Early Neuromuscular Electrical Stimulation After ACL Reconstruction

A clinical viewpoint (4) propose that clinicians should distinguish between three clinically meaningful levels of quadriceps activation to guide rehabilitation: A minimal, adequate, and fast activation.

• Minimal activation: The patient can perform a straight leg raise without an extension lag and can perceive a quadriceps contraction. However, activation remains inhibited and is often insufficient for weight-bearing or resisted tasks.
• Adequate activation: The patient can achieve and maintain full knee extension during the required activity and produce a high-quality voluntary quadriceps contraction throughout functional movement. This reflects sufficient activation to progressively load the quadriceps.
• Fast activation: The patient can generate force rapidly (high rate of force development), a quality that becomes increasingly important during the later stages of rehabilitation for running, plyometrics, and return-to-sport activities.

In addition, the severity of activation failure can be classified using the following clinical AMI grading system (4):

• Grade 0: Normal contraction
• Grade 1: Inhibited contraction without extension deficit
• Grade 2: Inhibited contraction with extension deficit
• Grade 3: Passive extension deficit (chronic limitation and may require surgery)

Rehabilitation goals: Graft protection, manage swelling, control pain, restore voluntary quadriceps activation, minimize muscle atrophy, improve passive and active knee extension, and achieve a straight leg raise without lag.

Role of NMES: This is the phase where NMES is likely to have its greatest clinical impact. During the first two postoperative weeks, AMI is at its most severe and the window for preserving muscle fiber size is narrow. At this stage, many patients are unable to generate an effective voluntary quadriceps contraction, making NMES a valuable adjunct to exercise as early as possible post-operatively.

At this stage, use NMES to:

• Facilitate voluntary quadriceps activation during and after sessions
• Maintain muscle fibre recruitment during the period of enforced rest
• Reduce the experience of joint pain (analgesic effect), allowing earlier engagement with exercise
• Support management of joint effusion

Clinical approach: Apply NMES together with isometric muscle contraction during exercises where possible. If the patient cannot yet co-contract voluntarily, passive NMES is appropriate while working toward superimposed use.

Monitoring: Assess activation quality at each session. Can the patient sustain a contraction? Is there visible VMO engagement?

Combine with cryotherapy. Cryotherapy may reduce afferent inhibitory signals from joint mechanoreceptors, temporarily lowering the AMI threshold and making NMES-evoked contractions more accessible (7). Apply ice for 15–20 minutes prior to NMES where feasible.

Rehabilitation goals: Graft protection, manage swelling, control pain, restore voluntary quadriceps activation, minimize muscle atrophy, improve passive and active knee extension, improve knee range of motion (0–120°), and begin progressive open and closed kinetic chain exercises.

Role of NMES: Continue NMES as a rehabilitation warm-up and superimposed during strengthening exercises. This is the phase in which reaching the target intensity (≥100 mA / visible tetany) should be prioritized.

Clinical approach: Superimpose NMES during isometric quadriceps contractions at 30°, 60°, and 90° of knee flexion, terminal knee extensions, short arc quads, progressive straight leg raises with weight, and the introduction of leg press (after week 3) and squat patterns.

For hamstring tendon graft patients, NMES to the hamstrings may be introduced from approximately 6 weeks post-operatively, once the harvest site has healed and isometric hamstring loading is clinically appropriate (7). Electrode placement directly over the harvest incision should be avoided until the wound is fully closed.

Monitoring: Track activation quality subjectively (contraction visibility, patient feedback) and objectively where possible.

Combine with BFR training. NMES and BFR target different mechanisms and are appropriate to use simultaneous.

Rehabilitation goals: Progressive voluntary muscle activation and rate of force development toward ≥70% limb symmetry index at 12–16 weeks when integrating running and plyometric exercises.

Role of NMES: TThe role of NMES diminishes progressively as voluntary activation improves. Traditional progressive resistance training becomes the primary driver of strength recovery. NMES may still be used in patients with persistent activation deficits or superimposed during higher-load exercises as a neuromuscular facilitation tool.

A concept that is often underemphasized in post-ACLR rehabilitation is the distinction between restoring adequate activation and restoring fast activation. Rate of force development (RFD), the speed at which force can be generated, is [ab2.1]largely determined by motor unit firing rate rather than maximal peak force and deserves dedicated focus as rehabilitation progresses.

Clinical decision-making: If a patient reaches 10–12 weeks post-operatively and still cannot produce a quality quadriceps contraction during loaded tasks, or their limb symmetry index on isometric testing remains below 70%, this is a flag for reassessment. Consider whether AMI remains a contributing factor (persistent effusion, inadequately managed pain) and maintain NMES alongside other modalities.

Use the highest intensity you can tolerate. Where possible, NMES should be delivered while the patient simultaneously attempts a voluntary contraction as superimposed NMES has consistently shown superior results to passive application. A progressive NMES hierarchy can be applied (6).

• Static passive NMES: NMES [ab3.1]delivered without an active voluntary contraction, progressing the intensity until a visible contraction is achieved
• Static superimposed NMES: NMES [ab4.1]delivered with a simultaneous voluntary contraction
• Dynamic superimposed NMES: NMES delivered progressively through closed and open chain. NMES can additionally be combined with blood flow restriction.

Effective use of NMES requires ongoing clinical assessment. Key markers to track:

Activation quality: can the patient produce a visible, sustained voluntary contraction? Is there VMO engagement? Does activation quality change under load?

Evoked force: the actual torque or contraction magnitude produced by the NMES stimulus. This is the major determinant of NMES effectiveness.

Intensity tolerance: How does the patient’s tolerable NMES intensity change session to session? Plateau or regression may indicate inadequate pain or effusion management

Effusion: Persistent joint effusion perpetuates AMI. Assessment with the stroke test at each session. If effusion increases with exercise load, the load is too high and AMI will worsen.

Limb symmetry index: Tracking isometric quadriceps strength symmetry provides objective data to guide NMES weaning and progression. A limb symmetry index below 70% should prompt continued or intensified NMES use.

NMES should not be used in patients with:

• A pacemaker or implanted electronic device
• Active blood clot (DVT)
• Active infection in the area
• Active bleeding disorder
• Cancer in the treatment area
• Untreated heart rhythm disorders (avoid chest area)
• Seizure disorders (avoid head/neck)
• Pregnancy (avoid abdomen/low back)

NMES should be introduced within the first 2–3 postoperative days. Delaying NMES by several weeks misses the period when quadriceps activation failure is most pronounced and muscle atrophy develops most rapidly. Start early, progress the intensity, and integrate NMES within a structured, criterion-based rehabilitation plan.