The present study was designed to compare the effects of the 6-week supervised whole-body TRT and FRT protocols with equal volume on the muscular fitness and physical performance in untrained healthy men. The main finding of this study was that both whole-body resistance training modalities (traditional and functional resistance training) produced similar training effects in untrained healthy young men over a 6-week intervention period. No significant differences were detected with the training-induced improvements between training protocols in pre to posttest maximal strength, repetitions of bench press and leg flexion, isometric force production, MBT distance, CMJ height, 30m sprint time, pull-ups, body fat and lean body mass. Regarding internal training load, no significant in MI, TS, WITL, WMITL between training groups were observed over the 6-week training protocols. In a study, Sparkes and Behm reported that unstable resistance training had a tendency for a smaller instability-induced force deficit compared to the force produced with the stable training. However, there was no difference between TRT and FRT groups when we assessing muscular fitness, so it was possible that unstable resistance training may be more effective in force production as well during a brief training period.
As mentioned previously, the most notable difference between the resistance training program used in the present study and other studies is the strictly controlled training volume of the two groups during the training process in the present study. Despite differences in the training intensity and condition, the increased muscular strength and lean body mass exhibited no difference between both groups in our study. Several studies have employed fewer exercises[23, 24], more sets, or lower training frequencies, all of which are not favorable for ideal muscular strength and lean body mass gains. Probably, skeletal muscle adaptations (strength and muscle mass) were determined in response to equal-volume resistance training with divergent training strategies. Kubo et al. reported that the increase in muscle volume was similar among the three training protocols, namely 4RM, 8RM, and 12RM, under equal training volume. Similarly, Colquhoun et al. found that three resistance training sessions per week provided similar increase in muscle strength and fat-free mass compared with six sessions per week under equal volume condition. Hence, we assume that the muscle adaptation status could be same in response to TRT and FRT protocols under an equal volume.
Two modalities of resistance training, which differed in terms of surface condition and intensity, were considered in the present study. The 1RM strength parameters of the upper and lower limb and muscle mass observed in our study are consistent with those indicated in similar studies[29–31]. Kibele and Behm reported that the traditional resistance training characteristics were to perform higher overload weights than in functional resistance training, which could also obtain similar muscle strength responses with the use of lower resistive load under unstable condition. The comparison of data between the two groups exhibited that despite forces were applied without overload to the upper and lower muscles in the FRT group when using an instability device for training, strength enhancements were probably related to the increase in trunk and lower muscle activation, sympathetic transmission, and recruitment of motor neurons, which may endorse intramuscular and intermuscular coordination and cooperation and make the agonist muscle activation more economic, thereby enhancing the strength performances. Furthermore, the greatest strength enhancements were observed in the lower limbs (e.g., Barbel Squat, 29.8% and 31.6% increase for the TRT and FRT groups, respectively) because the selected motor patterns in both groups were mainly standing and lower limbs such as the Bulgarian split squats. Peter reported that the center of gravity tends to swing as the body moves along a vertical axis, increasing the degree of lower limb instability, which could be conducive to trunk and lower limb muscle activation and intramuscular and intermuscular coordination.
However, there were limited literatures obtainable comparing the effects of TRT and FRT on muscular endurance, which highlights the significance of this study. Interestingly, improvements on repetition of bench press and leg flexion, TRT and FRT protocols were similar between groups. In relation to volume-load, improvements were also detected in the two groups after 6 weeks training. Although resistance training consisting of high-intensity compared to lower-intensity seem to elicit greater metabolic stress, according to the present study the specific stimuli provided by traditional protocol does not to appear to translate into enhanced muscular endurance. The evidence suggests that high repetitions (≥20RM) in lighter loads is efficient for enhancing muscular endurance under equal training volume. For example, Campos reported that no difference was observed between low, moderate and high repetition groups under approximately equal volume despite excellent muscular endurance for the high repetition group, and our results also test and verify this point. Therefore, we speculate that traditional high-intensity/instability low-intensity induces similar capillarization and mitochondrial adaptation, and the potential mechanism for enhanced muscular endurance following instability resistance training could also be the result of better tolerance of unstable condition.
Our study is the first to investigate the effect of whole-body FRT on the CMJ in untrained young men and compare the effects of 6 weeks TRT and FRT and indicates that both were equally beneficial in promoting the jumping height. So far, most studies have found that traditional resistance training improves jumping ability[4, 6, 36]. However, fewer studies focused on the effects of whole-body functional resistance training on vertical jumping ability. Two studies on athletes demonstrated that vertical CMJ was increased after long-term functional resistance training[6, 37], whereas these two studies do not seem to prove the idea that functional resistance training is of great advantage for improve explosive power, and it is important that a study from non-athletes has verified and supported this objective. Additionally, two studies results showed that FRT protocol did not improve jumping abilities[4, 15]. This finding is inconsistent with our study. We found that two main reasons are responsible for why the FRT protocol did not improve participants jumping abilities. Firstly, their FRT protocol mainly performed upper limb exercise, which is the biggest difference from our whole-body exercise. Secondly, their participants were previously trained men that may not be affected by the same degree of stimulation as the untrained young men. In view of the reasons mentioned above, the improvement of jumping abilities induced by our FRT protocol may be primarily related to neuromuscular coordination and adaptation. And meanwhile, previous studies indicated that strength training can promote jumping performance, approximately by 5-15%[39, 40]. Thus, our TRT protocol seem to increase the force produced by joint, which lead to an improvement of jumping ability.
