• Younger adults aged 18–35 years with a body mass index (BMI) of 18.5–34.9 kg/m² who have participated in lower-body resistance training at least twice per week for the previous 6 months.
• Older adults aged 70 years and older with a body mass index (BMI) of 18.5–34.9 kg/m² who have not participated in lower-body resistance training during the previous 6 months.

• Are a recreationally active (≥150 minutes per week of physical activity) male between the ages of 18-35.
• Are willing to come in for all five visits.

• Participants must either be defined as resistance trained (engaging in resistance exercise with weights at least 2x/week for the past 8 weeks) OR untrained (do not meet ACSM criteria for being physically active: strength training less than 2x/week and less than 150 minutes of aerobic exercise per week)
• 18-50 years old
• Healthy (no pain or injury)

• Adults aged 18 years and older,
• Had lower extremity musculoskeletal surgery within the previous 6 months.

• are between the ages of 18-35 years,
• either have had an anterior cruciate ligament reconstruction (ACLR) surgery or have not had an injury to either of your knees,
• have not been medically diagnosed with a concussion in the previous 6 months,
• have not been medically diagnosed with colorblindness,
• have no known neurological (nervous system) disorder (i.e. Multiple Sclerosis),
• have not taken opioid medication (pain medication) in the last 2 weeks,
• are not pregnant of planning to become present during study participation, and
• have no contraindications to brain MRI (you will complete a screening form).

Inclusion criteria – ACL group:

• Age 18-35 years
• History of primary unilateral ACL reconstruction (ACLR) within last 2 years
• No history of any lower limb surgery besides the current ACLR
• Medical clearance to initiate jogging

Inclusion criteria – Uninjured group:

• Age 18-35 years
• No history of lower extremity surgery
• No history of lower extremity injury (defined as seeking medical attention) within last 2 years

• At least 18 years old
• A licensed or certified healthcare provider
• A student in an accredited professional healthcare program

• Age 12-45 years
• History of knee injury (e.g., ACL tear, meniscus tear, patellofemoral pain, osteoarthritis) or surgery (e.g., ACL reconstruction, meniscectomy or repair, total knee arthroplasty) within the last 2 years