The goal of this project is to test our central hypothesis that the spinal cord neuroplasticity in females will be modulated by the level of estradiol concentration. under aim 1 we will determine the influence of estradiol fluctuations on spinal circuit excitability post afferent (sensory) mediated subthreshold motor priming in young healthy females and males. We will use an established repetitive peripheral nerve electrical stimulation with a stimulation intensity below the motor threshold to prime the spinal motor circuits. under aim 2 we seek to characterize the input output property of spinal circuit excitability after descending drive (motor) mediated priming in young healthy male participants. in aim 3 we will examine the influence of estradiol fluctuations on descending drive mediated motor priming in young healthy females.

FEMALES

Inclusion Criteria:

• Ages 18-39 years
• Eumenorrheic (regular monthly cycles of 24-35 days)
• Moderately active (less than 7 hours of vigorous physical activity per week)
• History of pregnancy is allowed if patient is in post-lactation phase

Exclusion Criteria:

• History of musculoskeletal or orthopedic injury of the spine, hip, knee, ankle or foot
• History of neurological injury of the peripheral or central nervous system
• Current smoker
• History of disordered eating
• History of stress fracture in the lower limb
• History of a connective tissue disorder (Marfan's syndrome, Ehlers-Danlos disease)
• Pacemaker, metal implants in the head and spine region
• Pregnancy
• On a hormonal contraceptive regimen (oral, transdermal or vaginal)
• History of menstrual dysfunction (primary or secondary amenorrhea, oligomenorrhea, anovulatory cycles, polycystic ovarian disease)
• Started or stopped taking oral contraceptives within the previous 6 months
• Exercise vigorously more than 7 hours per week or currently participating in competitive level sports.

MALES

Inclusion Criteria:

• Ages 18-39
• Moderately active (less than 7 hours of vigorous physical activity per week)

Exclusion Criteria:

• History of musculoskeletal or orthopedic injury of the spine, hip, knee, ankle or foot
• History of neurological injury of the peripheral or central nervous system
• Current smoker
• History of disordered eating
• History of stress fracture in the lower limb
• History of a connective tissue disorder (Marfan's syndrome, Ehlers-Danlos disease)
• Pacemaker, metal implants in the head and spine region

(1) Healthy females (2) Healthy males (control)