While tissue acidosis causes local deep-tissue pain, its effect on referred pain and mechanical muscle hyperalgesia is unknown. The aim of this study was to investigate a human experimental acidic muscle pain model using a randomized, controlled, single-blinded study design. 72 subjects (36 female) participated in three visits, each involving one 15 min intramuscular infusion into the anterior tibialis muscle: acidic phosphate buffer (5.2 pH) at 40 ml/hr (N=69) or 20 ml/hr (N=54), normal phosphate buffer (7.3 pH) at 40 ml/hr (N=70), or isotonic saline at 40 ml/hr (N=19). Pain ratings and pressure sensitivity of superficial and deep tissues were assessed before, during, and 20 min after infusion. Acidic buffer produced light to moderate, rate-dependent, muscle pain (not sex-dependent) compared to the control infusions, that referred pain to the ankle in 80% of women and 40% of men. Pain did not vary across selfreported menstrual phases. Pressure pain thresholds (PPTs) were reduced over the infused muscle with acidic infusion, defined as primary mechanical hyperalgesia. PPTs decreased at the ankle in those with referred pain in response to acidic buffer, i.e. referred mechanical hyperalgesia, but not at the foot. No pain or changes in PPTs occurred in the contralateral leg. These results demonstrate muscle acidosis can lead to local and referred pain and hyperalgesia, with significant sex differences in development of referred pain.

Rehabilitation after spinal cord injury (SCI) aims to preserve the integrity of the paralyzed musculoskeletal system. The suitability of ultrasound (US) for delineating training-related muscle/ tendon adaptations after SCI is unknown. The purpose of this study was to quantify within-and between-operator reliability for US and to determine its responsiveness to post-training muscle/ tendon adaptations in SCI subjects. Two novice operators and one experienced operator obtained sonographic images of the vastus lateralis, patellar tendon, soleus, and Achilles tendon from 7 SCI subjects and 16 controls. For control subjects, within-operator concordance (ICC(3,1)) ranged from 0.58 to 0.95 for novice operators and exceeded 0.86 for the experienced operator. Between-operator concordance (ICC (2,1)) ranged from 0.62 to 0.74. Ultrasound detected muscle hypertrophy (p < 0.05) following electrical stimulation training in subjects with SCI (responsiveness), but did not detect differences in tendon thickness. These error estimates support the utility of US in future post-SCI training studies.

Ratings of perceived discomfort (0 to 10 scale) have been used to estimate relative maximum holding times (%MHT), particularly for static tasks. A linear 1:10% ratio has been described, where a rating of 5 corresponds to 50%MHT. It is unknown whether this linear ratio is valid for dynamic tasks. Additionally, whether pain or exertion are the primary predictors of discomfort is not clear. Thus, the goal of this study was to investigate both pain and exertion ratings during static (50% maximum; N= 42) and dynamic (75% maximum; N=34) elbow flexion tasks until failure. Gender, self-reported physical activity, and peak torque were also assessed. Pain and exertion ratings reasonably matched the 1:10% ratio during the static task but not during the dynamic task. Exertion related more strongly to MHT than pain in both tasks. Neither gender nor activity level appeared to influence perceptual ratings, but peak torque explained approximately 20% of the variance in MHT.

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