Metabolite Accumulation And Power Output During And After Fatiguing Exercise In Young And Older Adults

Fatigability of limb muscles during high-intensity dynamic exercise is postulated to be determined by the accumulation of metabolites, namely hydrogen ions (H+) and inorganic phosphate (Pi). We previously showed that the increased fatigability of older adults during dynamic exercise was accompanied by a greater accumulation of metabolites compared with young adults. However, the time course of metabolite accumulation in relation to power output during and after dynamic fatiguing exercise is unknown. PURPOSE: Determine the time course of metabolite accumulation and changes in power during lower limb fatiguing exercise and recovery in young and older adults. METHODS: 9 young (20-36 yrs; 5 women) and 8 older adults (69-77 yrs; 4 women) performed a 4-min fatiguing knee extension exercise (1 maximal velocity contraction every 2 s) lifting a load equivalent to 20% of their maximal voluntary isometric contraction while prone in the bore of an MRI scanner. Recovery was measured with sets of 5 maximal dynamic contractions (1 every 2 s) at 30, 60, 150, 300, and 600 s after exercise. Intramuscular [Pi] and [H+] were measured with 31P-MRS, and statistical parametric mapping was used to evaluate the time course differences between the metabolite accumulation and power during exercise and recovery. RESULTS: Fatigability was ~1.6-fold greater in older than young adults (P < 0.001), which was accompanied by a greater decrease in pH (6.71 ± 0.07 vs. 6.62 ± 0.10, P = 0.043) and increase in Pi (24.7 ± 2.4 mM vs. 32.2 ± 5.6 mM, P = 0.002) by the end of exercise. Except for the first 32 s of exercise where pH increased while power decreased, the time course of H+ accumulation and power loss was similar. However, after exercise H+ continued to increase for 60s as power was recovering. In contrast, Pi accumulated more rapidly than the loss in power for the first 152 s of exercise, after which Pi no longer accumulated but power continued to decrease. After exercise, the recovery of power and Pi followed a similar time course. There were no age effects on the relationship between power output and metabolites at any time point during or after exercise. CONCLUSIONS: These data suggest that both H+ and Pi are essential metabolites disrupting power during and in recovery from fatiguing exercise and that the effects of these metabolites are similar in young and older adults. Supported by NIA R01 grant (AG048262) to Christopher Sundberg, Robert Fitts and Sandra Hunter.