Exercise, including eccentric contraction and any movement that a person is unaccustomed to, can result in exercise-induced muscle damage (EIMD) (Clarkson and Hubal, 2002; Cleak and Eston, 1992). In EIMD, muscle fibers are disrupted (Yu et al., 2003), leading to inflammation that can last as long as 30 d (Nosaka and Clarkson, 1996). This causes a decrease in muscle strength and range of motion (ROM) of the joints and also a decrease in physical performance, including impaired balance and sprint ability (Khan et al., 2016). The parameters used to measure the extent of EIMD include muscle soreness (SOR), strength, ROM, swelling, and biochemical analysis (Clarkson and Hubal, 2002; Hyldahl et al., 2017).
In a previous study, the extent of EIMD after 2, 6, and 24 eccentric exercises performed with maximum effort was examined. Relative to pre-exercise values, the maximum isometric torque 2 d after exercise reduced by approximately 20%, 33%, and 56%, respectively, and the peak values of serum creatine kinase (CK) activity increased by approximately 2-fold, 323-fold, and 1591-fold, respectively (Nosaka et al., 2001a). Conversely, although participants performed 100 repetitions (10 times x 10 sets) of the concentric exercise, elbow flexion, with maximum effort, the maximum isometric torque 2 d after exercise only reduced by approximately 10% compared with the pre-exercise value, and the CK activity hardly altered in any of the groups (Bottas et al., 2005).
In addition to the symptoms described above, it was recently reported that the N-terminal fragment of titin leaks into urine following eccentric contraction exercise (Kanda et al., 2017). Titin, a main determinant of the extensibility of sarcomeres in muscles, is a giant tandem modular protein anchored to myosin and the Z-disc in sarcomeres (Eckels et al., 2018). Urinary titin N-terminal fragment (UTF) was initially detected in patients with Duchenne muscular dystrophy (Rouillon et al., 2014), followed by that in healthy individuals after eccentric contraction (Kanda et al., 2017). UTF concentrations reach a peak at 96 h after eccentric exercise (Yamaguchi et al., in press), and this peak is significantly correlated with other indices of muscle damage, such as isometric strength (r [less than or equal to] -0.485), ROM (r [less than or equal to] -0.485), SOR (r [greater than or equal to] 0.549), and CK activity (r [greater than or equal to] 0.647) (Yamaguchi et al., 2020). Although UTF concentrations get altered following eccentric contraction, it is unclear whether any changes occur after concentric contraction.
Therefore, we aimed to investigate UTF concentrations after concentric and eccentric contraction exercises. Our hypothesis, which is based on the study by Bottas et al. (2005), was that UTF concentrations will remain unchanged after concentric exercise but would significantly increase after eccentric exercise. If UTF concentrations do not alter following exercise involving concentric contractions, which induce little muscle damage, UTF should only be detected specifically following EIMD caused by eccentric contraction exercises.
Participants and study design
We recruited nine healthy men (age: 22.9 [+ or -] 1.7 years; weight: 69.5 [+ or -] 9.2 kg; height: 172.5 [+ or -] 5.0 cm; mean [+ or -] SD) with no injuries of the muscles of their upper limbs and who had not performed regular resistance training in the 6 months prior to the experiment. We explained the purpose, procedures, risks, and benefits of the study to potential participants; they provided written informed consent before participating. The experiment was conducted according to the Declaration of Helsinki and was approved by the Waseda University ethics committee (2017-162). We frequently reminded the participants to abstain from performing any strenuous physical activities or activities that they were unaccustomed to and to refrain from taking anti-inflammatory medication during the experiment. Body weight was measured using a multifrequency impedance body composition analyzer (InBody720 Inbody Japan Inc., Tokyo, Japan), and height was measured using a wall-mounted stadiometer (SECA 213, Yagami Inc., Nagoya, Japan).
The participants performed a bout of concentric exercises (CON) comprising three sets of 10 maximal concentric contractions using the dominant arm and a bout of eccentric exercises (ECC) comprising the same number of contractions using the same arm but separated by >8 weeks. All participants were allowed to choose their dominant arm for exercise because Newton et al. (2013) demonstrated that muscle damage does not differ between the dominant and nondominant arms. Moreover, to unify the work intensities for both conditions, CON and ECC were performed at the same workload measured using an isokinetic dynamometer. The reason why we chose an interval of 8 weeks between bouts was that the results of a previous study revealed that EIMD has a protective effect against subsequent muscle damage; any muscle damage induced by eccentric exercise performed within several days of another exercise is significantly reduced (Hyldahl et al., 2017; Yamaguchi et al., in press). This protective effect can last for 12 weeks (Nosaka et al., 2005); stronger the degree of EIMD elicited by the initial exercise, stronger the effect (Chen et al., 2007). This effect is also observed when the initial exercise is concentric contraction (Nosaka and Newton, 2002a). The actual duration of the effect of concentric contraction remains unknown, but it is assumed that the duration of the adaptation after concentric exercise is shorter than that after eccentric exercise because the extent of the first EIMD affects the effect size of the next adaptation. Therefore, we asked the participants to perform eccentric elbow flexion exercises of at time points separated by >8 weeks following concentric condition.
The dependent variables included the maximum voluntary isometric contraction (MVIC) torque of the elbow flexions, ROM of the elbow joint, SOR, UTF concentration, and serum CK activity. The measurements of MVIC, ROM, SOR, and UTF concentration were obtained immediately before and after and 24, 48, 72, 96, 120, and 144 h after concentric and eccentric exercises (Lavender and Nosaka, 2006). Because serum CK activity barely change following concentric exercise (Lavender and Nosaka, 2006; Pedersen et al., 2001), serum CK activity was collected when the other dependent variables were measured only for the ECC condition. All dependent variables were measured by the same investigator to ensure that the results were consistent. Based on the intraclass correlation coefficient (R) and coefficient of variation (CV), the test--retest reliabilities of the study were 1.0 and 0.0% for SOR, 0.96 and 1.5% for ROM, 0.91 and 2.9% for MVIC, 0.99 and 3.4% for serum CK activity, and 0.94 and 4.5% for UTF, respectively.
Eccentric and concentric exercise
Each subject was seated on the chair of an isokinetic dynamometer (Biodex System 3, Biodex Medical Systems, Inc. Shirley, NY, USA), and the shoulder joint angle was set at 45[degrees] flexion with 0[degrees] abduction. The participants were asked to supinate their forearm and grasp the attachment connected to the lever arm of the dynamometer. In the concentric exercise task, the participants performed 3 sets of 10 repetitions with 60-s rest between the sets of elbow flexions. In the eccentric exercise task, the participants performed the same workload recorded during concentric exercise. We forcibly extended the elbow joint from a flexion (starting position: 90[degrees]) to an extension (end position: 180[degrees]) for over 3 s (Tsuchiya et al., 2018). To ensure that participants performed only the target actions, we set the dynamometer to conduct the flexion passively from the end position to start position for 3 s for ECC and vice versa for CON. We verbally encouraged participants during exercise to maximize their efforts. The peak torque and work of each contraction were calculated by the Biodex built-in software.
Maximal voluntary isometric contraction torque
The MVIC torque of the elbow flexions was measured by the isokinetic dynamometer (Biodex System 3, Biodex Medical Systems, Inc. Shirley, NY, USA). We set up the isokinetic dynamometer in the same position for each subject, as described earlier for exercise. We recorded the positions of the dynamometer and each attachment on the first day of the experiment and measured the MVIC values of the participants in the same...