The Effects of Anticipatory Stress on Pain Responses in Male and Female Athletes

Athletic competition elicits a stress response that is both physical and psychological in nature. Additionally, following athletic competition, an increase in pain threshold (analgesia) has been observed. However, the mechanisms that produce analgesia due to athletic competition have not been systematically investigated. The present study sought to determine if the psychological stress in anticipation of an athletic competition is alone able to induce an analgesic state by evaluating sympathetic nervous system measures (heart rate, blood pressure), cortisol levels, self-rated anxiety scores, and pain thresholds in male and females soccer players and in female basketball players prior to a game and a practice, and on a non-competitive day. The results suggest that there is an increase in pain threshold (analgesia) in anticipation of both an athletic contest and a team practice session and that this increase may be associated with increases in cortisol levels, sympathetic nervous system measures, and self-rated measures of anxiety. Future research on coaching styles, winning expectancies, and the social context surrounding the competition would help clarify the present findings. Pain Responses in Athletes 3 The Effects of Anticipatory Stress on Pain Responses in Male and Female Athletes Pain signals the body of tissue damage (real or potential). Feelings of pain are generated so that the body does not undergo more harm, attention can be paid to the painful area, and the problem can be fixed. However, during the stress response, mechanisms within the body that are potentially detrimental long-term are activated so that the body can more effectively respond to stress without being hindered by pain, a phenomenon known as stress-induced analgesia. For instance, during competition, athletes do not tend to exhibit a pain reaction to injuries that typically elicit large pain responses in other environmental circumstances. The present study evaluates the stress response in anticipation of athletic competition in male and female athletes. More specifically, this study sought to determine if pain thresholds are increased in anticipation of an athletic competition as compared with a non-competitive or practice session in male and female athletes as a result of stress-induced analgesia. Pain versus Nociception Pain signals the presence of potential or actual tissue damage and is therefore essential to the well being of a person. There is, however, a difference between the perceptual experience of pain and the physiological experience of nociception. Pain is a complex perception, the brain’s unique interpretation of sensory input influenced by both the environment and one’s emotional state (Basbaum & Jessell, 2000). Nociception, in contrast, is a physiological experience that refers to activity in the neural circuits. Pain Responses in Athletes 4 Interestingly, the brain itself lacks nociceptors, and therefore cannot experience pain (Carlson, 2004). Beecher (1956, as cited in Gatchel, 1999) first demonstrated that the pain experience could be modified by psychological factors through studies of World War II soldiers. He compared the requests of soldiers returning from battle for pain medication to the requests of civilians with similar wounds and found that only 25% of the soldiers requested pain medication as compared with over 80% of the civilians. Many soldiers expressed that their pain was not sufficient to require medication, suggesting the presence of a psychological component to pain. Even before Beecher, individuals have always had a desire to understand and to control pain. As far back as 4000 B.C., the ancient Egyptians attempted to treat pain; over 2000 years ago, acupuncture was used to alleviate pain in China. As time progressed, medical practitioners began to understand that pain was both physical and psychological. The Greek physician Hippocrates (400-300 B.C.), for instance, believed that pain and other physical or mental illnesses resulted from an imbalance of the four humors, or bodily fluids. By the 17 century, the study of medicine was more scientific, as Decartes argued that pain was a result of a specific type of activity in the sensory nervous system. More recently, 19 century scientists argued that one must better understand pain in order to treat it more effectively (Gatchel, 1999). Primary Afferent Nociceptors The understanding of pain has evolved since the 19 century. Scientists now understand that the perceptual experience of pain begins with primary afferent Pain Responses in Athletes 5 nociceptors, which are relatively unspecialized nerve cells throughout the skin and in deep tissues that respond exclusively to noxious stimuli. Primary afferent nociceptors project from the site of tissue damage (real or potential) to the dorsal root ganglion, a collection of cells that supply information to the spinal cord. There are three major types of nociceptors, all