31P Magnetic Resonance Spectroscopy Assessment of Muscle Bioenergetics as a Predictor of Gait Speed in the Baltimore Longitudinal Study of Aging

Abstract

Background: Aerobic fitness and muscle bioenergetic capacity decline with age; whether such declines explain age-related slowing of walking speed is unclear. We hypothesized that muscle energetics and aerobic capacity are independent correlates of walking speed in simple and challenging performance tests and that they account for the observed age-related decline in walking speed in these same tests.

Methods: Muscle bioenergetics was assessed as postexercise recovery rate of phosphocreatine (PCr), k _PCr, using phosphorus magnetic resonance spectroscopy (³¹P-MRS) in 126 participants (53 men) of the Baltimore Longitudinal Study of Aging aged 26-91 years (mean = 72 years). Four walking tasks were administered-usual pace over 6 m and 150 seconds and fast pace over 6 m and 400 m. Separately, aerobic fitness was assessed as peak oxygen consumption (peak VO₂) using a graded treadmill test.

Results: All gait speeds, k _PCr, and peak VO₂ were lower with older age. Independent of age, sex, height, and weight, both k _PCr and peak VO₂ were positively and significantly associated with fast pace and long distance walking but only peak VO₂ and not k _PCr was significantly associated with usual gait speed over 6 m. Both k _PCr and peak VO₂ substantially attenuated the association between age and gait speed for all but the least stressful walking task of 6 m at usual pace.

Conclusion: Muscle bioenergetics assessed using ³¹P-MRS is highly correlated with walking speed and partially explains age-related poorer performance in fast and long walking tasks.

Keywords: Bioenergetics; Lower extremity performance; Magnetic resonance spectroscopy; Muscle; Walking speed.

Figure 1.

Representative ³¹P spectra with inorganic phosphate (Pi), phosphocreatine (PCr), and ATP resonances indicated in panel B. Panels (A) and (B) show example baseline and postexercise spectra representing a typical level of exercise-induced PCr depletion (ΔPCr). Panels (C) and (D) show example baseline and postexercise spectra representing a minimally acceptable level of PCr depletion, with ΔPCr ~ 33%.

Figure 2.

Scatterplots of relationships between phosphocreatine recovery rate (k _PCr), peak oxygen consumption (peak VO₂), and age.

Figure 3.

Covariate-adjusted associations between age and gait speed displayed graphically using adjusted variable plots (see statistical analysis for explanation). The slopes in Model 4 were less steep than the slopes for the corresponding gait test in Model 1, especially for UGS-150s.