Several dominant factors determining the explosive strength performance, such as force produced by joints, force development rate/muscle power produced by muscles and neural coordination of movement. Considering that our FRT protocol belonged to whole-body exercises, it seems that, accord with the development of throwing abilities, the FRT group obtained enough training stimulation for this variable, which made their throwing ability significantly improved. We deduced that the improvement of throwing is mainly relevant to the neuromuscular coordination. Especially, when training using instable device, more emphasis is placed on trunk region control and muscular coordination. Seeing that multiple joints participate in action during MBT test, including eccentric-concentric contractions of the shoulders and trunk regions mostly. Therefore, significant improvement on throwing ability is logical by our FRT protocols.
In regard to the physical performance tests, we observer a significant improvement in 30-m sprint and pull-ups from baseline, and no difference was noted between the groups; therefor, the TRT and FRT protocols improved the performance of 30-m sprint and pull-ups. Previous studies showed that the functional resistance training yielded significant positive results on straight line sprint ability[6, 37]. The inconsistent study results probably caused by different training protocols. Especially, the effects on explosive performance may be result of improvements in functional status. Campa et al. indicated that functional movement patterns could improve the synergistic interaction between motor control and core stability compared with other resistance training methods. This finding may be attributed to the advantage of neuromuscular adaptation on eliciting better transfer of the strength growth to physical performance.
Regarding the body composition, our results indicated significant changes in body fat percentage and lean body mass between the groups, although the difference between the groups was nonsignificant in terms of these parameters. This result is consistent with that of a previous study. Several studies have demonstrated the efficiency of resistance training in neuromuscular and metabolic stimulation to endorse tissue structure changes such as reduced adipose and muscle tissues[43, 44]. Another interesting result of the present study is that the body weight and BMI decreased significantly in the TRT group but not in the FRT group, which does not seem logical. Previous studies on the metabolic response to FRT found an average caloric expenditure of approximately 10.1 kcal for one-minute functional resistance training, which is higher than the expenditure of 5−9 kcal/min reported in studies examining traditional resistance exercise[45, 46]. Probably, the reason should be that the TRT group in the present study performed a high-intensity workout (at least 4–5 sets of 12 repetitions per training) until exhaustion and produced more energy consumption. Thus, we assume that the caloric expenditure of the TRT protocol is somewhat higher than that of the FRT protocol.
Finally, in term of internal training load, no difference in MI, TS, WITL and WMITL in the TRT group were observed over the 6 weeks training. On the other hand, a significant increased in MI and TS in the FRT group in week four. The American College of Sports Medicine suggested that periodic monitoring the training load to mitigate the adverse adaptation, because excessive training loads associated with low recovery is one of the main manifestations of overtraining. Also, the adjustments and willingness of workload in accordance with the training status of participants is indicated an effective strategy for improving physiology adaptation and promoting increased performance. The result of present study showed that the WITL of FTT group and TRT group are 1289.3 and 1220.7, respectively, which is slightly below the results presented by previous researcher[47, 48] who used RPE to quantify internal training load in players. Gabbett demonstrated that the weekly internal training load of 3000-5000 increased more chance of injury by 50%—80%; however, less than 1250 per week also revealed the potential injury risk and did not improve physical fitness. Therefore, using PRE during training period can help trainer adjust the training load when necessary. At the same time, in order to avoid high MI and TS, respecting the personality of participants, which is also an effective method to limit injury, since abrupt raise stemming from MI and TS were related to the occurrence of disease. Ferrari et al.. reported significant associations between upper respiratory symptoms and TS throughout a competitive season in well-trained cyclists. However, the MI in two groups was higher than 2.0 (MI>2.0, means higher injury risk) in this study, no injury cases were observed, indicating that the use of PRE scale to monitor training load can reduce the adverse impact on participants
Some limitations in this study should be noted. First, this study involved a limited number of performance variables, it would be necessary to include additional motor tests, such as static and dynamic balance, and agility tests in future research, since it is possible that the FRT protocol produced even more changes than TRT protocols in certain variables we observed in this study. Therefore, it would be interesting to compare the effects on specific motor-performance variables induced with FTR and TRT protocols. Next, the study participants were limited to untrained young men; thus, the outcomes could not be generalized to women or experienced individuals such as athletes. Moreover, the intervention duration was relatively short (only 6 weeks). Therefore, it is hardly to cause significant difference in muscular fitness and physical performance between the two groups. Future studies with a larger sample size and different participant types, and longer study periods are required to determine the excellent resistance training pattern for health promotion.