The protocol is similar to the ASLR test, except that the participant is instructed to maintain the elevated leg off the ground for as long as possible. A biofeedback air cuff will be placed under the participant's lumbopelvic region to assess movement of the spine/pelvis. The cuff will be inflated to 40 mm Hg, and the participant instructed to keep the needle as close to 40 mm Hg as possible throughout the test; no information will be provided on how to affect cuff pressure, but visual feedback of cuff pressure will be provided throughout the task. Ratings of perceived exertion (RPE) and pain will be obtained from participants every 30-60 seconds. Task failure will be defined as a heel height ≤10 cm or a change in cuff pressure ≥20 mm Hg. Both limbs will be tested, but in different sessions. The order of limb testing (dominant vs non-dominant) will be randomized and counter-balanced. Limb dominance will be self-reported.

A straight leg raise maximal voluntary contraction (MVC) of each lower extremity an MVC of the hip extensors of the grounded limb will be performed in supine before and within 2 minutes of completing the ASLR fatigue task. The straight leg raise MVC will be performed with a custom-made load cell instrumented strength testing device, which consists of a rigid platform on which the participant will lie. At the foot of the device is a tower that houses 2 load cells connected to a lightly padded push plate.

The hip extension MVC will be performed on a force plate. Before the ASLR fatigue task, a minimum of 3 MVC trials will be performed for each muscle group, with a minimum of 1 minute rest between trials; the highest peak force will be considered the MVC. Only 1 MVC trial per muscle group will be performed following the ASLR Fatigue Task.

Real time ultrasound will be used to assess inter-recti distance above and below the umbilicus. Participants will be assessed with B Mode images in supine at rest, in supine while lifting their head, and in supine while performing a straight leg raise. The Principle Investigator has training and experience in musculoskeletal ultrasound.

For the ASLR Fatigue Task, the participant is instructed to maintain the elevated leg off the ground for as long as possible. Task failure will be defined as a heel height greater than or equal to 10 centimeters off the ground.

A straight leg raise maximal voluntary contraction (MVC) of each lower extremity will be performed in the supine position before and within 2 minutes of completing the ASLR fatigue task with a custom made load cell instrumented strength testing device. This device consists of a rigid platform on which the study participant will lie supine. At the foot of the device is a tower that houses 2 load cells and is connected to a lightly padded push plate. This push plate can be raised and lowered such that the participant can push up into the plate (contacts the anterior surface of the participant's shin) or push down onto the push plate (contacts the posterior aspect of the participant's heel).

Before the ASLR fatigue task, a minimum of 3 MVC trials will be performed on each leg, with a minimum of 1 minute rest between trials, in order to ensure a true maximum has been obtained; the highest peak force will be considered the MVC. Only 1 MVC trial per leg will be performed following the

A hip extension MVC will also be performed in supine (pushing down into the ground) on a force plate (1000 Hz) prior to performance of the ASLR fatigue task. Only the leg that will remain in contact with the force plate during the ASLR fatigue task (grounded limb) will perform the extension MVC, in order to normalize downward pressure into the force plate by the grounded limb during performance of the ASLR fatigue task. This will allow us to quantify activity of the grounded limb during the fatiguing task. A minimum of 3 hip extension MVCs will be performed, with 1 minute rest between trials. An MVC may also be assessed at the end of the fatiguing exercise bout.

An inertial measurement unit (Trigno, Delsys, Boston, MA) will be placed on the test limb to track movement of the limb during performance of the ASLR Fatigue Task. Fluctuations in limb acceleration will be measured and quantified as the standard deviation of acceleration. Limb steadiness will be collected online, stored, and analyzed offline with ConsensysPRO software. The investigators hypothesize that fluctuations in limb acceleration will be greater among postpartum women as compared to nulligravid women during the performance of the ASLR Fatigue Task.

Real time ultrasound will be used to assess inter-recti distance above and below the umbilicus. Participants will be assessed with B Mode images in supine at rest, in supine while lifting their head, and in supine while performing a straight leg raise.

For the ASLR Fatigue Task, the participant is instructed to maintain the elevated leg off the ground for as long as possible. A biofeedback air cuff placed under the participant's lumbopelvic region assesses movement of the spine/pelvis during the test. The cuff will be inflated to 40 mm Hg, and the participant will be instructed to keep the needle as close to 40 mm Hg as possible throughout the test. Pain Scores will be measured (0-10 with higher scores increased pan) every 30-60 seconds. Task failure will be defined as a heel height ≤10 cm or a change in cuff pressure ≥20 mm Hg. Both limbs will be tested in different sessions.

Physical activity habits around the time of testing will be assessed with a tri-axial accelerometer for 7 days outside of the laboratory, in part by measuring the TAC per day. TAC is the total number of filtered, full-wave rectified, integrated accelerations (all directions). These will then be averaged/day based on the total number of valid wear days for each participant.

