1. Eleven patients with the bone loss of ageing were treated with the vitamin D analogue 1 alpha-hydroxycholecalciferol and calcium for 3--6 months. 2. Muscle biopsies were taken from the vastus lateralis before and after the treatment and the activity of several enzymes was measured. Succinate dehydrogenase and total phosphorylase activities, which are a measure of the oxidative capacity, were low and increased significantly with the treatment. The lactate dehydrogenase activity, which can be taken as a measure of the anaerobic metabolism, was normal and did not change with treatment. The phosphagen stores, ATP and creatine phosphate were low and increased to normal with treatment. 3. Histochemical classification of the fibre composition revealed that the treatment induced an increase in the relative number of fast-twitch a (FTa or type II A) fibres accompanied by a reduction of the fast-twitch b (FTb or type II B) fibres. The cross-sectional area of the FTa fibres also increased with the treatment. 4. The present findings indicate that treatment with the active vitamin D analogue, 1 alpha-hydroxycholecalciferol, and calcium improves the myopathy associated with the bone loss of ageing.

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Summary Four full thickness cartilage defects, two linear and two elliptical, and four subchondral cavity defects were created at the point of weightbearing of the medial femoral condyle in four experimental ponies. This study showed that subchondral bone cysts can develop following full thickness (cartilage only) linear cartilage defects at a weightbearing location. Subchondral bone cysts did not develop following the removal of an elliptical piece of cartilage, exposing the subchondral bone. Primary subchondral defects created in communication with the joint cavity did not heal by replacement with bone over a six‐month period. Fibrous and cartilaginous repair occurred which appeared to be unrelated to the size or depth of the subchondral defect. Irrespective of the size of the defects, the surrounding bone showed marked woven bone formation with remodelling of existing and newly formed bone. The resulting osteosclerosis indicated a positive balance and a natural limitation of cavity expansion. The positive balance affected only the surrounding bone and did not lead to re‐establishment of the bone within the cavity during the period studied.