Thermal imaging to measure muscle activity during and soreness following exercise.

Experimental overview The study followed a within-participant design, requiring volunteers to visit the laboratory on four occasions. The initial visit (5–7 d before visit two) involved a comprehensive neuromuscular testing familiarisation. The second visit involved resting thermal images of the right and left anterior thighs, and maximal voluntary contraction (MVC) testing before and after an intense exercise protocol designed to induce acute muscle damage and delayed onset of muscle soreness (DOMS). Visits 3–4 involved a thermal image, and an MVC test. Testing was balanced for morning-afternoon, and within a participant all testing was performed at the same time of day (±1 hr). All testing was administered by an accredited exercise physiologist (AEP). Initial testing session Height and mass were recorded and participants completed the pre-exercise screening questionnaire (Exercise and Sports Science Australia Adult Pre-Exercise Screen Tool). After a standardised warm up (WU; see neuromuscular section), individuals completed multiple-sets of 5 s isometric MVC’s during which twitch interpolation was applied. Participants were considered familiarised after achieving a plateau in MVC torque for a full set (5 x 5 s) of contractions. On average familiarisation took 7 sets (range: 3–9). During the initial visit, participants were also familiarised to the perceptual soreness, session rating of perceived exertion (sRPE) and modified profile of mood states ‘POMS’ measures. Perceived soreness was rated on a 0 ‘no soreness’ to 10 ‘extremely sore’ visual analogue scale. The modified POMS comprised six-states (‘active’, ‘energetic’, ‘restless’, ‘fatigued’, ‘exhausted’, and ‘alert’) rated on a 1–5 Likert scale, which ranged from 0 ‘low’ to 5 ‘high’. POMS states were summed, providing a global indication of mood ‘Mood’. Session rating of perceived exertion (sRPE) was collected 10 min after exercise. Participants were given specific instructions for the days leading up to their first testing session so to ensure adequate preparation of thermal image skin sites. Namely, (a) avoid prolonged sun exposure five days prior to testing to prevent sun burn; (b) if applicable, remove hair from the anterior and posterior aspects of both thighs 36 hr before testing to prevent inflammation and/or skin surface damage (c) avoid exercise, caffeine and alcohol in the 24 hr prior to testing; and (d) avoid hot showers, ointments and cosmetics on each testing day. Thermal images Two thermal infrared cameras were utilised to assess resting (A305sc, FLIR Systems, Wilsonville, Orego, USA) and exercising (PI450, Optris Gmbh, Berlin, Germany) skin temperature. Measurements were undertaken in temperature-controlled, fluorescently lit rooms without the existence of electric heat generators, wind drafts or external radiation sources. Prior to the resting images being collected the participant rested for 20 min in a seated position in order to acclimate to the temperature of the room. Resting thermal images, with the FLIR camera, were then conducted with camera positioning (relative to participant), stabilisation, emissivity and image processing as previously reported (Bach AJ, Stewart IB, Disher AE, Costello JT. A comparison between conductive and infrared devices for measuring mean skin temperature at rest, during exercise in the heat, and recovery. PLoS One. 2015;10(2):e0117907). The exercising thermal images were conducted with the Optris camera mounted on a portal frame, downward looking and parallel with the anterior thigh regions while the participant was seated on the dynamometer. The distance between camera and the anterior surface of the thigh was set to 1.2 m to ensure both right and left thighs were in the field of view of the camera simultaneously. Data sampling and image processing were as previously reported (Moghadam P, editor 3D medical thermography device. SPIE Sensing Technology + Applications; 2015: SPIE; Vidas S, Moghadam P, Sridharan S. Real-Time Mobile 3D Temperature Mapping. IEEE Sensors Journal. 