Given the importance of core strength in athletic performance, its assessment should be considered an integral part of functional diagnostics. Above all, such testing should differentiate between athletes with different demands on the power and endurance of their trunk muscles and provide relevant information on the efficiency of sport-specific training (e.g., in rowing, canoeing, wrestling, judo, karate, golf).
Traditionally, these tests include isometric measures of endurance and isokinetic measures of strength and work (3,5,15,19). Furthermore, there are isoinertial tests such as those of trunk flexor endurance recommended by the American College of Sports Medicine and the National Strength and Conditioning Association. Most current field tests evaluate the endurance (e.g., trunk flexor and extensor endurance tests and lateral bridge test) rather than the strength and power component of trunk muscles.
Trunk rotation endurance is supposed be more important than strength alone in the prevention and treatment of low back pain. This has been documented by a study of Lindsay and Horton (14) who found significantly less endurance in the nondominant direction (the follow-through of the golf swing) in golfers with low back pain than in a healthy group. McGill et al. (18) reported that poor trunk muscular endurance, and aberrant flexor/extensor endurance ratios, correlates with a history of low back injury. Spinal extensor endurance has also been shown to correlate with decreases in injury risk for the low back (23).
However, the strength and power component of trunk muscles may better mimic the demands imposed by sports. Usually, single repetitions of a particular exercise with increasing weights stepwise up to the 1 repetition maximum (1RM) are performed to obtain individual force-velocity and power-velocity curves or to analyze power- and velocity-weight lifted relationship. It is known that with increasing weights there is a decrease in velocity in the concentric phase of lifting. Contrary to this, power increases from lower weights, reaches a peak, and then toward higher weights, decreases again. Such an optimal velocity, that is, the one allowing the production of the greatest power, depends on ratio of fast and slow twitch muscle fibers (27); thus, it may be hardly changed with training. However, the optimal weight at which maximal power is achieved increases significantly after the training. Moreover, the maximal power obtained from the testing procedure consisting of maximal effort single repetitions with increasing weights is less complicated and hence a more practical alternative for the assessment of strength capabilities than the traditional 1RM approach.
Alternatively, this weight may serve as the one used for endurance power tests. Monitoring power in a set of a determined number of repetitions performed with maximal effort in the concentric phase enables the quantification of fatigue in a way similar to other endurance tests. Peak power and mean power from entire set and the fatigue index (calculated as the ratio of the decrease in power from the peak to the lowest values and the peak power) are the most used parameters of strength endurance.
Yet, there are only few studies that have evaluated muscular strength and endurance during trunk flexion and extension motions (10,16,21,25,29). Typically, isokinetic machines (7,8,20) or electromyography (9,17,22) are used to measure strength characteristics during axial rotation movements. However, when using an isokinetic dynamometer with a torso rotation attachment, no significant differences in peak torque were found within or between groups of healthy individuals who do not play golf and those who are highly skilled at the sport (14). The authors also reported no significant difference in the endurance of trunk muscles between the healthy elite golfers and the non-golfing controls. Similarly, Suter and Lindsay (26) were unable to show any significant differences in the static holding times or a decline in the electromyography median frequency between low-handicap golfers with low back pain and healthy, age-matched controls who did not golf.
The limitation of these measurements is that torso rotation performed while sitting on the chair with straps around the back and legs provides artificial movement patterns. Additionally, most of the custom-made equipments are relatively expensive and not portable for use on the sporting field. To avoid these drawbacks, one can use a system that allows monitoring of basic biomechanical parameters during rotational movement of the trunk. So far, the study of Andre et al. (1) determined the test-retest reliability of the kinetic rotational characteristics of the pulley trainer when performing a rotational exercise of the axial skeleton in the transverse plane while sitting on a box. The authors found that a pulley system and an external dynamometer can be used together as a reliable research tool to assess rotational power. Although such a test is suitable for canoeing, for example, for many other sports, such as hockey or tennis, rotational movement performed during standing would be a more specific alternative. As athletes prefer free weights or weight exercise machines to improve the strength of their trunk muscles, the testing should be as close as possible to the movement used during training or competition. Presumably, the test adapted from the wood chop exercise may provide conditions similar to those imposed in many sports involving trunk rotation (baseball, golf, karate, etc.). However, the methodological issues associated with such testing have not been investigated yet. Such rotational movement allowing more involvement of the lower body may be less confined to the trunk, which in turn should increase the movement variability and influence the reliability.
