Investigating the Role of Central Pain Hypersensitivity in Skeletal Muscle Neural Drive

Pressure Pain Threshold as an index of central pain hypersensitivity.

Pressure Pain Threshold (PPT) performed with digital algometer, applied at proximal forearm. A gradually increasing pressure is applied, and participants indicate when the sensation of pressure turns to pain. PPDT measured in Kilopascals (kPa). Three tests applied, which are then mean averaged. Lower PPT indicates greater pain sensitivity.

Temporal Summation as an index of central pain hypersensitivity.

Temporal Summation (TS) assessed using a puncate stimulator (designed not to puncture the skin). Participants rate their experience of sharpness during the stimulus on a 10 cm visual analogue scale, first following one application, and second following 10 repeated applications. TS is calculated by subtracting first score from second (in cm). Higher positive scores indicate higher sensitivity.

Conditioned Pain Modulation as an index of central pain hypersensitivity.

Conditioned pain modulation (CPM) involves temporary ischaemic pain at one arm by way of blood pressure cuff. PPT simultaneously applied to opposite forearm. CPM calculated as conditioned PPT minus non-conditioned PPT in kilopascals (kPa). Lower positive or negative CPM indicates higher sensitivity.

A measure of physical performance of basic physical tasks, which is made up of for tasks:-

Standing balance: participants are timed as to how long they can maintain standing balance unassisted without walking aids. First with feet side by side, second in semi-tandem stance, third in full tandem-stance. Points are awarded if balance is maintained for 10 seconds.

Walking speed: participants are timed to walk a short distance (3 or 4 meters) at their usual walking pace. Allowed to use their usual walking aids. Points are awarded based on pre-set time scales for completing the walk.

Repeated sit to stand: participants timed as to how many times they can sit to stand 5 times from an average height chair. Points awarded based on pre-set durations.

Points are combined to give an overall score ranging from 0 - 12. A higher overall score = higher physical performance.

Motor unit firing characteristics will be captured using HDsEMG during a range of sub-maximal skeletal muscle contractions. HDsEMG sensor will be placed on the skin over the muscles tibialis anterior and biceps brachii bilaterally to record muscle activation. Established methods will allow analysis of motor unit firing characteristics in terms of size, number, complexity, reliability and firing rate.

A self completed questionnaire giving a central aspects of pain factor, an index of central nervous system aspects of pain. The questionnaire includes 8 items covering neuropathic-like pain, fatigue, cognitive difficulty, catastrophizing, anxiety, sleep disturbance, depression and widespread pain. For the former 7 items, respondents rate the frequency of experiencing symptoms over the past week with four options "never", "sometimes", "often", "always". The widespread pain item involves shading areas of a manakin relating to painful areas of the body experienced on most days in the last four weeks. All items are combined to give an overall score. Scores can range from 0 to 16. The higher the score, the greater the central aspects of pain factor.

A range of isometric muscle contraction patterns will be performed (tibialis anterior and biceps brachii) in terms of contraction strength, duration and rate of force generation. Neuromuscular control metrics will include force steadiness, and force accuracy.

To test whether experimental remote pain has an influence on neuromuscular functioning, a temporary pain stimulus will be applied to the arm via blood pressure cuff to a subjective level of 4/10 where '0' no pain and '10' worse pain or discomfort imaginable. A series of isometric (static) muscle contractions will be carried out during the remote discomfort stimulus lasting approximately 2 minutes. If discomfort becomes too much at any time the cuff will be removed immediately.

The effect of remote pain stimulus on neuromuscular functioning will be analysed in terms of neuromuscular control (including force steadiness, and force accuracy) and motor unit firing characteristics (using HDsEMG).

Pain intensity measured by self-report using an 11 point numerical rating scale from 0-10, where 0 is no pain and 10 is the worst pain imaginable.

Three items will be administered, using the 11 point scale detailed above:-

• Over the past 4 weeks, how strong was your strongest pain?
• Over the past 4 weeks, how strong was your pain on average?
• How would you assess your pain now, at this moment?

The RAND SF-36 PF is a sub-scale of the overall RAND SF-36 quality of life self-completed questionnaire. The SF-36 PF is a measure of Physical Functioning.

There are 10 question covering a range of every day physical functioning activities. Three responses are possible regarding the level of limitation experienced for each item i.e. "not limited at all" (three points), "limited a little" (2 points) or "limited a lot" (1 point). The overall SF-36 PF score is from 0-100, where 100 indicates the most favourable health state.

The HAQ-DI is a self-completed questionnaire designed to measure a person's functional ability over the past week. It covers eight categories of daily activities, including:- Dressing and Grooming; Arising; Eating; Walking; Hygiene; Reach; Grip and Activities. It also asks questions on the use of aids or devices and the need for help from other people. Each item is scored from 0-3. The overall HAQ-DI score is calculated by summing all the domains, and dividing by eight, providing an ordinal score from 0 (no-disability) to 3 (severe disability).

The MFIS is a self-completed questionnaire which provides a measure of how fatigue impacts on daily life. It has 21 items and provides an overall score out of 84 with higher scores indicating a greater impact of fatigue. Four sub-scales can be calculated: physical, cognitive and psychosocial, indicating how much fatigue impacts on these aspects of health and wellbeing.

The PASE is a self-completed questionnaire which measures physical activity levels over the past 7 days. It includes activities relating to leisure, household and occupation. It provides an overall score from 0-400 or more and a higher score indicates higher levels of physical activity.