Thus, the aim of present study was compare the effect of high-intensity intermittent exercise performed before strength training (CT group) with strength training alone (ST group) on maximum strength after 8 and 12 weeks. A secondary aim was to evaluate the relationship between acute strength training volume-load and long-term strength gains. We hypothesized that CT would present inferior strength gains compared to ST after 8 and 12 weeks, which would be related to lower strength volume-load in the CT group.
This study was carried out at Sao Paulo State University (UNESP), Presidente Prudente, SP, Brazil and performed according to the guidelines of the Declaration of Helsinki. The project was approved by the Ethics Research Group of the Sao Paulo State University (Protocol number: 22793414.7.0000.5402).
This was an experimental longitudinal study that compared the strength gains and maximal aerobic speed to typical training sessions in subjects assigned to either a concurrent training (CT) group or a strength training only (ST) group. Anthropometric testing, maximal aerobic speed, maximal strength, and isolated acute volume evaluations were performed at baseline and at week 8 and week 12 (Figure 1). An additional aerobic evaluation was conducted following four weeks of training in the CT group to allow for intensity adjustments in the high-intensity intermittent training (HIIT) protocol.
Inclusion criteria for participation in the study were: 1) participating in systematic strength training during the previous 6 months (Whaley et al., 2006); 2) age between 18 to 35 years; and 3) considered physically active through aerobic conditioning (minimum twice a week). Exclusion criteria were: 1) contraindications involving the cardiovascular system, muscles, joints, bones of the lower limbs or any musculoskeletal disorders that would limit the participation in strength training; and 2) use of nutritional supplements within the past 6 months (e.g., protein, amino acids, and creatine), prior anabolic steroid use, or use of any other illegal agents known to increase performance for the previous year. All subjects were asked to maintain their usual nutritional habits and to only engage in exercise as proposed by the study protocol.
Out of a total of 104 men who participated in the first screening, only 22 met all the inclusion/exclusion criteria and agreed to participate in the study protocol. Participants were randomized into two study groups: CT (n = 12) and ST (n = 10), using simple randomization techniques for allocation, which ensured that trial participants had an equal chance of being allocated to a given treatment group (Egbewale, 2014). During the 12 weeks of training, three men dropped out of the study (a dropout rate of 13.6%) and were excluded from the final analyses. The reasons for dropouts were: incompatible schedules (n = 1 from ST group) and declined participation with unspecified reasons (n = 2 from CT group).
Height was measured using a fixed stadiometer (Sanny brand, Sao Paulo, Brazil). The participants were barefoot and wore light clothing while standing at the base of the stadiometer, touching their shoulder blades, buttocks and heels to the equipments vertical support. Body mass was measured using an electronic scale (Filizola PL 50, Filizzola Ltda., Brazil), with a precision of 0.1 kg.
Maximal aerobic speed test
For determination of maximal aerobic speed, the subjects performed a maximal incremental test on a treadmill (Inbramed-ATL) until voluntary exhaustion. Each stage was composed of 2-min, with the first being performed at a speed of 8 km x [h.sup.-1] followed by 1 km x [h.sup.-1] increases at the end of each stage. In addition, heart rate was registered using a heart rate monitor (Polar Electro, model S810i or RS800, Finland). The maximal speed reached in the test was defined as maximal aerobic speed (MAS). When the subject was not able to finish the 2-min stage, the speed was expressed according to the accumulated time in the final stage, determined as follows: MAS = speed of penultimate stage + [(time, in seconds, remained in the final stage multiplied by 1 km x [h.sup.-1])/120s]. This test was conducted in an isolated session at baseline, week 8, and week 12 for both groups, and following four weeks in the CT group only, to adjust the speed of the HIIT sessions. All participants arrived at the laboratory early in the morning and the time of day and environmental conditions (temperature: 22 [+ or -] 2[degrees]C) were consistent between testing sessions.
Strength test procedures
One week prior to testing, the participants attended three familiarization sessions (Monday, Wednesday and Friday) in which they performed four sets of 12-15 repetitions of each exercise, to become accustomed to the equipment and testing protocols performed throughout the study (Ritti-Dias et al., 2011). During the subsequent week, approximately 72 hours after the aerobic test, the subjects performed a maximum dynamic strength test consisting of a one repetition maximum (1RM) half-squat using a Smith machine (Ipiranga[R], Sao Paulo/Brazil). The participants performed a five-minute general warm-up on a treadmill at 50% MAS followed by a specific warm-up consisting of 1 set of eight repetitions at 50% 1RM, and 1 set of three repetitions at 80% of 1RM on a Smith machine with 2 min rest between sets. After 3 min rest...