Tendon adaptation to bone shortening.

The ability of tendon to adapt its length to imposed conditions was tested in rat soleus. Shortening of one tibia left tendon insertions intact, but reduced the distance between them. Tendon lengths were found to decrease after a short period of recovery (1 or 2 months) whether surgery was performed in young or adult animals. Comparison of tibia and tendon length correlation in control rats of different ages and in experimental animals showed that adaptation was more complete in young rats than in adult rats. A long period of recovery seemed to improve adaptation only in young rats. Hydrothermal isometric tension measurements indicated that collagen remodeling occurred during tendon adaptation to bone length, with the process being more marked at the muscle-tendon junction.

Adaptation of connective tissue length to immobilization in the lengthened and shortened positions in cat soleus muscle.

1. The aim of this study was to investigate length adaptation of connective tissue in adult cat soleus muscle immobilized in the lengthened position for 28 days (group I) or in the shortened position for 14 (group II) or 28 days (group II). 2. The method, combining passive tension-length curves of the whole muscle with measurement of the length of the contractile part of isolated fibres, enabled separate determination of length of the very slightly compliant tendinous part, and of the highly compliant parallel elastic component (PEC) mainly constituted of belly connective tissue. 3. PEC length was found to increase in group I and to shorten in group II and III, showing that it adapted to the length imposed on the muscle. 4. Tendinous part lengths did not change significantly in any of the three groups compared to the controls. 5. Comparison of the respective adaptations of PEC and contractile tissue lengths showed that PEC adaptation was insufficient in groups II and III.

Experimental rapid sarcomere loss with concomitant hypoextensibility.

Slow soleus muscle in guinea pigs was actively shortened by constant electrical stimulation of the sciatic nerve for 12 hours. At the end of this period, hypoextensibility and a 25% decrease in the numbers of sarcomeres were observed. When a 36- or 48-hour period was allowed to elapse after the end of stimulation, some recovery occurred, and the sarcomere numbers were then found to be diminished by only 5%. If the stimulation was performed when the muscle was prevented from shortening (by ankle fixation in dorsiflexion), there was no decrease in sarcomere numbers and no hypoextensibility after neuroaxis lesions, when abnormal contractions maintain lasting shortening of the muscle.

Decrease of muscle extensibility and reduction of sarcomere number in soleus muscle following a local injection of tetanus toxin.

Slow soleus muscle in guinea pig developed within 4--6 days after local injection of a sublethal dose of tetanus toxin and 2--4 days after the first signs of local tetanus, a myostatic contracture characterized by a change in the passive tension--lengthening curve associated with a considerable decrease of the sarcomere number. It was demonstrated by clinical and EMG investigations that the soleus did have a tetanic spasm at least within the 2--4 day period of observation. When local tetanus was confined to slow soleus by functional suppression of rapid gastrocnemius and ankle flexor muscles, the decrease of the sarcomere number still persisted. This decrease failed to occur after section of the nerve supplying the soleus when associated with an injection of the tetanus toxin, and was much greater than when the soleus was passively shortened for the same period of time by plaster cast.

Effects of denervation on the reduction of sarcomere number in cat soleus muscle immobilized in shortened position during seven days.

1) Immobilization of cat soleus muscle in shortened position for only seven days was sufficient to elicit a marked reduction in the sarcomere number of individual muscle fibres and a considerable decrease in extensibility. 2) These effects were similar to those observed in previous experiments (TABARY et al., 1972) during which the muscles were immobilized for four weeks. 3) When a denervation was made just prior to immobilization, then one noted a marked difference in the change: the sarcomere number was only slightly reduced and there was a very small change of extensibility. The contralateral non denervated side showed the expected reduction of sarcomere number and decrease of extensibility. 4) These results contrast with previous observations made on denervated muscles immobilized for a period of four weeks. 5) It is proposed that the differences in sarcomere number adaptation observed in denervated and non denervated immobilized soleus are caused by differences of sarcomere length between the two muscles caused by the absence of presence of muscle contraction.

The relationship between sarcomere length in the soleus and tibialis anterior and the articular angle of the tibia-calcaneum in cats during growth.

