HHS Public Access Author manuscript Author Manuscript
J Pain. Author manuscript; available in PMC 2017 March 23. Published in final edited form as: J Pain. 2016 April ; 17(4 Suppl): S63. doi:10.1016/j.jpain.2016.01.259.
Differences in psychophysical responses to muscle stimulation in young and old healthy adults M.E. Hoeppli1,2, T.S. Thurston3, S.K. Sidhu4,5, T.S. Mangum3, J.C. Weavil3, T.J. Hureau4, W. Tang6, R.W. Hughen6, A.R. Light6, M. Amann4,6, and P. Schweinhardt1,2 1Alan
Edwards Center for Research on Pain, McGill University, Montreal, Canada
2Faculty
of Dentistry, McGill University, Montreal Canada
Author Manuscript
3Department
of Exercise and Sport Science, University of Utah, Salt Lake City, UT, USA
4Department
of Internal Medicine, University of Utah, Salt Lake City, USA
5School
of Medicine, Discipline of Physiology, University of Adelaide, Australia
6Department
of Anesthesiology, University of Utah, Salt Lake City, UT, USA
Author Manuscript Author Manuscript
Thin fiber muscle afferents (group III/IV) signal the sensations of muscle ache and fatigue. Aging may influence the sensitivity of these afferents. However, little is known about mechanisms and receptors involved in this age-related impact. Seventeen young (27±3 years) and thirteen old (68±5 years) matched (sex, BMI, and activity) healthy adults participated in this study. During a first session, participants were familiarized with a twodimensional (2D) scale adapted from the Gracely Box Scale. In a second session, participants received 3 intramuscular (left abductor pollicis brevis) infusions (each 2 mL over 5 minutes) of combinations of metabolites: 1) pH=7.4, ATP=300nmol, lactate=1mmol found in resting muscle (neutral concentration mixture), 2) pH=7.0, ATP=1μmol, lactate=15mmol - found during moderate exercise (low concentration mixture), and 3) pH=6.6, ATP=5μmol and lactate=50mmol - found during strenuous exercise (high concentration mixture). ATP activates P2X receptors, hydrogen ions (i.e., lowered pH) activate ASIC and TRPV1 receptors, and lactate enhances ASIC activation. These three metabolites act synergistically to activate group III/IV neurons. During the infusion, participants continuously rated their sensations of fatigue and ache on the 2D scale. Young participants reported significantly more fatigue and ache than the older adults with higher fatigue sensations during the infusion of both the low and high concentration mixture (both p
J Pain. Author manuscript; available in PMC 2017 March 23. Published in final edited form as: J Pain. 2016 April ; 17(4 Suppl): S63. doi:10.1016/j.jpain.2016.01.259.
Differences in psychophysical responses to muscle stimulation in young and old healthy adults M.E. Hoeppli1,2, T.S. Thurston3, S.K. Sidhu4,5, T.S. Mangum3, J.C. Weavil3, T.J. Hureau4, W. Tang6, R.W. Hughen6, A.R. Light6, M. Amann4,6, and P. Schweinhardt1,2 1Alan
Edwards Center for Research on Pain, McGill University, Montreal, Canada
2Faculty
of Dentistry, McGill University, Montreal Canada
Author Manuscript
3Department
of Exercise and Sport Science, University of Utah, Salt Lake City, UT, USA
4Department
of Internal Medicine, University of Utah, Salt Lake City, USA
5School
of Medicine, Discipline of Physiology, University of Adelaide, Australia
6Department
of Anesthesiology, University of Utah, Salt Lake City, UT, USA
Author Manuscript Author Manuscript
Thin fiber muscle afferents (group III/IV) signal the sensations of muscle ache and fatigue. Aging may influence the sensitivity of these afferents. However, little is known about mechanisms and receptors involved in this age-related impact. Seventeen young (27±3 years) and thirteen old (68±5 years) matched (sex, BMI, and activity) healthy adults participated in this study. During a first session, participants were familiarized with a twodimensional (2D) scale adapted from the Gracely Box Scale. In a second session, participants received 3 intramuscular (left abductor pollicis brevis) infusions (each 2 mL over 5 minutes) of combinations of metabolites: 1) pH=7.4, ATP=300nmol, lactate=1mmol found in resting muscle (neutral concentration mixture), 2) pH=7.0, ATP=1μmol, lactate=15mmol - found during moderate exercise (low concentration mixture), and 3) pH=6.6, ATP=5μmol and lactate=50mmol - found during strenuous exercise (high concentration mixture). ATP activates P2X receptors, hydrogen ions (i.e., lowered pH) activate ASIC and TRPV1 receptors, and lactate enhances ASIC activation. These three metabolites act synergistically to activate group III/IV neurons. During the infusion, participants continuously rated their sensations of fatigue and ache on the 2D scale. Young participants reported significantly more fatigue and ache than the older adults with higher fatigue sensations during the infusion of both the low and high concentration mixture (both p
