The purpose of this study was to examine the effects of 6 weeks of low-volume CT on muscle power, muscular strength, and Wmax in healthy active young adult men. The main results showed that all components of PF analysed, muscle power, muscular strength and maximal aerobic capacity improved significantly through low-volume CT. The VL had a significant and progressive tendency to increase between weeks, and without differences on internal load.
The neuromuscular component that seems to be most attenuated by CT is the muscle power or rapid/explosive force, due to a phenomenon called "interference effect", which affects chronic adaptations in power [7, 27–29]. Despite the greater interference effect, due to the possible limitation of neural activation speed [29], the CT can significantly increase the muscle power [7, 27–29]. However, [4] did not report any harmful effect at the jump height between the group that performed the CT in relation to what did only RT. A study [30] that assessed the jump height by the squat jump test also found a significant improvement of 6.6% after 12 weeks of CT, nevertheless the AT and RT were performed on alternate days, a strategy used to minimize the effect of interference between aerobic and neuromuscular adaptations [6, 7]. This study also found significant improvements of 9.4% in jump height. These results suggests that for young active adults low-volume CT may be enough to increase vertical jumping capacity, which is related to the anaerobic potency of lower limbs [31]. This chronic adaptation can be explained by improving the voluntary neural activation speed, related to the motor neuron recruitment speed and maximal motor unit discharge rate [32].
For muscular strength, it is already well documented in the literature, that CT increases maximal strength in the upper limbs and the lower limbs [5–7], which agree with the results obtained in this study either for 1RM in the bench press or for 1RM in the squat. In this study, the RT was done before than HIIT which it is the order that had better results to increase 1RM [5, 6]. A study of Winett et al. [33] found that even low-volume CT was able to improve strength in various exercises in untrained subjects. Another article methodologically similar to our study, which prescribed a higher daily volume for both of RT and AT, they found improvements in strength in the half squat [34]. The low weekly volume presented in this study appears to be effective to create neuromuscular adaptations and this result is supported by other articles that through prescription of single set with low weekly volume of RT were sufficient to increase maximal strength [15, 35]. This improvement can be explained by adaptations, especially neural, in relation to structural adaptations, due to the duration of the intervention period [36]. Some of these adaptions could be related to the increase of motor unit firing frequency and synchronization, motor unit recruitment thresholds, motoneurons excitability and an improvement in antagonist muscle coactivation [36]. Another interesting detail was the magnitude of the improvements in the present study (20.59% for bench press and 17.25% for squat) compared to other CT studies, which was an average of 16.55% increase in bench press [28, 37], and 23.6% in squat [38–40]. A possible interpretation of these results is that for upper limbs strength the low-volume CT can be as effective to increase strength as higher volume CT, however for the lower limbs the higher volume seems to be more effective for active individuals and untrained in RT. Despite this, more studies are required to be able to successfully compare the effects of CT of different volumes on muscle strength.
The CRF seems to be the component of PF with more positive responses to CT, due to a small or inexistent interference effect between training bouts, independently of exercise order [5, 6]. The Wmax is the maximum capacity to generate energy through the aerobic metabolism [41]. Improvements in Wmax are supported by this information and the positive association between CRF and Wmax [42]. This could be explained, particularly for untrained subjects, due to an increase in myofibrillar and mitochondrial protein synthesis, as well as mitochondrial biogenesis after CT [43]. An investigation led by Lee et al. [44], found that after 9 weeks of CT training with a higher weekly volume, moderately active young adults increased 7.1% of Wmax. The increase of 8.8% in Wmax was also obtained by Fyfe et al. [28] as a result of an 8 weeks of CT with a higher volume training protocol performed by active subjects. Another study with a duration of 12 weeks of CT conducted by Shamim et al. [30] in young active adults, resulted in a 14% increase in Wmax. The results reported in these studies are relevant when compared with our outcomes, because we had an increase of 10% in Wmax with a lower weekly training volume than the above-mentioned studies. This suggests that a smaller training volume may be as effective as to increase aerobic capacity in young active adults.
When measuring CRF through VO2max, there is more evidence pointing in the same way. A work of Winett et al. [33] who performed a low-volume CT protocol achieved significant improvements in VO2max in untrained subjects during 12 weeks of training. When addressing CT with a higher volume there is a wide literature that indicated to significant improvements of VO2max [38, 45–47].
For the measures of training load, the progressive increase in weekly VL between weeks was supported due to the application of the principle of progressive overload applied in the training sessions, which was the 2-by-2 rule [26]. This increments in VL could be associated to strength improvements in untrained men [48], this relationship may explain the increase in muscle strength, due to the increase in VL in the 6th week. Despite this increase in VL, the internal load did not increase and still presented a decreasing trend, which was not the case in the study of [28], which had some fluctuations in the internal load. That could be explain by the study design because the change the intensity zone of RM between the weeks of intervention, as well as its multifactorial response [49]. The internal load is a psychophysiological indicator where the effort of the entire training session is measured in a relative way [23] and a higher internal training load can increase vulnerability for diseases or injuries [50]. This situation gives us information that there was less effort and less perceived physiological stress while training and therefore is expected a less possibility to have an injurie or disease.
This investigation has some limitations which should be considered before drawing conclusions. The group sample is small and that can affect the statistical power and by being associated with a group of students of the sports degree who already practiced physical activity before and during the experimental period. Nevertheless, there was a CG that had the same characteristics of the EG, but they did not perform the training. The training protocol duration was only 6 weeks, although it was already possible to see significant differences in PF. Food, alcohol intake or tobacco intake were not controlled, although subjects were asked to keep their usual routines. The training sessions were not always given on the same days or at the same time, but the researchers always tried to ensure an interval of 48h between workouts to ensure a good recovery. Finally, all the results obtained can only be considered for the research population and should not draw conclusions for populations with other characteristics.
In future investigations it will be necessary to use a more representative sample and with another type of population so that we can draw more conclusions. Perhaps, in individuals with training experience to check if even with lower volumes of training can be achieved improvements. It may be interesting to use morphological evaluations to examine the musculoskeletal hypertrophic response to low-volume CT. Could be interesting to create an extra group that performs even less volume to draw more conclusions about the minimal dose of exercise. Longer investigations may lead to more significant results. However, this remains hypothetical and requires further investigation to elucidate these topics.