Elevated sodium intake and muscle cramp

Participants will increase their daily sodium intake by 150 mmol per day for seven days. Sodium intake will be increase using slow sodium tablets. The participants will take a total of 15 tablets, each tablet contains 10 mmol of sodium. The participants will be asked to interspace these regularly throughout the day i.e. a tablet approximately every awake hour. They will be advised to consume 70 mL of water per tablet as per the product label, with no requirements around food intake. Adherence will be assessed by verbal confirmation with each participant that they took the 15 tablets each day for seven days and they will be asked to return any remaining tablets. This the response and number of tablets remaining will be formally noted within the data collection sheets. The investigators will be in regular contact with the participants to remind them of their appointments. A investigator is present at all appointments and formal recording of appointment attendance is taken. Participants will also be encouraged to be in regular contact with the investigators about adherence to the tablets and any symptoms they may feel. Participants will attend clinical research rooms with the Department of Medicine, University of Otago, Dunedin hospital on five occasions. The first session is approximately 30 minutes to familiarise the participants to the measures and the mild-electrical stimulation protocol used to induce muscle cramp in the Abductor Hallucis muscle as with the attached published methodology study (DOI: 10.1088/1361-6579/ab8855). Following the familiarisation session the participants attend four experimental sessions, each approximately 1 hour long at the same time of day at baseline, days 2, 4 and 7 following increased daily sodium intake. The familiarisation session will occur within two weeks (usually a week) of the baseline session. The baseline session will occur within two days of the participants being required to commence taking the slow release sodium tablets. Briefly an incremental graded 150-stimulus train is used to induce muscle cramp, with a maximum current of 80 milliamps at a maximum stimulation frequency of 70 hertz targeting the Abductor Hallucis muscle using a constant current stimulator (DS7R, Digitimer Ltd, England). The cathode (Ambu Bluesensor M, Denmark; area ~79 mm2) is placed over point of the main motor point for the Abductor hallucis (within an area defined by the anatomical landmark of the navicular tuberosity - up to 1.5 cm posterior and 2 cm inferior). The anode an 8 × 6 cm carbon electrode covered with conductive gel (Spectra 360 Electrode Gel, NJ, USA) is secured on the opposite side of the participant’s foot to the motor point search area. Superficial EMG recording electrodes (Ambu Bluesensor M, Denmark; area ~79 mm2) is placed over abductor hallucis muscle belly anteriorly to the motor point; the positive electrode was placed posteriorly to the first metatarsophalangeal joint and the negative electrode was placed between the positive electrode and the anterior edge of the search area for abductor hallucis motor point. The participants leg and foot were placed within a turnbuckle ankle orthosis (product reference number: 66 426, AliMed Inc. Dedham, MA, USA) set to comfortably and consistently support and hold the ankle in plantarflexion and foot in inversion. The electrical stimulation will be administered by the investigators. Whilst continuously measuring electromyograph activity and electrocardiogram (via a standard limb lead II configuration) through Labchart 8 software and a Powerlab (3508/P) analogue-to-digital converter sampling at 10,000 Hz (ADInstruments, Dunedin, NZ), and intermittently measuring non-invasive brachial blood pressure (Connex ProBP 3400 series Welch Allyn).