Muscles subjected to unaccustomed eccentric exercise are susceptible to muscle damage that rely on sarcomeres disruption and impairment of the excitation-contraction (E-C) coupling system (Proske and Morgan, 2001). The well known results of this exercise-induced muscle damage are prolonged decrease in maximal force generated by the muscle during both voluntary and electrically evoked contractions and delayed onset muscle soreness (DOMS) (Janecki et al. 2014; Prasartwuth et al. 2005). It is well documented that a prior bout of eccentric exercise triggers a rapid adaptive response that significantly diminishes muscle damage and the muscle force recovery occurs faster after a second bout of the same exercise (Chen et al. 2009; Nosaka et al. 2001). This protection developed after the first bout of eccentric exercise is referred to as repeated bout effect (RBE) and mechanisms responsible for this phenomenon relies on neural, cellural and mechanical factors (McHugh, 2003). Another specific effect that occurs after eccentrically biased exercise is long-lasting increase in low-frequency fatigue (LFF) (Edwards et al. 1981). LFF is defined as the disproportionate decrease in force elicited with electrical stimulation at a low frequency, compared to a high frequency (Bruton et al. 1998). Jones (1996) has proposed that the primary mechanism responsible for LFF is a decrease in calcium release from sarcoplasmic reticulum that affects E-C coupling process. It has been confirmed with direct measure of calcium concentration changes in mouse skeletal muscle in the presence of LFF (Chin et al. 1997). Therefore, LFF reflects changes associated with exercise-induced impairment of the E-C process that affects contractile machinery responsible for the muscle force development. LFF could be measured by calculating the double pulse torque to single pulse torque (DT/ST ratio), which is an effective method to quantify LFF consistent with the literature using trains of high and low frequency stimulation (Ratkevicius et al. 1995; de Ruiter et al. 2005). The advantage of this method of LFF assessment is that DT/ST ratio has less influence on muscle fatigue (Rassier et al. 1999), and because is more comfortable when compared to longer stimulation trains, minimizes volitional reactions to the noxious stimulation and the post stimulus co-activation artifacts (Iguchi and Shields, 2010; Meszaros et al. 2010), that may affect strongly the force curve obtained from the elbow flexors muscles.
The study of Kamandulis et al. (2010) performed on the knee extensors muscles revealed repeated bout effect for the LFF assessed as a stimulation trains. However, it has been reported that elbow flexors muscles, are more susceptible to muscle damage than the knee extensors muscles (Jarmutas et al. 2005; Paschalis et al. 2010) what could effect on their time course of recovery and susceptibility to repeated damage. Because of their function (fast and accurate movements), prolonged low frequency force deficit could affect strongly performance of the daily task at low force level (Dundon et al. 2008; Smith and Newham, 2007).
To the best of our knowledge, there is no study that has measured LFF after repeated ECC of the elbow flexors using DT/ST ratio. Because this mode of stimulation minimizes potential artifacts, it could be a sensitive tool for indirect assessment of muscle damage, recovery and adaptation to ECC. Therefore, the aim of this study was to assess modifications of DT/ST ratio after two bouts of eccentric exercise of the elbow flexors. We hypothesized that the changes of DT/ST ratio will decrease and/or disappear faster after the second bout of ECC when compared to the first bout.
Sixteen untrained, right handed male volunteers (age 24 [+ or -] 4 yr, height 1.79 [+ or -] 0.09 m, body mass 75 [+ or -] 8 kg) took part in this study and gave their written consent, the study was approved by the local Ethics Committee and complied with the Helsinki Declaration. Some of the subjects were familiarized with resistance training but had not perform any resistance exercise for at least 6 months before this study. The participants did not have any neuromuscular disorders and were free from any injuries of the upper limbs. All subjects were instructed to keep their normal diet and not to take any anti-inflammatory drugs as they could affect recovery mechanisms associated with repeated bout effect (Kyparos et al., 2012; Lapointe et al., 2002).
A within-group repeated measures design was used to determine changes in LFF after two bouts of eccentric exercise separated by 2-3 weeks. Maximal voluntary isometric torque of the right elbow flexors during maximal voluntary contraction (MVC), electrically evoked responses to single pulse (single twitch--ST) and double pulse (double twitch--DT) and pain assessment were collected before (pre), immediately after (post), 24 and 48 hours (h) following each bout of ECC. The sequence of measurements was always in the same order: pain assessment, single twitch, double twitch and MVC tasks. This order was chosen to avoid the potential influence of maximal torque development on subsequent measurements (e.g. MVC effect on twitch potentiation) (Vandervoort et al., 1983).
The measurements of MVC and electrically evoked contractions of the elbow flexors of the right arm at 90[degrees] elbow joint angle were described previously (Janecki et al. 2014). Briefly, the MVC's were measured with the BIODYNA dynamometer, designed and built by Warsaw Technical University in Poland (Kedzior et al. 1987) (Figure 1). The device consists of: a chair with seat belts for stabilization, a column, and a moveable arm with a wrist handle. The wrist handle has a high sensitivity force transducer (SML-200, Interface, Scottsdale, Arizona, USA) inbuilt on one side and movable plate for the wrist stabilization from the other side. The adjustable wrist handle allows setting the lever arm individually to ensure that during maximal voluntary contraction measurements force is exerted by the subject's wrist against the force transducer at the level of styloid processes of the radius and ulna. The wrist handle placement on the lever arm was recorded before the first measure, and used as the site for the wrist handle setting on each subsequent measurement. The torque applied at the elbow joint was calculated by multiplying the measured force by the perpendicular distance between the force transducer and the center of rotation of the elbow. Before the study began the MSL-200 force transducer was calibrated on two different lever arms of the dynamometer using weights of known mass (0.5, 1, 2, 5 and 10 kg) and the calibration was linear within the tested range. An angle was measured with a precision potentiometer connected to the lever arm of the dynamometer.
Tests were conducted with the participants seated with their back supported and their...