Therefore, 3 specific research questions were addressed in the present study: (a) Is the reliability of data obtained from the standing cable wood chop exercise acceptable for its use in the evaluation of rotational power? (b) Are there individual differences in mean power during the standing cable wood chop exercise? If so, at which weight are these differences most pronounced? (c) Is endurance power test sensitive for distinguishing among healthy fit individuals with different levels of maximal power production? We hypothesized that the power produced during the standing cable wood chop exercise would be a reliable parameter able also to discriminate within-group differences. Verification of this hypothesis was accomplished by the estimation of the repeatability and sensitivity of a novel method for assessing the maximal power during the standing cable wood chop exercise with different weights and the endurance of the core muscles.
Experimental Approach to the Problem
This study adopted a fully controlled research design with repeated measurements on 2 separate occasions 1 week apart. Our first interest was to estimate the repeatability of a novel method for assessing the mean power during a standing cable wood chop exercise. Then, we conducted experiments with aims to determine the maximal value of mean power using maximal effort single repetitions of the standing cable wood chop exercise with increasing weights and to evaluate the endurance of the core muscles during a set of a determined number of repetitions performed at a previously established weight at which maximal power was achieved. To do this, young fit men performed maximal effort single repetitions of the standing cable wood chop exercise with weights increasing stepwise up to 1RM, and a set of 20 repetitions at a previously established weight at which maximal power was achieved. A FiTRO Dyne Premium system based on precise analogue velocity sensor with sampling rate of 100 Hz was used to monitor basic biomechanical parameters involved in the exercise. During both exercises, mean values of power were analyzed.
A group of 23 fit men (age 22.4 ± 1.9 years, height 184.0 ± 9.3 cm, and weight 79.6 ± 12.2 kg) volunteered to participate in the study. All of them had at last 4 years of experience with resistance training, including exercises to strengthen the trunk muscles. They provided information on their physical activity, health status, and history of neurological and musculoskeletal disorders or injuries. No subject had experienced low back pain for less than 12 months before the study. Individuals who had previously undergone surgery or other medically invasive procedures for low back pain were excluded from participation in the study. They were asked to avoid any strenuous exercises during the study. All of them were informed of the procedures and the main purpose of the study and gave their written informed consent. The procedures presented were in accordance with the ethical standards on human experimentation stated in compliance with the Declaration of Helsinki and approved by the head of the Institute where testing was carried out.
Subjects were exposed to a familiarization session during which the technique of the standing cable wood chop exercise was explained and trial sets were carried out. They performed slow practice repetitions of trunk rotation to become accustomed with the desired movement. Afterward they underwent trunk rotation strength and endurance testing. Both tests were carried out mid-morning. The testing procedure and time of day were identical for all subjects. The same experienced researchers conducted the measurements during testing sessions.
After the warm-up, subjects performed 2 repetitions (1 repetition at a higher weight) of the standing cable wood chop exercise with a stepwise increase in the weight up to 1RM. Their task was to rotate their torso forcefully until they reached a 90° rotation and then to slowly return to the starting position facing the weight stack machine. The test was then repeated for the opposite side of the body. The right torso rotation for a right-handed participant was categorized as the “dominant” rotation, whereas the left rotation was referred as the, “nondominant” rotation and vice versa for a left-handed participant. To estimate the repeatability of this strength test for trunk muscles, the test was undergone again 7 days after the initial testing session.
