To examine the effect of high intensity physical activity on the post-exercise PCr recovery rate (k_PCr), and test the hypothesis that the decline of muscle quality may be attributed to an age-related decline in muscle mitochondrial energetic capacity.

We investigated the relationship between the post-exercise phosphocreatine (PCr) recovery rate (k_PCr), and measures of muscle quality in the Baltimore Longitudinal Study of Aging (BLSA) participants (n = 258, age = 72 ± 13 years, male = 120, female = 138). In-vivo ³¹P MRS measurements were obtained using a 3T Philips Achieva MR scanner (Philips, Best, The Netherlands) and a 10-cm ³¹P-tuned surface coil (PulseTeq, Surrey, United Kingdom) fastened over the left thigh vastus lateralis muscle; participants performed a rapid ballistic knee extension exercise while lying supine in the bore of the magnet, following a protocol similar to that outlined by Coen et al.¹ A series of pulse-acquire ³¹P spectra were obtained before, during, and after the knee extension exercise. The pulse sequence consisted of adiabatic RF excitation pulses with a 90-degree flip angle, TR = 1.5 s, averaged over four scans, with 6 s between consecutive time points. A total of 75 dynamic time points were acquired, for a total of 7.5 minutes scan time. The duration of exercise was monitored to achieve between a 33 – 66% reduction in PCr signal amplitude; spectra were processed using jMRUI (version 5.0), and quantified using a nonlinear least squares algorithm (AMARES).^2,3 The recovery rate for phosphocreatine was calculated by fitting the time-dependent change in [PCr] to a mono-exponential function of the form PCr(t) = PCr(0) + {PCr_rest – PCr(0)}{1 – exp(-t/ τ_PCr)}; k_PCr, the recovery rate constant is defined as 1/ τ_PCr and is accepted as a marker of mitochondrial oxidative capacity.⁴

One functional metric of muscle quality (Nm/cm²) is defined as the ratio between knee-extension isokinetic strength (isokinetic dynamometer, 30 deg/s), and thigh muscle cross-sectional area (CMA) as measured by CT (computed topography). To account for variations in subject height, we define a normalized CMA, N_CMA = [(CMA_x/(h_x)²]*(h_mean)², where CMA_x and h_x denote the CMA and height, respectively, for the xth subject, and h_mean is the average height from all subjects in the population. Muscle quality is defined by N_NMQ = isokinetic peak torque/N_CMA.⁵

The amount of high intensity exercise is defined by the self-reported number of minutes per week spent participating in a high intensity physical activity.

The cross-sectional results from the BLSA show that the PCr recovery rate decreases with increasing age (Figure 1), but, along with age, the frequency of high intensity exercise per week is a significant predictor of k_PCr in both genders, when adjusting for age, height, and weight (Table 1), suggesting that the amount of intense physical activity modulates the mitochondrial oxidative capacity of skeletal muscle.

Muscle quality, N_NMQ, has previously been shown to decline with age; that relationship is demonstrated here again (Table 2).⁵ However, when adjusting for age and weight (N_NMQ is already normalized by height), k_PCr does not appear as a significant predictor of muscle quality in either gender. A linear regression model for N_NMQ was fit for predictors including age, weight, frequency of high intensity exercise and k_PCr; neither the exercise variable nor k_PCr were significant (data not shown). These results suggest that while bioenergetic capacity in skeletal muscle can be increased by high levels of activity, k_PCr does not act as a limiting factor for max knee extension torque. This work will be extended using other biochemical and functional measures available through the BLSA in an effort to further define the elements of the decline in physical function with age.

Age and frequency of high intensity exercise are predictors of the PCr recovery rate in both genders; frequency of high intensity physical activity appears to be positively correlated with k_PCr in men, and negatively correlated with k_PCr in women. However, k_PCr is not strongly associated with assessments of strength-defined muscle quality in either gender.