Objective: To evaluate the relationship between isokinetic strength, and aerobic aptitude in patients with chronic heart failure (CHF) and with chronic obstructive pulmonary disease (COPD). Methods: Seventeen stable CHF(n=11) and COPD (n=6) patients aged 51.12 ± 9.95 ys old (15 men, 2 women, CHF patients: NYHA class II: n=8 and III: n=3) underwent a dynamic isokinetic force evaluation of the quadriceps (IsoK) at an angular velocity of 180°s
$^{-1}$
, a symptom-limited cardiopulmonary exercise test with simultaneous monitoring of respiratory gases, and a 6-min walking test. Body composition was assessed by electrical bioimpedance. The relationships between muscle strength, maximal aerobic performance, results of endurance tests and anthropometric datas were assessed by simple regression analysis. Results: The isokinetic evaluation has been successfully performed in all patients. No cardiac arrhythmic event nor abnormal haemodynamic response was observed in either group of patients. In CHF patients, individual quadricipital muscle strength was significantly related to fat-free mass(r=0.862; p<0.01). However, neither peak O
$_2$
(global: r=0.562, p=NS or weight-adjusted: r=0.083, p=NS) nor endurance capacity(r=0.068, p=NS) were significantly related to the peripheral muscle strength. In COPD patients, quadricipital muscle strength was not significantly related to fat-free mass (r=0.242; p=NS) nor any other measured factor. Conclusion: Physiopathological mechanisms of exercise limitation in CHF and COPD patients are complex. Isokinetic strength and O
$_2$
peak are not related to each other and are therefore independent. Therefore, a systematic evaluation of isokinetic muscle strength may be of great value. Early degradations in aerobic capacity are probably followed by an alteration in muscle strength.
