Musculoskeletal adaptations in chronic spinal cord injury: effects of long-term soleus electrical stimulation training

Abstract

Objective: The purpose of this study was to determine whether long-term electrical stimulation training of the paralyzed soleus could change this muscle's physiological properties (torque, fatigue index, potentiation index, torque-time integral) and increase tibia bone mineral density.

Methods: Four men with chronic (>2 years) complete spinal cord injury (SCI; American Spinal Injury Association classification A) trained 1 soleus muscle using an isometric plantar flexion electrical stimulation protocol. The untrained limb served as a within-subject control. The protocol involved ~ 30 minutes of training each day, 5 days a week, for a period of 6 to 11 months. Mean compliance over 11 months of training was 91% for 3 subjects. A fourth subject achieved high compliance after only 5 months of training. Mean estimated compressive loads delivered to the tibia were approximately 110% of body weight. Over the 11 months of training, the muscle plantar flexion torque, fatigue index, potentiation index, and torque-time integral were evaluated periodically. Bone mineral density (dual-energy x-ray absorptiometry) was evaluated before and after the training program.

Results: The trained limb fatigue index, potentiation index, and torque-time integral showed rapid and robust training effects (P<.05). Soleus electrical stimulation training yielded no changes to the proximal tibia bone mineral density, as measured by dual-energy x-ray absorptiometry. The subject with low compliance experienced fatigue index and torque-time integral improvements only when his compliance surpassed 80%. In contrast, his potentiation index showed adaptations even when compliance was low.

Conclusions: These findings highlight the persistent adaptive capabilities of chronically paralyzed muscle but suggest that preventing musculoskeletal adaptations after SCI may be more effective than reversing changes in the chronic condition.

Figure 1

Representative example of plantar flexion torque produced by 10-pulse stimulus trains during a 125-train fatigue bout (stimulation started with the muscle in a non-fatigued state). A) Subject 1, month 0; FI = 40%. B) Subject 1, month 11; FI = 58%. The area under the torque curve for train 125 is indicative of the minimum integral for each month.

Figure 2

Mean (SE) percentage of baseline (month 0) values for 3 subjects with high compliance (>80%). Two subjects continued training until month 11. Their month 11 data are presented individually (Figure 3). PT = peak torque; FI = fatigue index; MI = minimum integral.

Figure 3

A) Peak torque. B) Fatigue index. C) Minimum torque-time integral for 3 subjects with high compliance in training. Filled symbols represent the trained limbs, and unfilled symbols represent the untrained limbs. Circles, squares, and triangles represent subjects 1, 3, and 4, respectively (Table 1).

Figure 4

Representative examples of plantar flexion torque produced by 10-pulse stimulus trains during the first 25 contractions of a 120-train fatigue bout performed 15 minutes after a fatigue protocol. A) Subject 1, month 0; potentiation index (PI) = 2.07. B) Subject 1, month 11; PI = 1.29.

Figure 5

Mean (SE) potentiation index for 3 subjects with high compliance (>80%; months 1–6). Two subjects continued training until month 11.

Figure 6

The single subject (subject 2 in Table 1) who demonstrated low compliance at the beginning of training. The dotted line represents 80% compliance. Data are given through month 7 to aid comparison between this subject’s magnitude of effect and that of the other 3 subjects. A) Torque, fatigue index, and minimum integral. B) Potentiation index.

Figure 7

Mean (SE) trained and untrained limb proximal tibia bone mineral density (BMD), before and after training. Black symbols represent each individual subject.