The influence of pressure stimulus intensity on pain perception and neuromuscular performance in healthy males

Participation in elite level sport is inherently linked with exposure to pain (Bartholemew et al, 1998; Heil, 1993) with those able to tolerate it whilst maintaining performance levels gaining a competitive edge over their opponents (Egan, 1987). Studies have shown that the presence of experimental pain reduces muscle performance (Farina et al, 2002; Farina et al, 2008) when pain is induced via hypertonic saline injection but this is an invasive method of pain induction whereby it is unclear if there is any interference of normal muscle functioning due to the injection of fluid into the muscle (Wing et al, 2011a). The series of studies presented within this thesis aimed to investigate whether this relationship existed when pain was induced via a non-invasive Gross Pressure Device (GPD) and subsequently the effect of severity of pain on muscle performance. The final stages of the research explored the possible location of inhibition; central or peripheral. The GPD was established to be a reliable method of pain induction at pain perception threshold level both inter-session and intra-session. A protocol for use was established in pilot testing which was followed throughout the research. Pain perception threshold level was established for all participants prior to undertaking the experimental trials as well as their maximal ramped contraction within an isometric knee extension measured on an isokinetic dynamometer which was subsequently used for normalisation. The experimental trials consisted of three explosive isometric voluntary contractions of which the peak maximal voluntary contraction (MVC) was selected for analysis. Electromyographic activity (EMG) of the vastus lateralis and semitendinosus were also measured during the contraction to indicate neural activation. Immediately following each painful trial participants were asked to rate the painfulness of the condition on a visual analogue scale (VAS). Inducing pain at perception threshold level produced a mean reduction in maximal force by 9%- 12% when pain was induced ipsi-laterally, and 11% when induced contra-laterally. When pressure exerted by GPD was doubled and then trebled from previously established pain perception threshold, a greater reduction in maximal force was reported (18% & 21% respectively). When placebo and nocebo conditions were introduced and therefore there was a disparity between the expected pain severity and the stimulus applied, while there was a significant difference in the perception of pain severity reported via VAS (F = 27.971; p < 0 05) no significant difference in force output was found (x2(2) = .452, p>0.05). The studies presented within this thesis have demonstrated that pain induced at perception threshold level reduced muscle performance even when a non-invasive method of pain induction was used. Furthermore it was found that there is an incremental decrease in performance relative to severity of pain induced. Concomitant reduction in force and EMG suggest that inhibition is likely to be neuronal but the level of this could not be established. However, the fact that pain induced contra-laterally produced similar reductions in force as ipsi-lateral pain induction suggests inhibition is unlikely to be at peripheral level. Furthermore manipulation of the severity of pain perceived was found not to alter muscle performance, suggesting that inhibition is unlikely to be at cortical level; rather, it is likely to be sub-conscious. Further work should be conducted to determine the mechanisms by which pain reduces muscle performance and the effect of pain on dynamic, multi-joint tasks in order that effective strategies can be developed for athletes to perform when experiencing pain.