This pilot study aims to investigate whether high intensity interval training can result in rapid improvements in physical fitness amongst the frail elderly (over 70 years old). Participants will undergo a wide range of physical fitness measures followed by a four week exercise protocol. Determination of improvement will be by repeated testing of the physical measures taken for baseline. The primary outcome measure will be anaerobic threshold. There will also be a subgroup of participants who will undergo muscle biopsy and D2O ingestion to allow an insight into the mechanistic basis behind exercise training response in this age group.

Inclusion Criteria:

• Aged over 70 years.
• Sufficient mobility to be able to exercise on a static exercise bike.
• Availability for the entire trial period.
• Sufficient capacity to consent for the trial.

Exclusion Criteria:

• Participants under the age of 70 years.

Participants with a significant past medical history of:

• Myocardial infarction (within last 6 months)
• Unstable Angina
• Heart failure (NYHA class III/IV)
• Uncontrolled Hypertension (BP>160/100)
• Previous stroke/TIA
• Severe respiratory disease inc. known pulmonary hypertension(>25 mmHg), Forced -Expiratory Volume in 1 second <1.5l.
• Brittle asthma / exercise induced asthma
• Known cerebral aneurysm or abdominal aortic aneurysm.
• Cognitive impairment, which may reduce individuals' ability to provide informed consent.
• Inclusion in a recent study which included any form of exercise, taking a drug or ionising radiation.

Exclusion criteria pertaining specifically to muscle biopsies:

• Current use of anticoagulation (i.e. Warfarin/Clopidogrel/Rivaroxaban)
• Any pre-existing clotting disorders known to the patient (i.e. haemophilia).
• Family history of severe bleeding requiring medical intervention.
• Any musculoskeletal deformity or skin conditions making the taking of a biopsy unsuitable as deemed by the medical practitioner taking that sample.
• Lack of capacity to fully consent for the procedure.