• Sex hormone fluctuations on muscle stretch reflex at relaxed and active states. [ Time Frame: 2 months ]
  Blood draw for Estradiol, Progesterone, and Testosterone.
• Tendon tap reflex testing. [ Time Frame: 2 months ]
  The muscle reflex will be evoked by tapping the patellar tendon with a hammer instrumented with a load cell. The load cell instrumented hammer will allow us to quantify the reflex gain by normalizing the reflex EMG by the tap force.
• Anterior knee laxity measurement. [ Time Frame: 2 months ]
  To assess the joint's connective tissue response at varying points in the cycle, anterior knee laxity (AKL) will be measured with the knee arthrometer.

• H-reflex testing. [ Time Frame: 2 months ]
  For the monosynaptic H-reflex testing, a stimulating bar bipolar electrode will be placed over the tibial nerve in the popliteal fossa with the anode positioned distally. The H-reflex and M-Wave will be recorded from the soleus muscle using surface EMG electrodes placed over the muscle belly at the myotendinous junction.
• Polysynaptic reflex testing. [ Time Frame: 2 months ]
  For polysynaptic reflex testing, stimulation will be applied behind the medial malleolus of the ankle to the distal tibial nerve using a Grass Instruments S48 Stimulator, a Grass SIU5 Stimulus Isolation Unit and a Grass Constant Current Unit. EMG recordings will be taken from the tibialis anterior muscle using self-adhesive surface electrodes. Data collection will include 20 series of 8 train pulses.

• Sex hormone fluctuations on cortical excitability using transcranial magnetic stimulation (TMS). [ Time Frame: 1 month ]
  MRI images will be acquired prior to the first TMS laboratory visit. The TMS coil will be positioned over the intersection of the central sulcus with the mid-sagittal plane using the subject's own anatomical MRI using an optical navigation system. Surface EMG will be recorded from muscles in the right leg. The paired-pulse paradigm isolates any modulatory influence of hormone concentration at cortex. With the subject seated with hip, knee, and ankle flexed at 90 degrees, TMS will be delivered to the leg area of the left motor cortex through using one of our TMS systems.
• Ovulation testing. [ Time Frame: 1 month ]
  Urine ovulation kit.

Inclusion Criteria:

• Females: ages 18-39 years, who are eumenorrheic (regular monthly cycles of 24-35 days) or on a stable hormonal contraceptive regimen for 6 months (oral, transdermal or vaginal), no history of pregnancy, moderately active (less than 7 hours of vigorous physical activity per week)
• Males: Ages 18-39

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).
• For female participants only: Point of care screening for anemia will be completed, and individuals with hemoglobin levels <11.6 g/dl will be excluded from participating in the study.
• Specific exclusion criteria for TMS (male and female): pacemaker, metal implants in the head region, history of epilepsy or seizures, skull fractures or skull deficits, concussion within the last 6 months, unexplained recurring headaches, medications that lower seizure threshold, and pregnancy.
• Additional exclusion criteria for female participants: History of menstrual dysfunction (primary or secondary amenorrhea, oligomenorrhea, anovulatory cycles, polycystic ovarian disease), current or past pregnancy, 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. The reason for excluding highly active or competitive athletes is due to the high rate of undiagnosed menstrual dysfunction in females of this population.
• This study will not include: adults unable to consent, Individuals who are not yet adults (infants, children, teenagers), pregnant women or prisoners.