Physical activity habits around the time of testing will be assessed with a tri-axial accelerometer for 7 days outside of the laboratory, in part by measuring mean steps per day.

This will be quantified as reported by the ActiGraph accelerometer and ActiLife analysis software, which uses the vertical axis data. The Algorithm used by ActiGraph is not publicly available, but ActiGraph accelerometers are frequently used in research quantifying physical activity.

Physical activity habits around the time of testing will be assessed with a tri-axial accelerometer for 7 days outside of the laboratory, in part by measuring mean duration per day of MVPA.

Intensity of physical activity assessed using tri-axial counts (per the accelerometer software) are as follows: Light physical activity is between 200 and 2690 counts/min, moderate intensity between 2690-6166 counts/min, and vigorous intensity is ≥ 6167 counts/min. MVPA is ≥2690.

Physical activity habits around the time of testing will be assessed with a tri-axial accelerometer for 7 days outside of the laboratory, in part by measuring mean duration per day of MVPA.

Intensity of physical activity assessed using vertical counts (per the accelerometer software) are as follows: Light physical activity is between 100 and 759 counts/min, moderate intensity is 760-5999 counts/min, and vigorous intensity is ≥ 6000 counts/min (MVPA is > 760 counts/min).

Physical activity habits around the time of testing will be assessed with a tri-axial accelerometer for 7 days outside of the laboratory, in part by measuring mean duration per day of all activity.

Average minutes per day of activity within each of the following categories: light = 25.8 mg - 100.5 mg; moderate = 100.6 mg - 428.7 mg; vigorous = > 428.8 mg

(mg is the vector magnitude of acceleration calculated in gravities using Euclidean Norm minus 1 metric)

Physical activity habits around the time of testing will be assessed with a tri-axial accelerometer for 7 days outside of the laboratory, in part by measuring mean duration per day of all activity.

Average minutes per day of activity within each of the following categories: light = 25.8 mg - 100.5 mg; moderate = 100.6 mg - 428.7 mg; vigorous = > 428.8 mg.

(mg is the vector magnitude of acceleration calculated in gravities using Euclidean Norm minus 1 metric)

EMG will be reported as the root mean square (RMS) value. The percent change in EMG from the start of the task to task failure will be examined.

EMG for the rectus femoris will be normalized to the maximum RMS EMG obtained during lower extremity strength testing. The percent change in EMG from the start of the task to task failure will be examined.

The PSQI assesses seven sleep domains: sleep quality, sleep latency, sleep duration, habitual sleep efficiency, sleep disturbances, use of sleeping medications, and daytime dysfunction. The seven components scores are then added to yield a global PSQI score in the range of zero to 21. Higher scores indicate worse sleep quality. A global score greater than five is diagnostic of poor sleep quality.

The PFDI-20 is composed of 20 items total on three scales (Pelvic Organ Prolapse, Colorectal-Anal Distress, Urinary Distress), each scored on a 5 point likert scale. Mean scores from each scale are calculated and multiplied by 25, then added together for a total possible range of scores from 0-300. Higher scores indicate increased pelvic floor distress.

This disability index measures how much lower back or leg pain affects the ability to manage everyday life. There are 10 sections, each section is scored from 0-5 for a total possible range of scores 0-50. The resultant score is divided by the total possible score and multiplied by 100 to get a percentage of disability.

The PCS is a measure of thoughts and feelings when one is in pain. It is a 13 item questionnaire scored on a 5 point likert scale. The total possible range of scores is 0-52 where higher scores indicate a higher degree of overwhelm with respect to pain.

The EPDS is a 10-item survey that screens for symptoms of depression that have been present in the previous 7 days. Each question is scored on a 4-point scale ranging from 0 to 3 points for a total possible range of scores between 0-30, where higher scores indicate increased depression. Only postpartum participants will complete this survey.

The PTSD Checklist for DSM-5 with Life Events Checklist for DSM-5 and Criterion A scale will be used to assess participant level of PTSD. The measure has a total range of scores between 0-80 with higher scores indicating greater PTSD symptoms.

Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5)

The TSK scale measures fear of movement. The total possible range of scores is 17-68, where higher score means higher degree of kinesiophobia.

The PGQ is a measure of how much pelvic girdle pain affects daily activities. It is a 25-item questionnaire scored on a 4 point likert scale for a total possible range of scores from 0-75 where higher scores indicate increased burden on daily activities.

Single motor unit behavior will be extracted from surface EMG recordings of the rectus abdominis and rectus femoris using a multi-channel array surface EMG electrodes during submaximal contractions and fatiguing exercise of the abdominal muscles, as an exploratory analysis. If successful, mean discharge rate of single motor units will be reported.