On the second test day, they performed the standing cable wood chop exercise at the previously established weight at which maximal power was achieved until a determined number of repetitions were completed. Subjects were instructed to perform repetitions with maximal effort in the concentric phase. Emphasis was placed on the proper position of the body during the exercise. They stood with their feet wider than shoulder width apart and toes slightly pointed outward while grasping the handle with both hands. They rotated the body from the right (or the left) toward the opposite side until the hands reached the end position in front of the body. They were asked to keep the elbows close to the body. They had to engage their abdominal/core muscles to stiffen the torso and stabilize the spine. Particularly in the endurance test, they had to kept their mid-section tense through the whole set. A laboratory assistant made sure that subjects remained upright throughout the movement and that their head, chest, and torso were aligned over their hips.
Basic biomechanical parameters during both tests were monitored by means of the FiTRO Dyne Premium (FiTRONiC, Slovakia). For this system, Jennings et al. (4) reported intraclass correlation coefficients (ICCs) of 0.97 (95% confidence interval [CI], 0.95–0.98) for maximal power during squat jumps and 0.97 (95% CI, 0.95–0.98) for biceps curls with the limits of agreement of −17 ± 96 W and 0.11 ± 13.90 W, respectively. A study by Zemková et al. (31) showed ICC and SEM% values in the range 0.97–0.98 and 7.6–7.7%, respectively, for mean power in the entire concentric phase of lifting, 0.96–0.98 and 9.1–9.6%, respectively, for mean power in the acceleration phase, and 0.94–0.97 and 9.2–10.0%, respectively, for peak power during bench presses with weights of 40, 60, and 80% 1RM. In particular, for rotational power, the intraclass correlation coefficients were 0.97 (at 9% of body weight), 0.94 (at 12% of body weight), and 0.95 (at 15% of body weight). When participants were separated by gender, similar ICC values were found (1). Thus, rotational exercises performed while seated on a box and holding the handle with both arms extended in front of the body can be considered to be a reliable research tool for assessing rotational power.
The system consists of a sensor unit based on a precise encoder mechanically coupled with a reel. While pulling the tether (connected by means of small hook to a barbell axis) out, the reel rotates and measures velocity. The rewinding of the reel is guaranteed by a string producing force of about 2 N. Signals from the sensor unit are conveyed to the PC by means of a USB cable. The system operates on Newton's law of universal gravitation (force equals mass multiplied by the gravitational constant) and Newton's law of motion (force equals mass multiplied by acceleration). Instant force while moving a barbell of a given mass in the vertical direction is calculated as a sum of the gravitational force (mass multiplied by gravitational constant) and the acceleration force (mass multiplied by acceleration). The acceleration of vertical movements (positive or negative) is obtained by the derivation of vertical velocity, measured by a highly precise device, mechanically coupled with the weight stack machine. Power is calculated as a product of the force and the velocity, and the actual position by the integral over the velocity. In both tests, mean values of power were analyzed. The device was placed on the floor and attached to the weights of the exercise machine by a nylon tether. Participants performed the wood chop exercise while pulling on a nylon tether of the device (Figure 1).
Data analyses were performed using the statistical program SPSS for Windows version 18.0 (SPSS, Inc., Chicago, IL, USA). Ordinary statistical methods including average and SD were used. Because the reliability of power during various resistance exercises, including a trunk rotational exercise, was provided, data analysis was limited to the estimation of the repeatability of power measurements during the standing cable wood chop exercise with different weights. Repeated measures of the analysis of variance (day × load) with Tukey's post hoc analyses were used to determine the differences in power output. The alpha level for significance was set a priori at p ≤ 0.05. The ICC values were calculated using the model 2,1 with 95% confidence intervals. A value above 0.80 was considered acceptable. In the endurance test, a paired t-test was used to determine the statistical significance of differences in power during initial and final repetitions of the standing cable wood chop exercise with different weights; p ≤ 0.05 was considered significant.
