Immature tendon adaptation to strenuous exercise.

White Leghorn roosters (3 wk old) were randomly assigned to runner or control groups. Runners were subjected to a progressive treadmill running program for 8 wk, 5 days/wk at 70-80% maximal O2 consumption (VO2 max). After 8 wk, runners showed a significant elevation in gastrocnemius fumarase activity (51%) and a 21% increase in VO2max compared with controls. The exercise program induced a significant increase in tendon collagen deposition (46%) without any changes in DNA, proteoglycan, and collagen concentrations or tendon dry weight. Also, tendon collagen from runners contained fewer (50%) pyridinoline cross-links. These results suggest that high-intensity exercise causes greater matrix-collagen turnover in growing chickens, resulting in reduced maturation of tendon collagen.

Adaptation of bone and tendon to prolonged hindlimb suspension in rats.

The rat hindlimb suspension model was used to ascertain the importance of ground reaction forces in maintaining bone and tendon homeostasis. Young female Sprague-Dawley rats were randomly assigned to either a suspended or a nonsuspended group. After 28 days, femur bones and patellar tendons were obtained for morphological and biochemical analyses. Prolonged suspension induced a significant change in the geometric configuration of the femur middiaphysis by increasing the minimum diameter (12%) without any significant alterations in cortical area, density, mineral, and collagen concentrations. Femur wet weight, length, DNA, and uronic acid concentrations of suspended animals were not significantly different from bones of nonsuspended rats. However, the collagen and proteoglycan concentrations in patellar tendons of suspended rats were 28% lower than the concentrations of matrix proteins in tissues obtained from nonsuspended animals. These data suggest that elimination of ground reaction forces induces alterations in tendon composition and femur diaphyseal shape by changing regional rates in bone remodeling and localized tendon strain. Therefore it appears that ground reaction forces are an important factor in the maintenance of cortical bone and patellar tendon homeostasis during weight-bearing conditions.

Rehabilitation of tendon injuries in sport.

Clinicians are faced with a growing number of athletes with injured tendons. Treatment of both acute and chronic injuries has proven to be quite complex. It is difficult to maintain the balance between resting the injured tendon and preventing atrophy of the surrounding muscles and joints. Questions also arise as to when the tendon should be strengthened and when the athlete is ready to return to full activity in sport. Through an awareness of the structural and mechanical properties of the tendon, an exercise programme for the rehabilitation of tendon injuries has been developed. It is recommended that this programme be used in combination with ice and other physical modalities. This approach will resolve most tendon injuries within 6 weeks of its implementation. The use of anti-inflammatory medications and surgery can only be recommended in select situations where more conservative measures are inadequate.

Regional and age variations in growing tendon.

Gastrocnemius tendons of 10 White Leghorn chickens at 6, 8, and 12 weeks of age were divided into proximal, middle, and distal portions to assess regional variability in composition and growth. Body weight increases approximately 150% during the period examined, whereas the lateral gastrocnemius muscle and tendon increase approximately 193% and 227%, respectively. No significant changes in cellularity (DNA concentration) or hydroxypyridinium (OHP) crosslinks occur with increasing age. Hydroxyproline (HYP) concentration increases by 12 weeks of age, as hexuronate, glucosamine, and galactosamine decrease. Composition shows some regional variation: the distal region of the tendon has a lower HYP concentration and increased GAGs and OHP crosslinks compared to either the proximal or middle regions, which do not differ from each other. The mean collagen fibril diameter increases with age, but the oldest tendons also contain more small diameter fibrils (< 40 nm). There is a unimodal fibril distribution at all three ages, although this has broadened by 12 weeks. The data from this study suggest that rapid tendon growth occurs throughout the time period examined and that changes characteristic of mature tendon, such as increased OHP crosslink concentration, have not yet developed in hatchlings because of the large amount of new tissue being produced. Whereas all three regions of the tendon are similar in size, composition of the distal region differs from that of the proximal and middle regions, suggesting that this portion of the tendon should be avoided when sampling a tendon.

Tendon hypertrophy is associated with increased hydroxylation of nonhelical lysine residues at two specific cross-linking sites in type I collagen.

This study was designed to investigate whether the changes in lysine hydroxylation known to occur in hypertrophic tendon occur randomly or at specific lysine residues in the type I collagen molecule. Peptides corresponding to the two known major cross-linking sites of type I collagen (a lysine (or hydroxylysine) at position 9N cross-linked to a hydroxylysine at 930 and a lysine (or hydroxylysine) at position 16C cross-linked to a hydroxylysine at position 87) were prepared by collagenase digestion, size fractionation, and separation by high performance liquid chromatography from normal chicken tendon and from chicken tendon subjected to increased tensile load as a result of muscle hypertrophy. The ratio of the difunctional cross-links dihydroxylysinonorleucine to hydroxylysinonorleucine in normal tendon is 0.75:1; this ratio is increased to 6:1 in hypertrophic tendon. The dihydroxylysinonorleucine to hydroxylysinonorleucine ratio is increased to the same extent in samples of the purified cross-linked peptides derived from both the N-terminal and C-terminal lysine aldehyde residues. On the other hand, the relative hydroxylysine content of preparations of the pooled larger helical peptides obtained by cyanogen bromide digestion of normal and hypertrophic tendons was essentially identical. These results demonstrate that there is a specific increase in hydroxylation of only the N- and C-terminal non-helical lysine residues that participate in the formation of the reducible difunctional cross-links of type I collagen in hypertrophic tendon, while the extent of hydroxylation of lysine residues in the helical regions is not affected. The specific mechanism by which the enzyme lysyl hydroxylase acting on its substrate can distinguish between lysine residues destined to be in non-helical versus helical regions in a nascent collagenous peptide that has not yet attained its final secondary structure remains to be defined.