The range of sarcomere lengths in the muscles of the human lower limb.

The lengths of the sarcomeres of some muscles of the human leg were determined for the anatomical position, using a method based on diffraction. Measurements were made of the muscle lengths and angles of pennation from cadavers, and these were used to predict sarcomere lengths at other limb positions. The measured and predicted sarcomere lengths were compared with the length-tension curve for human muscle, which showed the range of sarcomere length from both extremes of muscle length to cover the entire range of the length-tension curve.

Double-hyperbolic force-velocity relation in frog muscle fibres.

1. The relationship between force and velocity of shortening was studied at 2.10 micron sarcomere length during fused tetani (1-3 degrees C) in single fibres isolated from the anterior tibialis muscle of Rana temporaria. The speed of shortening was recorded from the whole fibre and, in some experiments, simultaneously from a short (ca. 0.6 mm) segment, while the preparation was released to shorten isotonically at selected force levels ('load-clamp' recording). The segment was defined by opaque markers of hair that were placed on the fibre surface. The distance between the markers was recorded by means of a photo-electric detector system. 2. The force-velocity relation had two distinct regions, each one exhibiting an upwards concave shape, that were located within the ranges 0-78 and 78-100% of the measured isometric force (P0), respectively. The two portions of the force-velocity relation could be fitted well by hyperbolic functions or by single-exponential functions. The curvature was more pronounced in the high-force region than at low-intermediate loads. The transition between the two portions of the force-velocity relation (the 'break point' of the force-velocity curve) occurred at 78.4 +/- 0.4% of P0 (mean +/- S.E. of mean, n = 12) corresponding to 10.9 +/- 0.4% of maximum velocity of shortening (Vmax). The general shape of the force-velocity curve, and the appearance of a break point near 78% of P0, was the same when measurements were made from the whole fibre and from a short segment along the same fibre. 3. The 'negative' branch of the force-velocity relation was delineated for loads ranging from P0 to 1.6-1.8 P0 in five experiments. The negative branch formed a smooth continuation of the force-velocity relation recorded between 0.78 P0 and P0. The force-velocity relation was nearly flat between 0.90 P0 and 1.20 P0, the difference in speed of shortening or elongation being 1.8 +/- 0.3% (mean +/- S.E. of mean, n = 5) of Vmax over this range. 4. An increase in sarcomere length from 1.85 to 2.60 micron did not affect Vmax but caused a steady decrease in curvature of the force-velocity relation, both at low-intermediate loads and in the high-force range. Similar changes in shape of the force-velocity relation were produced by osmotic compression of the fibre in a Ringer solution made hypertonic by addition of 98 mM-sucrose.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulation rate, potentiators, and sarcomere length-tension relationship of muscle.

Isometric tetani of single muscle fibers of Rana temporaria were studied as a function of stimulation rate, sarcomere length (1.7-2.3 micron), twitch-to-tetanus ratio, and exposure to twitch potentiators (Zn2+ and NO3-) at 20 degrees C. As the stimulation rate was decreased below a maximal level, tension generation decreased. This depression in tension generation was more pronounced at shorter sarcomere lengths. Therefore the magnitude and shape of the sarcomere length curve was dependent on stimulation rate. Although the depression in tension generation was always accompanied by a noticeable ripple in the tension record in fibers with large twitch-to-tetanus ratios, it could be observed even during well-fused tetani in fibers with low twitch-to-tetanus ratios. In all fibers, however, high stimulation rates or exposure to potentiators resulted in maximum tension generation at each length, and the sarcomere length-tension curve followed that found by Gordon, Huxley, and Julian. This indicates that the fall in tension between sarcomere lengths of 2.0 and 1.7 micron is not due to length-dependent activation but is more likely to be the result of mechanical interference in the force-generating interaction between cross bridges and thin filament sites.

Tension development and calcium sensitivity in skinned muscle fibres of the frog.

