Achieving a high cardio-respiratory fitness is pivotal in healthy and clinical populations, in fact it is a strong predictor of mortality; in addition, since the most common daily activities are carried out at submaximal intensities and require continuous transitions from one exercise intensity to another one, also the rate of adjustment of pulmonary O2 uptake (i.e. V ̇O2 kinetics) is a key feature that determine the tolerance to physical activities. Solid organ transplant recipients suffer from a reduced maximal O2 consumption (V ̇O2max) and previous studies suggested a remarkable contribution of the peripheral, muscular factors, to this impairment. Likewise, pulmonary V ̇O2 kinetics are slower in organ transplant recipients likely because of the oxidative metabolism defects of skeletal muscle that affect these patients. In fact, organ transplant patients suffer from defects of the skeletal muscle, resulting in the impairment of the factors determining the peripheral gas exchanges and these abnormalities are, likely, induced by the side effects of immunosuppressive therapies and the deconditioning/disuse. It is of note that endurance training (ET) protocols carried out with a small muscle mass (i.e. single leg cycling (SL)) results in greater improvements of the peripheral factors affecting O2 diffusion and utilization if compared to ET performed with large muscle masses (i.e. double leg cycling (DL)). Therefore, the effect of SL-ET vs DL-ET on V ̇O2max and pulmonary V ̇O2 kinetics (i.e. O2 deficit (O2Def), mean response time (MRT) and the amplitude of the pulmonary V ̇O2 slow component (SCamp)) were investigated in heart, kidney and liver transplanted recipients (HTx, KTx and LTx, respectively). Moreover, the role of the cardiovascular, central, and peripheral factors in affecting improvement after ET has never been quantified in these patients. Therefore, the application of the multifactorial model of V ̇O2max limitation is proposed in order to determine the contributions of cardiovascular and local factors to better quantify the origin of the main limitation of V ̇O2max in these patients. 33 patients (HTx = 13, KTx = 11 and LTx = 9) were recruited and divided in SL-ET and DL-ET groups and completed 24 sessions of ET. After the exercise program, the SL-ET group increased the V ̇O2max by 13.8% ± 8.7 (p < 0.001) because of a larger maximal O2 systemic extraction. The DL-ET group increased the V ̇O2max by 18.6% ± 12.7 (p < 0.001) due to the concomitant central and peripheral adaptations. However, no difference was found for the V ̇O2max improvement (p = 0.237) between the two groups. Furthermore, following the exercise program, SL-ET group decreased O2Def, MRT and SCamp by 16.4% (13.7) (p = 0.008), by 15.6% (13.7) (p = 0.004) and by 35% (31) (p = 0.002), respectively. Likewise, the DL-ET group, O2Def, MRT and SCamp dropped by 24.9% (16.2) (p < 0.0001), by 25.9% (13.6) (p < 0.0001) and by 38% (52) (p = 0.0003), respectively. Additionally, the magnitude of improvement for the O2Def, MRT and SCamp were not significantly different between SL-ET and DL-ET groups (p = 0.277, p = 0.083 and p = 0.601, respectively).
In conclusion the results indicate that SL-ET is a valid ET typology, in fact was as effective as DL-ET to improve i) V ̇O2max and ii) pulmonary V ̇O2 kinetics in HTx, KTx and LTx. Finally, it is suggested that the impaired peripheral O2 extraction and/or utilization play a remarkable and important role, but not superior to the central factors, in limiting V ̇O2max and exercise capacity in this type of patients.
