Pressure pain thresholds and electromyographically defined muscular fatigue induced by a muscular endurance test in normal women

Objective: This study was undertaken to examine the relation between muscul ar tenderness measured as pressure pain thresholds (PPTs) and electromyogra phic (EMG) signs of fatigue before and after a local standardized static mu scle contraction. Design: pressure pain thresholds were measured in the shoulder region befor e, immediately after, and 10 minutes after a standardized static endurance test while monitoring the EMG signs of local muscular fatigue and its recov ery. The study did not address local biochemical issues. Setting: The study was conducted at the Department of Rehabilitation, Lund University Hospital, Lund, Sweden. Subjects: Twenty-five healthy female volunteers without musculoskeletal pro blems participated in this study. Intervention: A static endurance test was performed, which consisted of a s ubmaximal unilateral activation of the right trapezius and deltoid muscles for as long as possible. Outcome Measures: Bilateral PPTs over the trapezius and deltoid muscles wer e measured with an electronic pressure algometer. Established surface EMG p arameters of local muscular fatigue were assessed. The Borg Rating of Perce ived Exertion scale was used. Results: The average endurance time was 330 seconds, immediately after the test, significant bilateral increases in the normalized PPTs over both musc les were found, although the increase was more pronounced on the test side: over the right trapezius muscle by 13% (p <0.001), over the right deltoid muscle by 23% (p <0.001), and over the left trapezius and deltoid muscles b y 6% (p = 0.04) and(p = 0.009), respectively. These increases persisted IO minutes after the end of the test. The subjects developed significant signs of fatigue as defined by EMG criteria in both muscles on the right side du ring the test. The recovery from fatigue was approximately half complete 15 seconds after the end of the test and complete or almost complete 10 minut es thereafter. Conclusions: Pressure pain thresholds over shoulder muscles remained elevat ed up to 10 minutes after a unilateral static endurance test. This time cou rse was completely different from that of EMG-defined muscle fatigue, which showed a fast recovery. These findings indicate that the mechanisms of rec overy from fatigue and nociception are independent of each other. The bilat eral PPT increases might be explained by central antinociceptive mechanisms activated by static muscle work.