Na⁺,K⁺‐ATPase concentration and fiber type distribution after spinal cord injury

Abstract

Complete spinal cord injury (SCI) is characterized, in part, by reduced fatigue-resistance of the paralyzed skeletal muscle during stimulated contractions, but the underlying mechanisms are not fully understood. The effects of complete SCI on skeletal muscle Na(+),K(+)-adenosine triphosphatase (ATPase) concentration, and fiber type distribution were therefore investigated. Six individuals (aged 32.0 +/- 5.3 years) with complete paraplegia (T4-T10; 1-19 years since injury) participated. There was a significantly lower Na(+),K(+)-ATPase concentration in the paralyzed vastus lateralis (VL) when compared to either the subjects' own unaffected deltoid or literature values (from our laboratory, utilizing the same methodology) of VL Na(+),K(+)-ATPase concentration for the healthy able-bodied (141.6 +/- 50.0, 213.4 +/- 23.9, 339 +/- 16 pmol/g wet wt., respectively; P < 0.05). There was also a significant negative correlation between the Na(+),K(+)-ATPase concentration in the paralyzed VL and years since injury (r = -0.75, P < 0.05). These findings are clinically relevant as they suggest that reductions in Na(+),K(+)-ATPase contribute to the fatigability of paralyzed muscle after SCI. Unexpectedly, the VL muscles of our subjects had a higher proportion of their area represented by type I fibers compared to literature values for the VL of the healthy able-bodied (52.6 +/- 25.3% vs. 36 +/- 11.3%, respectively; P < 0.05). As all our subjects had upper motor neuron injuries and, therefore, experienced muscle spasticity, our findings warrant further investigation into the relationship between muscle spasticity and fiber type expression after SCI.

Spasticity may defend skeletal muscle size and composition after incomplete spinal cord injury

“…23 There is also a suggestion that spasticity contributes to the predominance of slow fibers in individuals with SCI. 40 Slow fibers are characterized by greater mitochondrial density compared to fast fibers. This could have resulted in partitioning of IMF towards oxidation in the HLI group rather than storage in the LLI group.…”

The length–tension relationship of human dorsiflexor and plantarflexor muscles after spinal cord injury

“…Previous reports have suggested a preservation of contractile properties and/or an increased expression of slow twitch fibres in muscle that experiences a significant degree of spasticity after SCI. 15,16 Furthermore, Stein et al 17 suggested that if spasticity occurs more frequently in the extensor muscles (such as the soleus), the tibialis anterior muscle would be placed in a stretched position, thus preserving the torque-generating capacity of that muscle. In contrast, the extensor muscles would be exposed to more shortened positions because of both the prolonged sitting Table 3 M-wave amplitude (mV) at various joint angles for able-bodied (AB) and spinal cord-injured (SCI) participants position in a wheelchair and the repeated bouts of spasticity.…”

The effects of body-weight supported treadmill training on cardiovascular regulation in individuals with motor-complete SCI

“…3,4 In addition, although functional electrically stimulated (FES) exercise has shown great promise as a cardiovascular stimulus, [5][6][7] it carries the risks of burns to the skin, autonomic dysreflexia and bone fracture, and further, stimulated contractions may not be evoked in those with lower motor neuron injuries. 8 The cardiovascular benefit of FES exercise may also be limited as individuals with SCI are more prone to muscle fatigue, [9][10][11] and may only perform such exercise for relatively short durations. 11 In the last decade, body-weight supported treadmill training (BWSTT) has shown promise as a means of enhancing gait recovery in SCI individuals with partially spared motor function, [12][13][14][15][16] but not in those with motorcomplete SCI.…”