Sarcomere number and sarcomere length were studied in six groups of kittens ranging in age from 10 minutes to 5 months and compared with those of adult cats. Although the soleus muscle is a slow contracting muscle and the tibialis anterior a fast contracting muscle, both have previously been shown to have the same range relative to ankle movement. For a given angle of articulation the sarcomere length was found to be the same at all ages except perhaps for the newly born. In contrast, the sarcomere number differed considerably, being much higher in the older animals. The relationship between active tension and muscle length was also measured, and again no difference was found between the muscles at any age, although the shape of the curves for the soleus and tibialis anterior was different.

Functional adaptation of sarcomere number of normal cat muscle.

1. Physiological and histological data were obtained from soleus and tibialis anterior muscles of normal adult cats to study to what extent fibre length and sarcomere number varied between animals and how this was related to the physiological characteristics of the muscles. 2. For a given muscle, the variation in the sarcomere number of individual muscle fibres between animals was found to be about 25%. These difference could partly be explained by comparing the number of sarcomeres and the length of the fibula, which was chosen as an index of the size of the animal. The average sarcomere number in the tibialis anterior muscle was about 60% greater than in the soleus. The variations between animals and between the anterior tibialis and soleus muscles were significantly greater than the variations observed within the same muscle. 3. The sarcomere length is dependent upon the articular angle, that is to say, the length change imposed on the muscle. These length changes are more extensive in the soleus muscle (100%) than in the tibialis anterior muscle (60%). A very significant correlation was found between articular angle and sarcomere length (for soleus r = 0.98, for tibialis anterior r = 0.94). 4. Although fibre length did vary considerably within a given muscle, the sarcomere length showed only minor variations at any given angle. This suggests that sarcomere number is determined in each individual muscle fibre. Such an adaptation implies a concomitant adaptation of the tendinous part of the fibre. This adaptation, resulting in definite sarcomere length at a definite angle, has obvious physiological implications. 5. Conventional length vs. active tension curves were established for the soleus and the tibialis anterior muscles. It is suggested that the difference between the sarcomere number of the two muscles may result in the difference between the shape of curves of these two muscles. 6. Active torque-angle curves were established for the two muscles in situ. The shapes of the curves for soleus and tibialis anterior are similar in spite of the different mechanical conditions of the two muscles. This fact helps to explain why the two muscles, despite their similar articular range, had very different sarcomere number.

Effect of denervation on the adaptation of sarcomere number and muscle extensibility to the functional length of the muscle.

1. The effects of denervation on the response of the cat soleus muscle to immobilization at different lengths by plaster casts has been investigated for a period of 4 weeks.2. The passive length-tension properties of the denervated immobilized muscles were not significantly different from those of non-denervated muscles. Muscles immobilized in the shortened position showed a marked decrease in extensibility whether they were denervated or not. In all the other cases the length-tension curves were not significantly differetn from those of normal muscles.3. The denervated soleus muscle immobilized in the lengthened position was found to produce 25% more sarcomeres in series, whilst those immobilized in the shortened position lost 35%. This adaptation was essentially the same as in muscles that had been immobilized but not denervated.4. Denervation was found to have no effect on the recovery of muscles that had been subjected to 4 weeks immobilization in the shortened position. In these muscles the sarcomere increased back to the normal level within 4 weeks after removal of the plaster cast.5. The adjustment of sarcomere number to the functional length of the muscles does not therefore seem to be directly under neuronal control. It appears to be a myogenic response to the amount of passive tension the muscle is subjected to.

Physiological and structural changes in the cat's soleus muscle due to immobilization at different lengths by plaster casts.

1. Passive length-tension curves were established for cat soleus muscles that had been immobilized in different positions. Muscles that had been immobilized in the lengthened position showed no difference in their length-tension properties to those of normal muscles. However, those immobilized in the shortened position showed a considerable decrease in extensibility.2. Muscle fibre length, sarcomere length and the total number of sarcomeres along single teased fibres were also determined for muscles immobilized in different positions. Soleus muscles immobilized in the lengthened position were found to have 20% more sarcomeres in series than normal muscles whilst those immobilized in the shortened position had 40% less than normal muscles.3. When the plaster casts were removed from muscles that had been immobilized in the shortened position, the length-tension curves and sarcomere number returned to normal within 4 weeks. Muscles that were immobilized in a shortened position and then immobilized in a second position were found to rapidly adjust to the second position with respect to their passive length-tension properties and sarcomere number.4. A change in the number of sarcomere in series seems to be the way in which the sarcomere length of the muscle is adjusted to its new functional length. The change in the length-tension properties which accompanies a decrease in sarcomere number appears to be the mechanism which prevents the muscle from being overstretched.