, Michael GallagherJr.2 and Robert J. Robertson3
(1)
Lock Haven University of Pennsylvania, Lock Haven, PA, USA
(2)
University of Central Arkansas, Conway, AR, USA
(3)
University of Pittsburgh, Pittsburgh, PA, USA
Pain has been defined by the International Association for the Study of Pain as an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage (Merskey and Bogduk 1994). This definition implies that the pain experience is subjective, has an affective component, and may not require actual tissue damage (Borg 1998; O’Connor and Cook 1999). The purpose of this laboratory manual is to focus on the intensity of pain that occurs naturally in active skeletal muscles during exercise, even in individuals who are healthy and injury-free. This perceptual construct is independent of perceived exertion, but the two variables have been measured concurrently during both aerobic and resistance exercise in adults and children (Cook et al. 1997, 1998; Kane et al. 2010; Robertson et al. 2009). Similar to perceived exertion, individuals can rate their intensity of perceived pain by selecting a rating from a range of numerical categories displayed on a scale. Some scales, such as the Pain Intensity Scale and the Children’s OMNI Muscle Hurt Scale, include construct-specific verbal and pictorial descriptors placed in juxtaposition to numerical categories representing the range of perceptual responsiveness from no pain at all to unbearable pain. Exercise-induced muscle pain may be an important variable to monitor during exercise testing and prescription because of its potentially powerful implications for the adoption and maintenance of regular PA.
3.1 Pain Threshold During Exercise
In addition to exercise-induced muscle pain intensity, other aspects of pain that have been examined include pain threshold, pain tolerance and the affective components of pain (Cook et al. 1997). The threshold of naturally occurring muscle pain has been assessed during exercise as the time-point at which pain is “just noticeable”. This has been measured using a timer that the subject activates to indicate the time-point during an exercise test at which pain sensation is detected (Cook et al. 1997). In contrast, the construct of perceived exertion does not allow for an onset threshold. It is generally expected that as soon as exercise begins, a certain level of exertion is perceived, the intensity of which can range from very low to very high. However, the onset of muscle pain varies greatly between individuals and in some cases may not occur until 90 % of peak exercise intensity (Cook et al. 1998).
3.2 Mechanisms for Pain During Exercise
Muscle pain during exercise may occur as a result of the stimulation of two separate nociceptive pathways: mechanoreceptors and chemoreceptors. Both of these afferent pathways send information to the central nervous system regarding tissue damage or the potential for tissue damage (O’Connor and Cook 1999). With respect to mechanoreceptor pathways, as physical measures of exercise such as power output (PO), break resistance, or weight lifted increase, there is a corresponding deformation of nociceptive mechanoreceptors. Second, as specific noxious byproducts of metabolism accumulate, such as bradykinin, there may be a greater stimulation of nociceptive chemoreceptors as well as a sensitization of the aforementioned mechanoreceptors. Metabolites such as hydrogen ions sensitize both types of nerve fibers, increase in contracting skeletal muscle as a function of time, and have been shown to accumulate at a faster rate with a greater anaerobic contribution to energy metabolism (Stebbins et al. 1990). In addition, increasing the amount of active musculature during exercise may increase stimulation of both of these nociceptive pathways (Cook et al. 1998).
Noxious sensations detected by the body during exercise are interpreted as a specific level of perceived exertion. As exertional perceptions increase during exercise, they can also be accompanied by increases in muscle-specific pain sensation. The onset of muscle pain sensation usually occurs at higher exercise intensity or at a later point in time during exercise. Muscular sensations of pain change in quality when the noxious stimulus increases in strength, stimulating nociceptors. This neurophysiological sequence results in pain and subsequent voluntary actions or even reflexes to withdraw from the stimulus (Borg 1998). However, it is important to effectively differentiate between perceptions of exertion and pain, especially when measuring them concurrently, as they are not isomorphic constructs. This can be accomplished by using standardized scaling instructions and separate, construct specific scales (Cook et al. 1997; Robertson et al. 2009).
3.3 Clinical Conditions and Pain
For certain clinical conditions, it is important to differentiate between naturally occurring muscle pain during exercise and pain that is symptomatic of a disease or disorder. An individual with cardiovascular disease can experience pain as a result of ischemia (or decreased tissue oxygen supply relative to demand) that is often reproducible during exercise at a specific level of exertion or exercise intensity. This includes angina pectoris, which is chest pain secondary to coronary artery disease, and intermittent claudication, which is pain in the legs secondary to peripheral artery disease. Pain is also a common symptom in diabetic patients with peripheral neuropathies and various types of arthritis. There are also certain disorders that are primarily characterized by pain, such as fibromyalgia and lower back pain. For these conditions, a well-supervised PA program or exercise prescription may be an important aspect of the treatment plan to help improve fitness levels and psychological well-being. Depending on the clinical condition and the degree of functional limitation as a result of a particular disease state, it may be more important to monitor clinically specific pain rather than naturally occurring muscle pain during exercise.
3.4 Rating Exercise-Induced Muscle Pain
Although the concept of pain and related clinical symptoms has been recognized for centuries, the study of naturally occurring muscle pain during exercise is relatively new (Cook et al. 1997). The earliest known investigations include those of Lloyd (1972), who reported pain threshold and pain tolerance during isometric biceps contractions, and Weiser and colleagues (1973), who asked subjects to rate the intensity of “leg aches” and “leg cramps” using a 5-point Likert scale immediately following moderate intensity cycle ergometer exercise. Prior to these studies, research involving the measurement of pain during exercise focused on pain that occurred as a symptom of a disease process, such as claudication secondary to peripheral artery disease. Because research that involved the measurement of muscle and limb pain intensity in healthy individuals employed a mechanical occlusion model to induce claudication pain, the findings of such experiments cannot be considered naturally occurring muscle pain during exercise (O’Connor and Cook 1999).
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