As no significant differences in peak torque in strength testing or in total work in rotational endurance testing between dominant and nondominant rotations were found in healthy golfers, control group, and golfers with low back pain (14), we assumed no side-to-side differences in power output in healthy, fit adults. Nevertheless, a paired t-test was used to determine the statistical significance of the differences between power output during the standing cable wood chop exercise on the left and the right sides. The criterion level for significance was set at p ≤ 0.05. Lindsay and Horton (14) also showed a moderate correlation between body weight and peak torque that supports the findings of Newton et al. (20), suggesting that rotational strength and endurance data can be presented in absolute terms (not normalized) when making between-subject comparisons.
As expected, there were no significant differences in the mean power between the dominant and nondominant sides of rotation with all weights used. Therefore, average values of better trial on each side were used for analysis. Analysis of variance showed no significant differences in mean power during the standing cable wood chop exercise with all weights used between the 2 testing sessions. The ICC values were above 0.90 (Table 1), which is comparable with those obtained for rotational power during the exercise where participants were seated on a box and held the handle with both arms extended in front of their body (1).
Further analysis showed that mean power increased from lower weights, reached a peak, and then, toward higher weights, decreased again (Figure 2). However, there were substantial individual differences not only in mean power, especially at higher weights, but also in its maximal values. For most participants, the maximal values of mean power during the standing cable wood chop exercise were achieved at about 75% of 1RM (462.2 ± 57.4 W, n = 11), whereas for others, it was at 67% of 1RM (327.2 ± 49.7 W, n = 7) or at 83% of 1RM (524.0 ± 63.2 W, n = 5).
The trunk endurance test was performed for a determined number of repetitions instead of performing repetitions until exhaustion. This is because fatigue during the final repetitions of the standing cable wood chop exercise impaired the ability to produce power more profoundly at lower than at higher weights. It is probably due to a greater number of repetitions with lower than higher weights (Figure 3). This resulted in conflicting effects of fatigue occurring at the end of the repeated wood chop exercise with different weights and different numbers of repetitions. As setting the appropriate weight and number of repetitions is one of the critical issues of trunk endurance testing, using a predetermined set of 20 repetitions for this purpose has been shown to be a better approach.
The rationale for this procedure may be corroborated by significant differences between the initial and the final repetitions of the standing cable wood chop exercise with higher weights (≥40 kg), whereas with lower weights (<40 kg), subjects were able to produce similar power throughout the set of 20 repetitions (Figure 4). Moreover, there were no significant within-group differences when using lower weights, indicating that this method is not sensitive enough to discriminate between individuals with different levels of trunk muscle endurance. Contrary to this, when a weight of ≥50 kg was used, most subjects were not able to complete the entire set of 20 repetitions. The weight at which maximal power was achieved should be thereafter used for the trunk endurance test (i.e., 40, 45, and 50 kg). When subjects performed the test with these weights (e.g., 40 kg was used for subjects who achieved the maximal values of power at 40 kg), they were able to complete, on average, 20 repetitions. At these weights, significant differences between the initial and the final repetitions of the wood chop exercise were found (13.9%, p = 0.025; 10.2%, p = 0.036; and 13.8%, p = 0.028, respectively).
It has been found that mean power produced during the standing cable wood chop exercise is a reliable parameter able to discriminate within-group differences. These findings are in agreement with the previous study by Andre et al. (1), which reported ICC values above 0.90 for rotational power measured during motion of the torso with arms extended in front of the body while sitting on a box. Contrary to this, subjects in the present study performed trunk rotations in standing position while keeping the elbows close to the body. We obtained maximal values of mean power from the testing procedure consisting of maximal effort single repetitions with increasing weights. This may represent a more specific and hence more appropriate alternative than the traditional 1RM approach. Such assessment of rotational power in the form of standing cable wood chop exercise may be applied in sports requiring the production of maximal force over a short period and in fitness-oriented training involving core exercises.
