The goal of this clinical trial is to evaluate how different exercise programs alter bone density, structure, and strength in adults. It will also collect data on hormones, factors released from skeletal muscle, and body composition.

The main questions it aims to answer are:

• Do exercise programs that are frequent with high-impact loading will cause greater improvements in bone health?
• What changes in hormones, factors released from skeletal muscle, and body composition contribute to bone adaptations?

Researchers will compare different exercise groups to see if the type of exercise influences bone adaptation compared to a recreationally active control group.

Participants are asked to:

• complete questionnaires, 5 blood draws across the study (3 at rest, 2 after strenuous exercise), dual energy x-ray absorptiometry (DXA) scans (to assess bone mineral density and body composition), high resolution tibial scans to assess bone structure and geometry, and physical performance testing.
• attend up to 7 testing visits.
• (for those randomized to the exercise training intervention) participate in exercise training sessions for 6 months.

The overall objective of this study is to determine the bone density, structure and strength adaptations that result from various exercise programs which differ based on the frequency and intensity of bone loading. Bone adaptation is measured with both dual-energy x-ray absorptiometry (DXA), the standard-of-care equipment used to evaluate bone mineral density (BMD), and high-resolution peripheral quantitative computed tomography (HRpQCT), a state-of-the-science imaging technology providing the resolution necessary to detect small but clinically relevant changes.

Additionally, the research team seeks to determine the physiological responses and adaptations to training that contribute to improved bone health by evaluating changes in hormones, factors released from active skeletal muscle, and body composition. Our hypothesis is that exercise programs emphasizing frequent, high impact loading will cause greater improvements in bone health. Additionally, we believe that exercise training can induce specific changes in hormones, skeletal muscle released signaling factors and body composition that contribute to these bone adaptations.

Participants who enrolled in this study were randomized to either an exercise training group or a control group who were instructed to maintain their current level of exercise and track this to report to the study team. Study completers were those who complied with the parameters of the group they were assigned and reported to all 7 testing visits.

Inclusion Criteria:

1. Age 18-40 years
2. Body mass index (BMI) 18-28 kg/m2
3. Weight stable (±10 lbs) in past 2 months
4. Takes part in moderate physical activity for at least 150 minutes/week
5. Currently free of upper or lower body /extremity injury or impairment
6. Comfortable with multiple blood draws
7. Able to commit to the 6-month study duration
8. Agrees to adhere to study requirements.

Exclusion Criteria:

1. Current smoker
2. Current clinical diagnosis of an eating disorder
3. Chronic use of non-steroidal anti-inflammatory drugs (NSAIDs). NSAID use is defined as chronic if these medications are taken more than three times a week for more than 12 consecutive weeks.
4. Use of medication incompatible with measurement of reproductive, metabolic, or bone-related hormones (including thyroid medications, glucocorticoids) or which includes anti-coagulants and may interfere with any of the study outcomes
5. Current oligo/amenorrhea in women, not explained by contraceptive use
6. Any metabolic or endocrine disease that affects bone; Individuals taking hormonal therapy for transgender care
7. Currently training for a competitive sporting event or planning to start
8. Weight has fluctuated by 10 pounds or more in the past 2 months based on subject self-report
9. Has been diagnosed with a medical condition, physical or psychological, that currently prevents exercise participation
10. Has suffered a musculoskeletal injury removing them from physical activity for more than a month, within the past two years
11. Currently pregnant or becomes pregnant during the study (as assessed by urine pregnancy tests at 0, 3 & 6 months)
12. Current asthma diagnosis, defined as experienced symptoms or been prescribed any form of treatment within the last 4 years, or those who have required more than one course of oral steroids or more than one nebulization since the age of 5 (JSP 950), will require a note/permission from treating physician to participate in study. Those who have had a single admission to intensive care or high dependency, or multiple admissions to hospital will be excluded from participating (JSP 950)
13. History of heart condition OR high blood pressure
14. Feeling of pain in the chest during rest, activities of daily living, or physical activity
15. Have lost balance or consciousness due to dizziness in the last 12 months (with the exception of dizziness due to hyperventilation)
16. Treating physician requires subject participates in medically supervised physical activity only
17. History of drug addiction or habitual recreational drug use (2 or more times per month in each of the previous six months)
18. Low bone mineral density indicated as a Z-score <-1.0 measured by dual-energy x-ray absorptiometry (DXA)

Individual participant data that underlie the results reported, after identification (text, tables, figures, and appendices).

Beginning 1 year and ending 5 years following publication.

Investigators whose proposed use of the data has been approved by an independent review committee ("learned intermediary") identified for this purpose. For individual participant data meta-analysis. Proposals should be directed to bnindl@pitt.edu. To gain access, data requesters will need to sign a data access agreement.