Differential fatigue of paralyzed thenar muscles by stimuli of different intensities

Abstract

Muscles paralyzed by injury or disease fatigue excessively when stimulated. This study examined whether the first few paralyzed thenar motor units recruited by electrical stimulation of the median nerve were more fatigue resistant than the total thenar motor unit population. The paralyzed thenar muscles of four subjects with chronic cervical spinal cord injury were fatigued by a 2-min intermittent 40-HZ protocol on 2 days. One experiment involved submaximal stimulation, the other supramaximal stimulation. These stimuli resulted in activation of part and all of the thenar muscles, respectively. Relative force loss, force-time integral decline, and slowing of half-relaxation time were always significantly less when only part rather than all of the muscles was fatigued. The part of the paralyzed muscles that was activated was also relatively fatigue resistant compared with control single thenar motor units. Thus, a reversal of recruitment order from fatigable to fatigue-resistant units cannot explain the extreme fatigability of paralyzed muscles. Use of submaximal stimulation during functional electrical stimulation may therefore help to reduce muscle fatigue because it recruits the more fatigue-resistant units.

Maximal versus submaximal intensity stimulation with variable patterns

“…Submaximal nerve stimulation recruits motor units with fast‐conducting axons before slow‐conducting ones (in contrast to voluntary contraction) in animal models 9, 15. However, in humans, motor unit recruitment has been found to be fast to slow44 or random35 in healthy muscle, and fatigue‐resistant to fatigable (slow to fast) in paralyzed muscle 17. The types of motor units activated during submaximal stimulation in our study are unclear.…”

“…During the 20‐H Z protocol, forces during the fatigue test decreased by an average of 42% during maximal stimulation and 29% during submaximal stimulation, indicating that more fast‐fatigable motor units may have been recruited during the maximal stimulation. Similarly, Godfrey et al17 found that paralyzed muscle was more fatigable when using maximal rather than submaximal nerve stimulation.…”

“…Patients with spinal cord injury or stroke frequently lose grasp, pinch, and manipulation skills that are vital for independent living 33. Although previous work has examined variable patterns of stimulation and the fatigue effects of NMES in the upper extremity,3, 17–20, 36–38 the specific NMES parameters that are optimal for reducing motor deficits in the neurologically impaired hand remain unclear.…”

Fatigue of muscles weakened by death of motoneurons

“…It is more common to submaximally activate muscles35, 56, 149 because of the potential for bone fracture and the discomfort that accompanies supramaximal stimulation in individuals with intact sensation. With submaximal contractions, it is unclear whether the force decline reflects the intrinsic fatigability of the paralyzed fibers, a decrease in muscle activation due to reductions in axon excitability,95 or both possibilities 57. Reductions in axon excitability probably explain why EMG and force decline together in some studies of paralyzed muscles 124…”

Minimizing muscle fatigue through optimization of electrical stimulation parameters

“…[18,38] Nevertheless, other researchers have also used a threshold of 50%. [34,39,40] Because In the present results, at lower levels of injected PA×PD, a low signal-to-noise ratio combined with low security in torque production resulted in low bilateral symmetry (see Table 1) and higher sensitivity to measurement error (e.g., when low torque levels [< 10 Nm] are generated in Figure 3). However, our measurement precision was sufficiently high at higher levels of injected PA×PD, where both torque production and fatigue were substantial that has higher clinical relevance.…”