Core exercises incorporated into strength and power training regimens require bilateral agonist-antagonist coactivation to produce movement and stabilize the spine. When the trunk muscles must be co-activated to stabilize the spine, that exercise is by definition a core stability exercise (11). Core stability is the ability of the lumbopelvic-hip structures and musculature to withstand compressive forces on the spine and return the body to equilibrium after perturbation (30). Factors such as the endurance, strength, power, and coordination of the abdominal, hip, and spine musculature are important components of core stability. The result of the study of Keogh et al. (6) suggests that similar to strength, core stability exhibits relatively high levels of task specificity. The implication of this is that once some initial conditioning of the core musculature is obtained, core stability training should be as specific as other aspects of the conditioning program if functional performance is to be improved. It could be argued that one way to achieve this would be the use of functional total body exercises that mimic in some respects actual movements that are routinely performed by the athletes in their sports. These total body exercises may also be used to assess functional core stability. The challenge remains as to what aspects of performance in these total body tasks would be assessed and how this would be quantified in an objective manner.
Selecting a single appropriate test to fully evaluate core stability is difficult, given the complex interaction of the lumbopelvic-hip structures and musculature. A number of static single-joint core stability measures and ratios were unable to distinguish resistance-trained subjects with high and low strength and power levels or to evaluate the efficiency of training involving complex dynamic core exercises.
Thus, there was a need to develop new, sport-specific tests evaluating rotational power of the trunk. It was especially important to develop tests that require little or no equipment and hence are inexpensive and fast to administer. Most current tests evaluate the endurance (e.g., trunk flexor and extensor endurance tests and the lateral bridge test) rather than the strength and power component of core stability. Given that rotational power is a better predictor of sport performance, tests that measure this component of the core may be more useful, especially because they may better mimic the demands imposed by sports.
The value of this novel method for assessing muscle power during the standing cable wood chop exercise is that it simulates movement that occurs in many sports, for instance, swinging a bat or stick or throwing while twisting the torso (baseball, basketball, cricket, golf, hockey, tennis, soccer, etc.). The difference is mainly the velocity of the trunk movement and the unloading or loading conditions under which it is being performed.
Core strength does have a significant effect on an athlete's ability to create and transfer forces to the extremities (24). It is obvious that the effective execution of the tennis stroke or golf swing not only requires rapid movement of the extremities but also substantial rotational power and/or velocity of the trunk muscles. Trunk extensors, flexors, rotators, and lateral bend agonists are active throughout the stroke in baseball and tennis as well as the golf swing. Watkins et al. (28) measured muscle activity in the erector spinae, abdominal oblique, and rectus abdominis. These authors found that all trunk muscles were relatively active during the acceleration phase of the golf swing with the trail-side abdominal oblique muscles showing the highest level of activity. Because of the muscular demands of sports, such as tennis, golf, ice-hockey, etc., testing should include the assessment of power over the entire motion of the trunk and in its acceleration and deceleration phase. The diagnostic system used in the present study allows assessment of peak power and mean power in the entire concentric phase and in its acceleration segment.
Although for some athletes the rotational power may represent a sport-specific pattern (e.g., a karate stroke), for others it may be the strength endurance of trunk muscles that is important for their athletic performance (e.g., canoe slalom). In sports requiring repetitive trunk rotations, it is therefore important to evaluate muscular endurance. Assessing the mean power in a set of a predetermined number of repetitions at a previously established weight enables the quantification of the effect of fatigue on core performance. This may be expressed as a decline of power values from the initial to the final repetitions. The weight at which maximal power is achieved has been found appropriate for the endurance test of the trunk muscles.
Given the relative importance of the trunk muscles in these sports, particularly in generating powerful trunk rotations, repetitive play and practice might contribute to the enhancement of the rotational power and endurance of the trunk muscles. However, the asymmetric pattern of trunk rotation during the tennis stroke or golf swing may cause side-to-side imbalances in rotational strength and endurance. These potential imbalances may contribute to an increased susceptibility to developing low back pain. Indeed, our preliminary study showed higher values of mean velocity in the acceleration phase of trunk rotation in the dominant than nondominant side in golfers and tennis players, suggesting that this parameter may be considered specific to asymmetric loading of trunk rotation (32).
