Postfatigue potentiation of the paralyzed soleus muscle: evidence for adaptation with long-term electrical stimulation training

Abstract

Understanding the torque output behavior of paralyzed muscle has important implications for the use of functional neuromuscular electrical stimulation systems. Postfatigue potentiation is an augmentation of peak muscle torque during repetitive activation after a fatigue protocol. The purposes of this study were 1) to quantify postfatigue potentiation in the acutely and chronically paralyzed soleus and 2) to determine the effect of long-term soleus electrical stimulation training on the potentiation characteristics of recently paralyzed soleus muscle. Five subjects with chronic paralysis (>2 yr) demonstrated significant postfatigue potentiation during a repetitive soleus activation protocol that induced low-frequency fatigue. Ten subjects with acute paralysis (<6 mo) demonstrated no torque potentiation in response to repetitive stimulation. Seven of these acute subjects completed 2 yr of home-based isometric soleus electrical stimulation training of one limb (compliance = 83%; 8,300 contractions/wk). With the early implementation of electrically stimulated training, potentiation characteristics of trained soleus muscles were preserved as in the acute postinjury state. In contrast, untrained limbs showed marked postfatigue potentiation at 2 yr after spinal cord injury (SCI). A single acute SCI subject who was followed longitudinally developed potentiation characteristics very similar to the untrained limbs of the training subjects. The results of the present investigation support that postfatigue potentiation is a characteristic of fast-fatigable muscle and can be prevented by timely neuromuscular electrical stimulation training. Potentiation is an important consideration in the design of functional electrical stimulation control systems for people with SCI.

Fig. 1

Representative examples for torque and electromyograph over several trains. Subject numbers refer to Table 1. A: chronic spinal cord injury torque (subject C1). Inset, point during the summated train at which we obtained the rate of torque development. B: M waves elicited by the first stimulus pulse of the trains depicted in A. The stability of the M waves suggests that potentiation of torque between trains 1 and 25 was not due to changes in neuromuscular propagation. C: subject A7, acute spinal cord injury pretraining torque. D: M waves elicited by the first stimulus pulse of the trains depicted in C.

Fig. 2

A: mean (SE) torque for acute and chronic spinal cord-injured subjects; n, no. of subjects. B: mean (SE) torque expressed as a percentage of train 1; n, no. of subjects. C: single subject who was tested during the acute stage of spinal cord injury and who returned 1.5 yr later for testing as a chronic spinal cord injury subject. The subject did no electrical stimulation training in the interim.

Fig. 3

Potentiation index for acute and chronic spinal cord-injured subjects. *Significant difference, P < 0.05.

Fig. 4

A: representative examples of the final 3 summated twitches of trains 1 and 25 for the acute spinal cord-injured and chronic spinal cord-injured cohorts. Solid lines, train 1; dashed lines, train 25. B: mean (SE) ratio of the absolute and normalized rate of torque development at train 30 to the rate of torque development at train 1. *Significant difference from acute condition, P < 0.05. **Significant difference from chronic normalized condition, P < 0.05.

Fig. 5

Representative examples for torque and electromyograph over several trains. A: trained limb after 2 yr of training. B: untrained control limb after 2 yr of contralateral limb training.

Fig. 6

A: mean (SE) torque for the trained and untrained limbs at 2 yr of training; n, no. of subjects. B: mean (SE) torque expressed as a percentage of train 1; n, no. of subjects.

Fig. 7

Potentiation index for trained and untrained limbs of the training group. *Significant difference, P < 0.05.

Fig. 8

A: representative examples of the final 3 summated twitches for train 1 and train 25 for the trained and untrained limbs at 2 yr of training. Solid lines, train 1; dashed lines, train 25. B: mean (SE) ratio of the absolute and normalized rate of torque development at train 30 to the rate of torque development at train 1.