Inclusion Criteria:  Have provided written informed consent to participate; Be able to comply with protocol; Be able to adequately understand written and spoken English; Be aged between 18 and 36 years; Have a BMI ≥18.5 and ≤28.0 kg/m2; Regularly perform a minimum of 3 sessions / week of 40 minutes or more of moderate or vigorous exercise; Be without any clinically significant illness (free from immune, cardiovascular or metabolic diseases and absence of medical conditions that would be a bar to joining the Army), Have baseline blood tests within reference range as per screening assessment table (table 1). Minor abnormalities will be assessed by the PI. Participants will still be eligible if these are felt to be of no clinical importance and this decision is documented by the PI; Be vitamin D replete (25(OH)D ≥50 nmol/L). Potential participants presenting with 25(OH)D <50 nmol/L I will be offered supplementation and may be re-screened once their course of treatment is complete  Exclusion Criteria:  Hypersensitivity to the active PTH or any of the excipients listed in any of the SmPCs. Pre-existing hypercalcaemia Participants with skeletal malignancies or bone metastases. Significant musculoskeletal injury or fracture within the last 6 months. Significance to be determined by the PI. Digoxin and any other concurrent therapy that, in the investigators' opinion, would interfere with the evaluation of the safety or efficacy of the study medication. Pregnancy, suspected pregnancy or breastfeeding. Female participants must have a negative urine pregnancy test at screening and be willing and able to use a medically accepted method of birth control (reliable use of oral contraceptive, hormonal intrauterine device, non-hormonal intrauterine device with condom, diaphragm with condom, or condom with spermicide) from the screening visit through to the end of the experimental testing visit or declare that they are abstaining from sexual intercourse from the screening visit through the end of the experimental testing visit or are surgically sterile (have undergone bilateral tubal ligation, bilateral oophorectomy, or hysterectomy). Less than 6 months post-partum. Less than 6 months post-breastfeeding. Severe renal impairment (eGFR <30mLs/min/1.73m2). Pre-existing metabolic bone diseases including hyperparathyroidism and Paget's disease of the bone. Unexplained elevations of total ALP. Prior external beam or implant radiation therapy to the skeleton. Participating in a concurrent drug trial. Potential participants will be excluded if they have been in receipt of an investigational drug or biological agent within the last 4 weeks (or 5 times the half-life if this is longer) prior to study entry.