The findings of these studies support the view that tests that enable the assessment of the rotational power and endurance of the trunk muscles during complex dynamic tasks are needed. So far, a variety of core stability tests have been developed for use in both clinical and research settings (2,12,13). Despite this, data are scarce regarding trunk rotation strength and endurance capabilities in athletes and in the general population. According to Kumar (7), the scarcity of data for trunk rotation is directly attributable to the lack of suitable, accurate, standardized, and affordable devices to carry out such measurements.
Furthermore, the problematic reliability and validity of current diagnostic methods for evaluating the strength of the trunk muscles limits their practical application. To our knowledge, this study is the first to examine the test-retest reliability and sensitivity of the maximal power and endurance of core muscles during the standing cable wood chop exercise on a weight stack machine. This study also dealt with methodological issues associated with such testing. The testing protocol should consist of 2 trials (one at higher weights) on each side of rotation with weights increasing stepwise up to 1RM. A practice trial is recommended before measurement to become familiar with the technique of the exercise and to attenuate the possibility of a learning effect. For the endurance test of the trunk muscles, 20 repetitions should be performed with the previously established weight at which maximal values of power were achieved. Assessment of the maximal power and endurance of the trunk muscles in this way may represent a more specific and therefore more appropriate alternative for athletes practicing core exercises with free weights or on weight stack machines as part of their training routine or competition. It can be effectively used by strength and conditioning practitioners in evaluating the power performance during the rotational movement of the trunk. However, further studies are needed to establish normative data by measuring trunk rotational power and muscular endurance in healthy individuals and age-matched athletes in relevant sports. Collection of these data would permit comparisons between individuals with different demands on the core musculature and possible side-to-side imbalances because of the asymmetric nature of sport-specific movements (e.g., tennis stroke and golf swing).
Evaluation of the maximal power and endurance of core muscles during the standing cable wood chop exercise on a weight stack machine is both a reliable method and sensitive to differences among physically active individuals. Mean power during the standing cable wood chop exercise is a reliable parameter with ICC values above 0.90 at all weights tested. It is also a sensitive parameter able to discriminate within-group differences in the maximal values of mean power and the endurance of core muscles. Substantial individual differences are observed in the mean power produced, especially at higher weights, and in its maximal values achieved at about 75, 67, and 83% of 1RM. At these weights, significant differences between the initial and the final repetitions of the wood chop exercise can also be found. Therefore, this method of assessing (a) maximal power using maximal effort single repetitions of the standing cable wood chop exercise with increasing weights and (b) the endurance of the core muscles using a set of a predetermined number of repetitions performed at a previously established weight at which maximal power was achieved may be used in functional performance testing, namely, for athletes who require the production of rotational power during training or competition (e.g., tennis players, ice-hockey players, and canoeists).
To date, no study has documented either the intersession test consistency or the sensitivity of power measurement during the standing cable wood chop exercise on a weight stack machine. We estimated the test-retest reliability of rotational power when performing a standing cable wood chop exercise with different weights. We also established the methodology for assessing the maximal power and endurance of core muscles. Taking into account the individual differences in maximal values of mean power during the standing cable wood chop exercise with stepwise-increasing weights, and in the decline of the mean power during the trunk endurance test, this method may be considered to be a suitable and practical alternative for sport-specific and fitness-oriented testing.
This work was supported by the Scientific Grant Agency of the Ministry of Education, Science, Research and Sport of the Slovak Republic and the Slovak Academy of Sciences (No. 1/0373/14). The results of this study do not constitute endorsement of the product by the authors or the National Strength and Conditioning Association. The authors declare that there is no conflict of interest.
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