Neuromechanical contributions to hamstring stiffness dysregulation and injury

Abstract

Hamstring strain injuries (HSI) remain one of the most commonly occurring medical problems in sport and recreation and are associated with high rates of missed playing time and re-injury. Despite increased research interest over recent decades, the financial cost and incidence of HSI continue to rise. A number of risk factors and mechanisms have been explored, but evidence remains inconclusive. The relationship between the nervous system and hamstring muscle properties has not been thoroughly examined. Evidence demonstrates deafferentation following other musculoskeletal injuries with negative implications on proprioception and function. Additionally, impaired muscle activation and stiffness regulation have also been linked to altered neural input from afferent receptors. However, it is unclear if similar maladaptations occur after HSI and if there is an association with the persistent symptoms, inability to return to pre-injury levels, and likelihood for re-injury following return-to-play. Further, it is unknown how common sports related experiences that are suspected to predispose to HSI, such as fatigue and competitive anxiety, interact with the neural and muscular properties of the hamstrings. Therefore, the purpose of this dissertation was to examine how hamstring neuromechanics become instantaneously decoupled in previously injured limbs and during fatigue and anxiety, as measured through patient reported outcomes, proprioception, and stiffness regulation. The results of this study demonstrate that: 1) sensory disconnect occurs following HSI between patient-reported tightness and objective extensibility, which is also linked to decreased proprioceptive acuity; 2) proprioceptive deficits exist following HSI with associated stiffness dysregulation and poor functional outcomes scores; 3) fatigue and anxiety contribute to stiffness dysregulation, with certain alterations amplified by previous HSI. These findings suggest that neural maladaptations continue to linger after HSI, possibly representative of mechanoreceptor trauma or central processing errors resulting from straining mechanisms. These neural alterations could interfere with the hamstrings’ ability to absorb and dissipate energy during rapid eccentric contractions, thereby increasing the risk of injury. This study indicates that examination of proprioceptive function and stiffness regulation can identify abnormal neuromechanical function in previously injured hamstrings. Based on the findings of this study, we provide new areas of hamstring prevention and assessment that have not been thoroughly considered and highlight that interventions may be needed to attenuate the neuromechanical consequences of fatigue and anxiety.