2015;15:1145-52). Neuromuscular function Neuromuscular function was assessed via maximal voluntary contraction (MVC) and evoked twitch properties of the right knee extensors using a Biodex isokinetic dynamometer (Systems 3, Biodex Medical Systems, New York, USA). Participants were seated in an upright position, chair backrest adjusted to 95° from the horizontal plane, and tightly secured with waist, shoulder, hip and thigh straps. The lateral epicondyle of the femur was aligned with the axis of rotation of the dynamometer, and the right knee was positioned at 90°, with 0° being full extension. The lower leg was firmly strapped to the lever arm, approximately 2 cm above the lateral malleolus of the ankle. Before testing, participants completed 15 isometric knee extension WU contractions, 5 at 40% perceived maximal effort, 5 at 60%, 3 at 80% and 2 at 90%, with 10 s rest between each contraction. Muscle activation of the right knee extensors was achieved by percutaneous stimulation of the femoral nerve using a self-adhesive electrode (anode, 3.2 cm diameter; Pals, Axelgaard Manufacturing Co. Ltd., Fallbrook, USA). A second electrode was placed on the border of the gluteal fold (cathode, 5 x 9 cm; Pals, Axelgaard Manufacturing Co. Ltd., Fallbrook, USA). During MVC, a single square-wave pulse width of 100 µs (400 V with a current of 400–700 mA) was delivered via a stimulator (DS7AH; Digitimer Ltd., Welwyn Garden City, England) at 120% of maximal peak twitch torque. The required current was determined via a twitch ramp procedure commencing at 50 mA, thereby increasing 50 mA every 30 s until a plateau in peak (evoked) twitch torque was achieved. Within ~2 s following MVC a second stimulus was delivered to examine muscle contractile properties. To maximise voluntary activation and enhance motivation, strong verbal encouragement and visual force feedback was provided during MVC’s. Neuromuscular data were sampled at 1,000 Hz and recorded into LabChart (LabChart 8.0; AD Instruments, Sydney, Australia) via a PowerLab system (16-bit PowerLab 26T; AD Instruments, Sydney, Australia). Maximal voluntary torque was considered the mean value in the 25 ms period preceding the electric stimuli. Superimposed torque was considered the peak value in the 100 ms after the stimuli. The level of voluntary activation (VA) was determined for each MVC using the twitch interpolation technique, with VA calculated as: VA (%) = [(1-superimposed twitch/potentiated twitch)*100]. MVC repetitions were excluded from analysis if: (1) no plateau prior to stimulation was achieved; (2) the superimposed stimulus was delivered at a sub-maximal force; or (3) stimulation occurred at a non-maximal effort. Peak twitch torque, rate of torque development (RTD), contraction time (CT), and half-relaxation time (HRT) were determined for each twitch response, and alterations in these properties were used to infer acute muscular fatigue, and the presence of DOMS. Exercise protocol Following the pre-exercise MVC test (MVC 1), participants undertook an intense single-leg exercise protocol consisting of a series of maximal concentric (CON) and eccentric (CON) contractions of the right knee extensors. Participants were seated upright and secured as per MVC testing. The protocol involved 6 sets of 25 maximal CON/ECC contractions at an angular velocity of 60 (CON) and 120°·s-1 (ECC), performed within a range of 15° to 80° knee flexion, with 0° being full knee extension. A 5 min rest period separated each set. A similar exercise protocol has previously been shown to induce acute muscle fatigue and DOMS in resistance trained male athletes (Pointon M, Duffield R, Cannon J, Marino FE. Cold application for neuromuscular recovery following intense lower-body exercise. Eur J Appl Physiol. 2011;111(12):2977-86). Participants were instructed to provide maximal effort across the range of each CON/EEC contraction. Loud verbal encouragement was provided throughout, and visual force feedback was displayed on a computer monitor situated 1.5 m in front of the dynamometer. Five min after protocol completion post-exercise MVC (MVC 2) testing was performed, and 10 min after exercise votes of sRPE and soreness were collected. Follow-up testing Participants returned to the laboratory 24 hr and 48 hr following the baseline testing session. Subjective scales of muscle soreness and POMS were collected and right thigh girth was assessed (F10-02DM, KDS, Malaysia). Following a 30 min acclimation period, thermal images of both thighs were conducted before undertaking the MVC protocol. Approximately 10 min after the MVC protocol a sRPE was collected.