of which transmit nociceptive information fairly slowly. However, there are both relatively fast and relatively slow nociceptive pathways which are each important for different reasons. The three types of nociceptors are Aδ mechanothermal, Aδ mechanosensitive, and polymodal C fibers; each plays a unique role in the perception of pain. Aδ mechanothermal and Aδ mechanosensitive nociceptors both have fairly small diameters, are thinly myelinated, and conduct relatively quickly (as compared with other nociceptors) at speeds of approximately twenty meters per second (Fields, 1987). Aδ mechanothermal and Aδ mechanosensitive nociceptors are activated by different stimuli; Aδ mechanothermal nociceptors are stimulated by extreme temperatures of greater than 45°C or less than 5°C whereas Aδ mechanosensitive nociceptors are activated by intense pressure. Polymodal C fibers also have small diameters, but are nonmyelinated, and therefore conduct at much slower rates than the other two (about two meters per second). Nociceptive C fibers are classified as polymodal because they are activated by mechanical, chemical, and thermal stimuli (Basbaum & Jessell, 2000). All nociceptors have two main functions: transduction and transmission. Initially, during transduction, a noxious stimulus (chemical, mechanical, or thermal) depolarizes the nociceptor and creates a nerve impulse that can be recognized by the brain. Subsequently, through the process of transmission, nerve impulses are conducted to the Pain Responses in Athletes 6 spinal cord and to other central nervous system structures to create the sensation of pain. Transmission generates several distinct pain responses such as withdrawal reflexes or the subjective reaction to the event (Fields, 1987). Application of a noxious stimulus generates two separate sensations. An early, sharp, brief prickling described as the first pain, is produced by lower threshold Aδ fibers (mechanothermal or mechanosensitive depending on the nature of the stimulus). Following the first pain is a dull and prolonged second pain, for which the higher threshold polymodal C fibers are responsible (Fields, 1987). Therefore, each type of fiber contributes uniquely to an individual’s perception of pain. Nociceptive Pathways in the Central Nervous System Primary afferent nociceptors project to the dorsal horn of the spinal cord where they interact with spinal cord neurons. The axons of spinal cord neurons cross to the anterolateral quadrant of the spinal cord on the side opposite the nociceptors and project to higher centers in the brain that contribute to the pain response (Fields, 1987). The majority of primary nociceptive afferents enter the spinal cord through the dorsal root. Axons in the dorsal root carry sensory information to the spinal cord and the brain. As the dorsal root approaches the spinal cord, at the ventrolateral section of the dorsal root there is a separation between the small diameter myelinated axons and the unmyelinated axons. Consequently, when the primary afferent nociceptors enter the dorsal horn of the spinal cord, large diameter and small diameter nociceptive fibers are completely separated (Fields, 1987). Pain Responses in Athletes 7 In the dorsal horn of the spinal cord, there are three types of neurons: projection neurons, excitatory interneurons, and inhibitory interneurons. Each type of neuron contributes uniquely to the experience of nociception. Primarily found in sections of the dorsal horn that receive direct input from primary afferent nociceptors (laminae I, II, and V), projection neurons fire maximally in response to noxious stimuli and send nociceptive information to higher brain centers that play a role in pain perception. Excitatory interneurons, in contrast, send nociceptive information to projection cells, to other interneurons, or to motor neurons that control spinal reflexes. Inhibitory interneurons too send nociceptive information to other neurons and play a role in the control of nociceptive transmission (Fields, 1987). The dorsal horn is divided into ten distinct sections, or laminae. The neurons in the superficial layers of the dorsal horn (primarily laminae I, II, and V) play an important role in nociception. Lamina I neurons are predominately projection cells that receive input directly from myelinated and unmyelinated nociceptors and that respond maximally to noxious stimulation. Most importantly, lamina I projection neurons extend to higher brain centers, such as the thalamus, that have an important role in pain processing that will be discussed later (Fields, 1987). The majority of the neurons in lamina II are interneurons (excitatory and inhibitory) that respond to both nociceptive and nonnociceptive information. Lamina II interneurons make connections with other lamina II interneurons or projection neurons in laminae I or V. Stalk cells of lamina II are excitatory interneurons that send axons to neurons in lamina I projection cells (Fields, 1987). Pain Responses in Athletes 8 Like lam