The INVOS probe is a small adhesive patch that is stuck to the skin. The probe is sterile and will be attached using the same sterile technique as the compartment pressure catheter. Where the overlying skin is hairy the hair overlying the area for the probe should be clipped before attaching the probe, to ensure adequate adhesion and accurate readings. The probe is to be sited just below the compartment pressure catheter, although if there is an underlying haematoma (blood clot), then the probe should be positioned so at to avoid this but still remaining over the front outer part of the leg. This technique conforms with the manufacturers recommended use of the equipment and is similar protocol to previous studies.