In order to investigate the difficulty of quadriceps training in the presence of an effusion into the knee we examined 13 patients with chronic effusions by recording isometric muscle strength. Maximal strength was markedly lower in the presence of an effusion, and aspiration of the effusion produced a 13.6% increase in strength (p less than 0.01). A further, small increase of 8% was recorded after intra-articular lignocaine injection. Isometric strength and surface integrated EMG correlated well in six patients. Two reflex mechanisms seem to inhibit quadriceps innervation in the presence of a persistent knee effusion, one mediated by pressure sensitive receptors, the other still unknown. Joint aspiration and systemic or intra-articular anti-inflammatory drug treatments are advised before any programme of quadriceps training to allow maximum effects to be achieved.