The investigators are interested in determining what treatments are effective in reducing inhibition after anterior cruciate ligament (ACL) surgery and helping people increase quadriceps strength. Two interventions that are often used clinically are neuromuscular electrical stimulation (NMES) and vibration. This is a study of how effective these treatments are, whether they work the same for everyone, and if one is better than the other. 22 participants will be enrolled and on study for 2 to 3 weeks.

After a screening visit, participants will be randomized to 2 intervention conditions over 2, 60-minute visits at least 1 week apart where they will complete:

• a 5 minute low intensity warm-up on a stationary bike

• a series of patient-reported outcomes surveys

• a pre-intervention quadriceps performance assessment

• the intervention condition, one each visit, either:

  • ~20 minutes of NMES while completing rehabilitation exercises, OR
  • ~20 minutes of hamstring muscle vibration

• a post-intervention quadriceps performance assessment

The primary objective of the study is to characterize the acute effects of interventions designed to mitigate the influence of inhibition (NMES and hamstrings muscle vibration) on quadriceps muscle performance in people post-anterior cruciate ligament reconstruction (ACLR) who have significant potential to benefit from these interventions (e.g. less than 4 months post-surgery).

• Specific Aim 1: To determine the acute effects of quadriceps NMES and hamstrings muscle vibration on quadriceps muscle performance in people 2-4 months post-ACLR
• Specific Aim 2: To determine if individuals with signs of more pronounced neural inhibition (quantified via assessment of knee extensor torque steadiness) have greater responses to NMES and hamstrings muscle vibration

Hypothesis 1: Quadriceps muscle strength will increase in people 2-4 months post-ACLR after 20 minutes of quadriceps NMES or hamstrings muscle vibration. The magnitude of increase in strength will be similar for both interventions.

Hypothesis 2: Surgical limb knee extensor torque steadiness will be strongly positively correlated with changes in quadriceps strength after NMES or hamstrings muscle vibration. People with greater steadiness impairments will demonstrate greater acute increases in quadriceps strength.

Inclusion Criteria:

• Have undergone ACLR within the previous 4 months
• Are at least 2 months post-ACLR

Exclusion Criteria:

• A history of ACLR on both legs

• Since the most recent ACLR:

  • secondary lower extremity surgery
  • leg or foot fracture

• Unable to walk without an assistive device

• Pain of greater than 5/10 during quadriceps strengthening exercises with the surgical limb

• Cannot achieve 90 degrees of knee flexion with the surgical limb

• People with contraindications to NMES

• Anyone who had a previous skin reaction to NMES or attempted but could not tolerate NMES

• Peripheral neuropathy or other neurological conditions