Background Skeletal muscle dysfunction is a clinically relevant extra-pulmonary manifestation of COPD.1 While muscle strength is undoubtedly important in functional performance, the ability to perform extended physical activity is also dependent upon muscular endurance. While previous studies have shown a correlation between quadriceps endurance and exercise test performance,2 we wished to investigate the same correlation across a wider range of functional outcomes. In addition, we have previously explored the clinical meaning of a “distance-desaturation product” in field tests and data have indicated increased clinical value of the measure. We hypothesised, therefore, that measures of skeletal muscle isokinetic endurance might add clinical value to measures of strength, especially in day-to-day or submaximal activities.
Methods A prospective cohort of 11 patients with COPD (age median 66, range 58–79; FEV1 median 0.81 L, range 0.68–1.41 L) was studied. We compared all 11 patients’ performance in functional tests (6-minute walk test (6MWT), incremental CPET, endurance CPET, and activity data) with the following measures of isokinetic quadriceps function:
Endurance (the peak torque of voluntary quadriceps contraction after 40 maximal reps, as a fraction of initial peak torque).
A putative “strength-endurance product” (SEP), as a novel measure to better reflect the overall functional performance of the musculature.
Results Somewhat surprisingly, isokinetic quadriceps endurance was not significantly associated with any parameter across all 4 exercise tests. Furthermore, combining strength and endurance in the SEP yielded only a minor improvement: only resting energy expenditure was significantly correlated (p < 0.05).
Discussion Understandably it appears that quadriceps endurance is a poor predictor of performance in exercise tests, however limited added value of a combined “SEP” was evident. The clinical meaning of endurance measures remain unclear.
References 1 Van’t Hul A, Harlaar J, Gosselink R, et al. Quadriceps muscle endurance in patients with chronic obstructive pulmonary disease. Muscle Nerve 2009;29(2):267–74
2 Vilaro J, Rabinovich R, Gonzalez-deSuso JM, et al. Clinical assessment of peripheral muscle function in patients with chronic obstructive pulmonary disease. Am J Phys Med Rehabil. 2009;88(1):39–46