The Role of Negative Emotions in the Aetiology of Non-Impact Sports Injuries: Interactions of the Stress Response and Motor Control Systems

Athletes of all sports are subject to many injuries, which interfere with their training programs, progress and performance, and are a major cause for career-ending withdrawal from competitive sport. The incidence and severity of sports injuries are related to the competitive level and pre- competition and competition time and are strongly associated with stressful life events. The Stress-Injury model, similarly to other psycho-social theories, provides an insightful explanation of the complex interacting factors associated with stress as well as the cognitive/emotional mechanisms, which underlie attentional dysregulation and inadequate control of goal orientated behaviour (in this particular case – execution of complex movements) and predispose athletes to injuries. The weak point in all these theoretical models is the oversimplification of the exact neuro-physiological mechanisms, which link negative emotional states (stress) with motor control regulation of highly skilful, complex and physically challenging movements, as is the reality for athletes from beginner to highly advanced Olympic competitor level.From whole body system’s perspective, the most fundamental principle that underlies behaviour is the principle of dynamic integrity and stability (both structural and functional). The Autonomic nervous system (ANS) through the function of its two branches (Sympathetic and Parasympathetic) and neuro-endocrine and immunological regulation preserves the dynamic stability of the internal environment in the face of constant change, as part of the general activity of the Central nervous system (CNS). The motor control system (MCS), as part of CNS, preserves the mechanical stability and integrity of the body in the face of movement. In these terms the stress response can be viewed as behaviour, which disrupts the dynamic stability of the entire CNS and involves both the ANS and MCS.Emotion, behaviour and stress responseIn the past few decades many different stress theories have evolved, approaching the complex interaction stimulus-psychological/physiological response from sometimes opposing angles. All these theories have many valid and common points, but when it comes to the specifics of meaning and use of common terminology like ‘stress’, ‘emotion’, ‘arousal’ and ‘balance/homeostasis’, the differences of their conceptual approaches become apparent, adding further confusion to this, already complex subject-area.It is well accepted that the stress response alters the activity of the whole neuro-axis: from sensory and emotional/motivational and memory association cortices; through thalamus, hypothalamus and brainstem structures; to spinal cord and peripheral autonomic and somatic nerves. The principal structures involved with emotional and motivational modulation and memory are the limbic structures of the Amygdala (cAmg), Bed nucleus of the Stria Terminalis (BNST) and the Hippocampus, which initiate and maintain fear and anxiety, while the ventral pre-frontal cortex (PFC) is associated with goal setting and motivation. The general activating (arousal) physiological response is executed by Hypothalamic (mainly, but not exclusively Paraventricular Nucleus, PVN) and mid-brain and brainstem nuclear groups like Locus Coeruleus (LC), Raphe nuclear groups, Central and Para-aquaductal Grey and many parts of the Reticular Formation (RF), which activate the Sympathetic branch of the ANS (SNS) and the systemic secretion of adrenalin from the adrenal medulla. It also leads to activation of the pituitary by CRH, release of ACTH, which triggers the secretion of Glucocorticoids from the adrenal cortex with multi-system alteration of metabolic function.The physiological arousal executed by the altered activity of the HPA axis in connection with the activation of the SNS is part of the stress response and should not be equalled with the stress response itself. What defines the stress response in comparison to straightforward SNS-HPA physiological arousal is the elucidation of inadequate behaviour of helplessness, resignation and withdrawal, which is illustrated by a specific activation pattern of the neuro-axis (Autonomic Nervous System – Central Regulator). This maladaptive state of Central-Autonomic dysregulation, well-illustrated in Panic Disorder and severe depression, is also connected with reduced tone of the Para-Sympathetic Nervous System (PSNS), heightened and rigid metabolic state and impaired cognitive/emotional capacity. The most fundamental feature of the stress response is the low activity of the PSNS, which is the principle stabiliser of CNS function - Central-Autonomic regulation, where metabolic stability is the opposite of metabolic rigidity.Emotion, posture and motor controlThe Motor Control system (MCS), similar to the autonomic regulation system, is another typical example of an open, non-linear system. The MCS co-ordinates musculo-skeletal function for the dual purpose of maintaining mechanical stability and integrity, while producing movement. Its effectiveness and efficiency are reflected in the strategies it employs to achieve stability without producing rigidity.Multiple studies have consistently demonstrated that emotional factors, especially negative emotional states, alter Motor Control and posture regulation. The principle features of the dysfunctional motor behaviour, associated with negative emotional states and stress, are shifting the body stabilisation from deep axial muscles to co-contraction of opposing superficial muscles – state of body rigidity rather than stability, which interferes with the fine tuning of the execution of movements, increases the chance of acute injury (abnormal loading of MSK structures) and can lead to chronic degenerative changes.The frequent occurrence of clinical co-morbidity of balance disturbances and anxiety has led to groundswell of research in that area. In a 2001 review, Balaban and Thayer refined and extended existing theories of CNS processing of afferent exteroceptive, interoceptive and proprioceptive information, as well as the central-autonomic and central-motor neural connections which control the two aspects of behaviour – metabolic and somatic. Central role in this circuitry is played by the Parabrachial nuclear (PBN) groups in the pons, which on the one hand receive sensory, vestibular, proprioceptive, interoceptive and emotional information, while on the other hand are closely connected with circuits executing autonomic regulation. PBN also integrates ‘gravitational’ information, which is fundamental to the sensory self-determination of the organism and plays a central stabilising and pivotal role in all functions of the CNS and especially in respect to the MCS. The PBN network links the sense of gravity, the affective and metabolic state of the body with the posture stabilising function of the MCS. So any disturbance in either emotional state, metabolic state, or ‘gravitational’ (postural) state simultaneously dis-regulates the fine tuning of control of the CNS.ConclusionsThe human body is a self-regulating open system, where homeorhesis (dynamic homeostatic balance) is achieved by continuous fluctuations of many inter-related variables around a central axis of stability. The autonomic stability of metabolism relies on robust and resilient PSNS, while the MCS achieves mechanical stability through employing the deep axial musculature.PBN network is central in the integration of sensory (especially gravitational) and emotional information, as well as regulation of balance and posture. Alteration of the activity of PBN network by negative emotional and mood states in stress is the direct mechanism of dysregulation of the MCS. As modern sport is characterised by performing physically challenging movements in conditions of heightened psychological pressure, it is not surprising the high occurrence of both acute and overuse sports injuries.Apart from athletes, many millions engage in physical exercise on a regular basis in order to reduce stress. It would be interesting to investigate how the rates of sports injuries in non-athletes relate to the specifics of their stress - the potential impact this could have on health resources and effectiveness of implementation of exercise and health promotion programs is substantial.