Table 7.

Summary of the Literature and Recommendations for Use of NMES in Anterior Cruciate Ligament Reconstruction

Indication	Parameter Recommendations	Outcome Measures Demonstrating Benefit
ACL reconstruction	Electrode placement: No standardized location reported in the literature. Recommended placement based on a synthesis of the literature: (1) quads on femoral nerve or muscle belly of rec fem or vastus intermedius and on MP or muscle belly of VM95–97or (2) quads (as above) and on hams (over muscle bellies of biceps femoris and semitendinosis or semimembranosis).98–101 Some studies placed electrodes on VL.102,103 Limb position: knee flexed to ~65° NMES waveform: low-frequency biphasic95,97,98,101,104–107 or medium-frequency burst-modulated AC99,103,108–110 Frequency: 30–50 Hz PC95,97,101,104–107 or 2500 Hz AC in 50 Hz bursts99,110,111 Pulse duration: 250–400 μs97,100,102,103,105–107,112,113 Current amplitude: individual max tolerated intensity; minimum at strong but comfortable muscle contraction95,97,99,100,105,106,109,112,113 Work–rest cycle: ON:OFF 6–10:12–50 s;95,98,101,103,105,106 use lower duty cycle–e.g., work–rest 1:3–1:5–if the muscle is weaker to limit fatigue associated with an electrically induced muscle contraction Treatment schedule: initiate ideally within 1 wk post-op:98–101 12–15 contractions/session98,99,102,103,108–110,112 Session frequency: 3×wk over 4–6 wk, particularly in the first 6 wk post-op98,101,110	Reduced pain (NPRS, VAS)98,105 Improved muscle strength (isometric and isokinetic, dynamometry, tensiometry)99,100,102–104,107–110,112,113 Reduction in loss of muscle volume or thickness (CT, MRI, US imaging)100,107,113 Self-reported function (ADL scale)108 Gait parameters (motion analysis)103 Achieving clinical milestones108 Limb circumference (tape measure)105,110 Functional performance (lateral step-up, anterior reach)100,101
Rationale for recommended NMES protocol	When reviewing the studies, difference in methodologies is obvious. It is evident that regardless of whether the stimulator used was a low-frequency PC or a medium-frequency burst-modulated AC device, the authors used some common parameters: (1) initiation of NMES on POD 1–2 and in some studies 1 wk post-op, (2) amplitude raised to max tolerated, and (3) 10–20 contractions/session in most cases. A study that used 300 contractions/d for 12 wk showed no advantage for strength until 52 wk post-op.104 For athletes who had not fully recovered strength at 6 or more mo post-op, initiating NMES at 6 or more mo post-op was beneficial.107 With respect to ON:OFF parameters, the studies show that short OFF periods (2–20 s) were applied only when ON times were short (5–6 s), frequency was low (20–30 Hz), or both. Short ON time or low frequency of stimulation results in motor unit sparing and thus slower onset of fatigue, which, in turn, reduces the OFF time needed for recovery. The literature does not show that strength improves using short ON and OFF times. 2 studies using short OFF periods95,107 compared 2 contrasting NMES protocols without a CON group; thus, the relative usefulness of these 2 protocols for strengthening cannot be elucidated. A further 2 studies97,106 showed no strength gain. Eriksson and Häggmark,96 with 5:6.5 s ON:OFF and an unusually high frequency of 200 Hz, used oxidative capacity as the only outcome measure, perhaps reflecting their intent to use Ex training to improve endurance, not strength. Strength was not measured in 2 other studies.101,105 Accordingly, our recommendations for strengthening quads are to initiate NMES as early as possible, even on POD 1 and ensure that the intensity elicits a maximum tolerated contraction; 10–15 contractions, 10–15 s ON:OFF duration, 3–5 times that of the ON time. Position the limb within the resting length of the quads (e.g., 65° flexion) to facilitate max force production.114 Some earlier studies used full extension, which is not advised because it places high strain on the ACL. In addition, studies with the knee <30° flexion have produced inferior outcomes.108
Physiological effect of NMES	In animal models, there is cellular and molecular evidence of positive changes in muscle with NMES after ACL surgery. NMES minimized connective tissue density in muscles and reduced MMP-2, increased both type IV collagen mRNA and protein levels,91 and minimized the accumulation of atrogenes and myostatin as well as prevented reduction in muscle mass early post-transection.115
Critical review of research evidence	We reviewed the individual RCTs identified by our search protocol as well as 2 SRs. Conclusions of the SRs were that the addition of NMES to rehabilitation Ex can improve strength92,116 and function92 at 6–8 wk post-op but is inconclusive for functional performance at 6 wk and self-reported function at 12–16 wk post-op.116 Earlier trials focused on use of NMES to reduce atrophy secondary to the prolonged immobilization post-surgery. Current ACL reconstruction protocols have significantly reduced duration of immobilization; accordingly, recent trials have focused on the use of NMES to address quads weakness secondary to both the original trauma and that incurred during surgery. NMES (using optimized parameters)+Ex is better than CON (Ex alone or Ex+sham NMES), especially when initiating treatment earlier post-op. Individual RCTs have limitations: In some cases, there is a risk of bias due to subjects, therapists, or outcome assessors being not blinded to group allocation. Some trials used parameters, particularly intensity, that are unlikely to induce improvements in strength. There are some instances of incomplete reporting or management of missing data points. Interpretation of the findings is complicated by differences in knee position, electrode position, type of stimulator (battery powered vs. console109), stimulation parameters, type of graft (quads vs. hams), duration of immobilization, time delay in initiating NMES (POD 2 vs. 3 wk vs. 24 wk), and failure to track compliance. Feasibility has been demonstrated: Recent studies have shown that patients tolerate NMES well even when initiated on POD 1–2. No adverse effects have been associated with NMES in this population.

NMES=neuromuscular electrical stimulation; ACL=anterior cruciate ligament; quads=quadriceps muscle; rec fem=rectus femoris muscle; MP=motor point; VM=vastus medialis; hams=hamstring muscles; VL=vastus lateralis; AC=alternating current; PC=pulsed current; max=maximum; post-op=post- operative; NPRS=numerical pain rating scale; VAS=visual analogue scale; CT=computed tomography; MRI=magnetic resonance imaging; US=ultrasound; ADL=activities of daily living; POD=post-operative day; Ex=exercise; MMP-2=matrixmetalloproteinase-2; mRNA=messenger ribonucleic acid; RCT=randomized controlled trial; SR=systematic review; CON=control.