This study aimed to compare the capacity to produce force rapidly with trunk muscles, measured as RTD, between women with experience in three different strength training modalities as well as physically inactive women. The obtained results showed that women with experience in CrossFit training have significantly higher isometric trunk extension and flexion IPT and RTD compared to women with no experience with exercise training. Trunk flexion IPT was also significantly greater for participants who practiced CrossFit compared to those who practiced traditional resistance training. Curiously, trunk extension and flexion RTD were not different between participants with experience in Pilates, traditional resistance training, and no experience with exercise training. Furthermore, CF group presented significantly higher RTD values at all analyzed time intervals (30, 50, 100, 150, 200 and 250 ms) when compared with the other groups, confirming our initial hypothesis. These data confirm that women who are engaged in CrossFit training benefit from greater capacity to produce rapid strength with the trunk flexors and extensors, even when compared to women who practice other strength training modalities.
Robust evidence supports the strengthening trunk muscles[1, 34], while performance in such tests is frequently reported to be associated with quality of life and control of lower back pain[35]. However, the role of rapid force development of trunk muscles and its association with exercise performance and quality of life are still unclear.
The data obtained in the present study suggests that maximal trunk extension and flexion strength, assessed as maximal isometric peak torque, is not different between untrained individuals and those who practice traditional resistance training and Pilates. Moreover, participants in the CrossFit group exhibited greater maximal trunk flexion strength compared to those in the traditional resistance training group. It would be expected that individuals who engage in any type of strength training regimens present better trunk strength than those who are sedentary, which was not the case in the present study. The similarities between trunk extension and flexion IPT between the ST/PL and the CON groups could be due to specific set configurations within these modalities. It has been widely accepted that heavy loads are required to improve maximal strength[36], and many CrossFit exercises apply such specific stress to the trunk extensors and flexors while it may not be the case for Pilates and traditional resistance training exercises.
It is well accepted that RTD is positively influenced by resistance training. Recent evidence indicates an important influence of movement velocity on isometric RTD adaptations, with fast exercise movements being more advantageous than slow movements[37]. Besides actual velocity of the movement, the intention to produce force rapidly during exercises is also considered a key feature of successful training interventions designed to improve RTD[37]. Among the exercise modalities investigated in this study, CrossFit is the one that involves a greater amount of explosive whole-body movements which clearly depend on a rapid recruitment of trunk muscles for force transfer and spine stabilization[38]. Opposingly, there is a tendency for adopting slow muscle actions during traditional resistance training to supposedly provide greater activation of targeted muscles[39], although existing evidence indicate that slow muscle contractions may not be more effective than fast muscle contractions in promoting positive neuromuscular adaptations[40]. Some authors suggest that the lack of increase (or reduction) in RFD after traditional strength training might result from the slow movement speeds during this training modality[19]. Based on the results obtained in the present study, this appears to also be the case for Pilates.
The benefits of Pilates for the core complex are well documented, especially when variables such as strength and flexibility are observed[26]. During Pilates training sessions the respiratory pattern must be continuously controlled to provide satisfactory trunk stabilization[41]. Consequently, Pilates movements are widely known to be performed with slow and controlled contraction velocities. This appears to explain the lack of differences in trunk muscle RTD observed between the PL and CON groups in the present study.
The choice of exercises focused on the core complex in each training modality may have an important influence in trunk muscles requirement. In traditional resistance training programs, core complex muscles are frequently strengthened through isolated exercises focusing on developing tension in target trunk muscles (e.g., rectus abdominis and obliques). During the execution of whole-body exercises, core complex muscles act as a kinetic link between the upper and lower extremities, allowing the transfer of torque and angular momentum between these structures[42]. A combination of isolated and whole-body exercises during traditional strength training sessions results in an accumulation of stimuli to core complex that culminate in the strengthening of this complex. However, the velocity of the contractions performed during traditional strength training does not appear to improve rapid force production of the trunk flexors and extensors as evidenced by the lack of significant differences in the RTD of these muscles between resistance trained and untrained women in this study. The similar RTD values observed for the RT, PL and CON groups in the present study are supported by the exercise training principle of specificity[36], which proposes that the adaptations to exercise training are specific to a plethora of variables inherent to the training protocol, with movement velocity being one of them.
Since it has been initially studied, in the 1970s [43], RTD has been considered an important functional marker due to its association with daily activities[44, 45]. The ability to produce a rapid increase in contractile strength during the initial phase of a voluntary contraction (0-300 ms) is vital not only for athletes, but also for older adults who need to neutralize sudden disturbances in postural balance [46, 10]. RTD also appears to improve functional capacity and reduce pain[47, 12]. However, few studies investigated RTD in trunk muscles. Rossi[48], showed that asymptomatic individuals have greater capacity to generate strength, and extend the trunk more quickly, when compared to subjects with chronic low back pain.
Although none of our participants reported lower back pain, when comparing our data with those of Rossi[48], only the CF group showed greater trunk extension RTD compared to untrained individuals and resistance- and Pilates-trained individuals, while trunk extension RTD values for these groups were similar to those presented by people with lower back pain[48]. This suggests that the capacity to produce strength rapidly does not appear to be associated with the incidence of lower back pain, as none of the individuals recruited for the present study reported lower back pain and trunk muscle RTD was different between groups. Furthermore, if health and quality of life are to be considered, it is important to notice that increased trunk RTD can result in quicker postural adjustments during daily activities, which could prevent falls and unexpected incidents.