Five out of 6 patients completed strength training as prescribed for the NMES group. All 5 patients completed training as prescribed in the volitional group. The dam from 1 NMES patient were excluded from final analysis because of a localized irritation at the lead passing site of the left medial gastrocnemius electrode; this patient did not complete the NMES strength training as structured on the training calendar. This patient, on multiple occasions during the last week, performed 2 sets of 15 repetitions for each muscle group to obtain the goal number of training sessions. This made the patient sore and most likely impaired her force-generating ability.
There were no differences in initial body mass (NMES = 40.12 [SD, 10.68] kg, volitional = 34.04 [SD, 8.88] kg; t = 0.88; P= 20) or change in body mass between the 2 groups over the 12-week intervention (NMES = 0.48 [SD, 1.24] kg, volitional = 0.74 [SD, 1.93] kg; t = −0.22; P = .414). The quadriceps and triceps surae NMES training dose was measured relative to the patient’s MVIC obtained at 3 time points: pretraining, after 6 weeks of training, and posttraining. The quadriceps NMES training dose increased from 81.8% to 118.9% of the MVIC after 6 weeks of training (). We were not, however, on average able to increase the NMES training dose for the triceps surae muscles at the 6-week time point owing to suboptimal electrode placement at implantation or owing to patient tolerance ().
Neuromuscular Electrical Stimulation Training Doses (% maximum voluntary isometric contraction force)
The quadriceps lemons MVIC force normalized to body weight increased more over the 12-week strength-training intervention for the NMES group compared to the volitional group (; NMES +1.61 N/kg, volitional +0.52 N/kg; t = 1.69; P = .065). A similar pattern of change in normalized force production occurred for the triceps surae muscles with the NMES group increasing force generation more than the volitional group (; NMES +0.80 N/kg, volitional −0.29 N/kg; t = 1.88; P = .049).
Figure 3 Quadriceps femoris (A) and triceps surae (B) force production normalized to body weight (N/kg) before (open bars) and after (solid bars) neuromuscular electrical stimulation (NMES; n = 5) and volitional (n = 5) 12-week strength-training programs in children (more ...)
In an attempt to explain the changes in force production, we examined the changes in voluntary muscle activation and muscle CSA. We had hypothesized that the volitional group would experience a greater increase in voluntary muscle activation because of a practice effect from their training; however, no between-group differences were observed for the quadriceps (; t = 1.20; P = .13) or triceps surae (; t = −1.25; P = .12). Both groups, however, had significant gains in voluntary activation of the quadriceps (NMES +0.057, t = −1.678, P = .084; volitional +0.134, t = −2.478, P = .034) but not in voluntary activation of the triceps surae (NMES +0.104, t = −1.136, P = 160; volitional −0.098, t = 0.732, P = .252). For muscle CSA, we had hypothesized that the NMES group would show greater increases in CSA than the volitional group because we predicted that we would produce greater training forces with NMES. We found that training with NMES produced greater changes in quadriceps CSA than volitional training (; NMES +4.42 cm2, volitional +2.36 cm2; t = 2.52, P = .023), but not in triceps surae CSA (; NMES +1.28 cm2, volitional +0.35 cm2; t = .77, P = .232). For the triceps surae, neither group showed significant muscle hypertrophy following training (NMES t = −1.223, P = .144; volitional t = −1.368, P = .132).
To gain a better sense of the contribution that voluntary activation and muscle CSA made toward changes in force production, linear regressions were used to compare the percentage change in voluntary activation or muscle CSA to the percentage change in MVIC force pooled across both muscle groups for each treatment group. The variability in voluntary activation of the agonists explained approximately 67% of the variability in the MVIC force for the NMES group (; r2 = .67; F = 15.944; P = .004) and approximately 37% of the variability in MVIC force in the volitional group (; r2 = .37, F = 4.737, P = .061). No relationship was seen, however, between the percent change in maximum CSA and the percentage change in MVIC force for either the NMES (r2 = .001, F = 0,007, P = .933) or volitional groups (r2 = .074, F = 0.481, P = .514).
Figure 4 The relationship between the percentage change in voluntary activation and the percentage change in maximum voluntary isometric contraction force in the (A) neuromuscular electrical stimulation (NMES) and (B) volitional strength-training groups after (more ...)
Walking speed was used to examine if either NMES or volitional training had an impact on gait performance (). There was no pre-post intervention difference between the NMES and volitional groups (t = 1.149; P = .142). The NMES group, however, showed a within-group increase in walking speed from 80.47 (SD, 16.80) cm/s at baseline to 96.37 (SD, 25.78) cm/s after 12 weeks of training (t = 2,671; P = .028), whereas the volitional group did not (85.57 [SD, 27.85] cm/s to 89.99 [SD, 23.14] cm/s; t= 0.551; P = .306).
Self-selected walking speed before (open bars) and after (solid bars) a 12-week strength-training program that uses either neuromuscular electrical stimulation (NMES) or volitional isometric contractions. * P < .1.