Predictive model of muscle fatigue after spinal cord injury in humans

Abstract

The fatigability of paralyzed muscle limits its ability to deliver physiological loads to paralyzed extremities during repetitive electrical stimulation. The purposes of this study were to determine the reliability of measuring paralyzed muscle fatigue and to develop a model to predict the temporal changes in muscle fatigue that occur after spinal cord injury (SCI). Thirty-four subjects underwent soleus fatigue testing with a modified Burke electrical stimulation fatigue protocol. The between-day reliability of this protocol was high (intraclass correlation, 0.96). We fit the fatigue index (FI) data to a quadratic-linear segmental polynomial model. FI declined rapidly (0.3854 per year) for the first 1.7 years, and more slowly (0.01 per year) thereafter. The rapid decline of FI immediately after SCI implies that a "window of opportunity" exists for the clinician if the goal is to prevent these changes. Understanding the timing of change in muscle endurance properties (and, therefore, load-generating capacity) after SCI may assist clinicians when developing therapeutic interventions to maintain musculoskeletal integrity. Keywords electrical stimulation; muscle fibers; plasticity; reliability; spinal cord injury Paralysis after spinal cord injury (SCI) deprives the musculoskeletal system of important stresses necessary to maintain the health of the paralyzed extremities. 30,33 Without these normal stresses, muscle atrophy and severe osteoporosis develop rapidly. 30 Electrically induced muscle contractions are an integral part of contemporary methods designed to place physiological stresses on the paralyzed extremities. 3,4,6,17,24 However, as muscle adapts after SCI it loses its fatigue resistance 29,36 and becomes a less effective source for generating stress to the paralyzed extremities. Establishing the time course of muscle fatigability may aid rehabilitation practitioners when developing comprehensive rehabilitation plans to maintain musculoskeletal properties after SCI. To our knowledge, no previous study has developed a model based on longitudinal data to predict the temporal changes in muscle fatigue after SCI.The chronically paralyzed soleus muscle transforms to highly fatigable fibers, while the acutely paralyzed muscle is fatigue resistant. 29 optimum stimulation parameters to attenuate this rapid fatigue in chronically paralyzed muscle. 9,13,15,37 Rarely has the goal been to prevent the muscle from becoming highly fatigable by intervening early after SCI, before the fibers become fatigable. Accordingly, a window likely exists after SCI that represents a key period during which an intervention would need to be initiated in order to maintain the endurance properties of paralyzed muscle.The most direct approach to quantifying muscle fatigue is to measure the torque generated at the beginning and end of a series of repetitive contractions. The quotient of these torques, the Burke fatigue index (FI), differs according to the type of motor unit being activated (being higher for slow and lower for fast...

Muscle fatigue characteristics in paralyzed muscle after spinal cord injury

“…This increased fatiguability in paralyzed muscle has been reported previously in the soleus and quadriceps muscle groups. [1][2][3] However, based on the M-wave results of this study, and from earlier work, 2,7 a failure in Na þ /K þ pump activity does not appear to play a significant role in the increased fatiguability during a relatively short period of maximum stimulation in people with SCI. This was an unexpected finding given the reported decrease in Na þ /K þ pump concentration after SCI.…”

“…Fatigue resistance has been shown to decrease rapidly for the first 1.7 years post-injury, with smaller decreases occurring in subsequent years. 3 Although the mechanism for the increased muscle fatiguability in the SCI population is unknown, a current theory of neuromuscular fatigue in the able-bodied (AB) population involves a decrease in muscle excitability due to increased extracellular K þ . The sodium potassium pump (Na þ /K þ ATPase) is largely responsible for maintaining concentration gradients of Na þ and K þ across the sarcolemmal membrane, allowing for muscle excitability to be maintained during repetitive activation.…”

Altered mRNA expression after long‐term soleus electrical stimulation training in humans with paralysis

“…Spinal cord injury (SCI) induces severe atrophy of paralyzed muscles as well as an increase in contractile speed and a reduction in peak torque, fatigue index, and torque–time integral 1, 2. These changes are central features of the chronic musculoskeletal deterioration that contributes to morbidity and mortality in SCI subjects 3…”

“…Muscle training, accomplished via transcutaneous electrical stimulation of paralyzed muscle, can prevent or reverse the effects of SCI on muscle mass and function 1, 2. However, the molecular mechanisms of electrical stimulation–mediated muscle training remain unknown, as do the majority of post‐SCI muscle molecular adaptations 4–8.…”

Depressive Mood and Fatigue in Iranian Patients With Spinal Cord Injury and Spared Walking Ability

“…In this field, classification of patients with SCI according to their ability to walk (with canes, walker or orthoses) and their spinal cord independence score (SCIM) lead to a proper categorization of these patients into subgroups based on their abilities. Previously, development of fatigue and depression were described in patients with SCI, regardless of their abilities (8). Here, we investigated only individuals with SCI with ability to walk (assisted or nonassisted) with high SCIM score, so that their restricted physical ability could be related to fatigue development and not to innerva-tion-induced immobility.…”