Evidence has suggested that the rating of perceived exertion (RPE) is correlated with various measurements of physiological processes, including cardiopulmonary factors [heart rate (HR), oxygen uptake (V[O.sub.2])] and peripheral factors (blood lactate and muscular strain) (Borg, 1982; Hampson et al., 2001). The functional link between RPE and physiological factors has been applied to exercise training and used in clinical settings. A sense of muscular strain is an important factor in the perception of effort (Robertson and Noble, 1997). Previously published studies have indicated that RPE is associated with active muscle strain, as measured using electromyography (EMG), during resistance training (Duncan et al., 2006; Lagally et al., 2002; 2004) and dynamic cycling exercise (Fontes et al., 2010; Macdonald et al., 2008). Furthermore, muscle activity, as measured using EMG, unrelated to the task, such as frowning, has also been shown to be associated with RPE (Blanchfield et al., 2014; de Morree and Marcora, 2010; 2012).
The face of effort is a type of facial expression that reflects how hard people are working during a physical task. The facial expressions often appearing as a frown or grimace reflect the subject's level of effort during a physical work task (Rejeski and Lowe, 1980). Specific facial expressions may convey physical effort, thereby contribute to an independent observers' estimate of an individual's momentary perception of exertion. Robertson et al. (2006) used changes in facial appearance (i.e. sweating, redness and grimacing) as one of the keys to observe and to rate exertion intensity. The procedure of observing exertion was that the observers were instructed to link increases in the subject's exertion with increases in facial redness and grimacing. Thus, the facial observation keys served as visual indicators of changes in the perception of exertion; these observation keys could visually aid in providing standard perceived exertion scaling procedures.
The facial expression of effort has been proposed as a method to predict and monitor exercise intensity, and it may provide an important non-verbal method to communicate the performance status of athletes (de Morree and Marcora, 2010). de Morree and Marcora (2010) first demonstrated that facial muscle activity associated with frowning was indicative of effort during physical tasks. In their study, the effect of workload (a progressive and systematic increase in workload) on EMG activity of the corrugator muscles increased concomitantly with overall RPE. In addition, they demonstrated a positive correlation between the perception of exertion and frowning muscle activity during leg extension exercise or aerobic exercise (de Morree and Marcora, 2010; 2012).
It is somewhat common for people to clench their teeth (jaw) while exerting maximal muscular effort during tasks such as lifting a heavy weight or participating in a sport that requires maximum effort (Ebben, 2006; Wallman and Sacco, 2007). Several studies have indicated a possible correlation between muscle activity associated with oral motor function and somatic motor function in other parts of the body (Ebben et al., 2008a; Kimura et al., 2007; Miyahara et al., 1996; Takada et al., 2000). Jaw clenching augments leg extensor force production and countermovement jumps, which could be associated with effort (Ebben et al., 2008b). According to these studies, jaw clenching activity may correlate with the perception of effort as well as frowning activity, which is one of the hypotheses investigated in the present study.
A positive correlation between RPE and frowning expression involving EMG evidence of facial muscle activity during leg extension and constant workload cycling exercises has been shown (de Morree and Marcora, 2010; 2012). However, this relationship has not been examined during incremental workload cycling exercise. In addition, jaw clenching activity has not been validated as being correlated to perceived exertion during aerobic exercise. Furthermore, it is not known which facial muscle activity is a good indicator of effort.
The aims of the present study were (i) to examine the effect of exercise intensity on EMG activity of two facial muscles and to examine whether EMG activity reflects the perception of effort as well as involved muscle activity during incremental workload cycling exercise and (ii) to determine the correlations between EMG activity of corrugator supercilii (CS) muscle (associated with frowning), that of the masseter muscle (associated with chewing), that of the vastus lateralis (VL) muscle, HR and RPE during incremental workload cycling exercise.
Thirty-three volunteers, including 18 males (age: 22 [+ or -] 2 years; height: 1.73 [+ or -] 0.06 m; weight: 67 [+ or -] 7 kg) and 15 females (age: 22 [+ or -] 2 years; height: 1.61 [+ or -] 0.05 m; weight: 51 [+ or -] 6 kg), were recruited from a university population to participate in the study. All participants reported being moderately physically active (leisurely exercise at least twice a week), healthy and asymptomatic of illness and having no pre-existing injuries. Written informed consent was obtained from all participants, and the study purpose (but not the purpose of the facial electrodes) and protocol were explained to them. The experimental procedures were approved by the Institutional Review Board of Chang Gung Medical Foundation.
Each participant performed an incremental workload exercise protocol on an electromagnetically braked cycle ergometer (Corival 906900, Lode BV, Groningen, The Netherlands). This incremental workload exercise allowed our hypotheses to be tested over a wide range of exercise intensities. Exercise included a 5-min warm-up at 0 watts (W), followed by a continuous and incremental increase in workload (starting at 40 W) by 40 W every 3-min. Participants were instructed to maintain a cycling cadence between 60 and 70 rpm by watching the digital signal of the cycle ergometer control panel throughout the exercise test. When a participant reached exhaustion or the inability to sustain a target pedal cadence greater than 55 rpm for a period of 5 s, the exercise test was terminated.
The subject's HR was continuously monitored during exercise by a wireless chest strap telemetry system (Polar Wear Link System and Polar FT4 HR monitor, Polar Electro Oy, Kempele, Finland). During the final 30 s of each 3-min stage of cycle ergometer exercise, participants were asked to report their overall feeling of exertion on the adult OMNI-Cycle RPE (Robertson et al., 2004).
Perceived exertion was defined as the subjective intensity of effort. All participants did not have previous experience in the use of a category rating scale of perceived exertion. Chen et al. (2013) showed that the familiarity of participants regarding the use of the RPE scale did not affect the scores. Before the experiment, all subjects were well instructed regarding the use of the scale and OMNI RPE (Robertson et al., 2004). Because there is no validated Chinese-translated version of the OMNI scale, a verbal description of the OMNI scale was shown both in Chinese and English. Although the anticipation of subsequent workload stages during the incremental test could potentially be a confounding factor of perceived exertion, Coquart et al. (2009) and Skinner et al. (1973) noted that the perceptually based values were not significantly different between the incremental test and a test with randomised workloads. Thus, this suggests that cyclists were able to perceive differences in intensity on a cycle ergometer that were not influenced by demand bias. Activities of the CS, masseter and VL muscles were continuously recorded using EMG during exercise.
All participants were given written instructions to avoid intense exercise, caffeine and alcohol consumption during the 24 h preceding the test. In addition, they were asked to sleep for at least 7 h before the test and consume a light meal approximately 2 h before the test.
During the exercise test, the surface EMGs of the...