Calcium activated isometric tension development was measured in single skinned muscle fibres of the ileofibularis muscle of the frog. The experiments were carried out at 5 degrees C, pH = 6.9, 1 mM free Mg2+ and an ionic strength of 160 mM. A Hill curve was fitted to the isometrically developed tension at different Ca2+ concentrations by means of a non-linear least mean square approximation. At a sarcomere length of 2.15 micron, the Ca2+ concentration for half maximum tension (K) was 1.6 microM. This Ca2+ concentration decreased with increasing sarcomere length; at 2.7 micron, K was 1.1 microM and at 3.1 micron, K was 0.9 microM. Therefore, Ca sensitivity is increased at larger sarcomere lengths. Consequently, the optimal sarcomere length for tension development shifted to larger values when the Ca2+ concentration was lowered. Osmotic compression of the fibre at 2.15 micron by means of 5% Dextran also caused an increase in Ca sensitivity (K was 1.0 microM). At 2.7 micron, addition of 5% Dextran hardly affected the Ca sensitivity. The possible role of the interfilament spacing in the explanation of these results discussed.

A laser diffraction method for measuring muscle sarcomere length in vivo for application to tendon transfers.

A technique that uses laser light diffraction to measure muscle sarcomere length allows direct determination of optimal muscle length during tendon transfers. Forearm muscle sarcomere length with the hand in the position of function is 2.4, and muscle length corresponds directly to sarcomere length. We have used these observations to restore optimal muscle length during tendon transfers. Standard high radial nerve tendon transfers in six fresh cadaver forearms demonstrated the efficacy of the laser diffraction method in accurately measuring sarcomere length. In two clinical trials with the laser, standard high radial nerve palsy tendon transfers were performed. In each case the clinical tendency was to overpull the muscle during the transfer. With the laser it was possible to identify excessive muscle stretch and restore optimal muscle length.

Architecture of the human gastrocnemius muscle and some functional consequences.

Measurements were performed on the m. gastrocnemius of eight human cadavers in order to describe, in some detail, the architecture of the muscle and its heads. The fibres of the lateral head contain more sarcomeres than those of the medial head. The effect of this difference on the length-force curve of the muscle, calculated with a planimetric muscle model, is diminished by the effect of the difference of fibre angle with respect to the line of pull of the muscle. Within the heads some variation of the number of sarcomeres of the proximal and distal fibres occurs in all muscles. In the lateral head the distal fibres contain fewer sarcomeres than the proximal fibres. In the medial head this is also found in some heads, while others show the reverse. In the lateral head the longer fibres have smaller angles of attachment to the tendon plate and vice versa, while in the medial heads this relationship is only found occasionally. Some variation in the number of sarcomeres is found in the fibres of one bundle. The effects of variations in the number of sarcomeres on the length-force curve are probably insignificant at greater muscle lengths, but may have some importance for the individual with relatively small muscle lengths.

Dependence of shortening heat on sarcomere length in frog muscle and fiber bundles.

The relation between shortening heat and sarcomere length was studied using fiber bundles from frog semitendinosus muscles as well as using whole muscles. The initial sarcomere length was varied between 2.0 and 3.66 micron. Shortening heat was estimated as the excess heat produced after a rapid isovelocity release in a 3 sec tetanus at 0 degrees C. The isometric control heat was measured in the same tetanus, before and after the period of shortening. The unstimulated whole muscles showed a large thermoelastic absorption of heat when released at sarcomere lengths longer than 2.5 micron, and the apparent shortening heat was negative at very long sarcomere lengths. The apparent shortening heat was corrected by subtracting the thermoelastic heat absorption by assuming that the thermoelastic effect was also present in releases of active muscles. The corrected shortening heat decreased linearly with increasing sarcomere length in the range 2.29-3.66 micron, intersecting the length axis at 3.73 +/- 0.21 micron. The thermoelastic heat absorption at long sarcomere lengths was substantially reduced in fiber bundles, suggesting that the parallel elasticity responsible for the thermoelastic effect is mainly present outside muscle cells. The corrected shortening heat in fiber bundles also decreased linearly with increasing sarcomere length, intersecting the length axis at 3.84 +/- 0.25 micron. Thus the results on fiber bundles, also based on correction but the extent of which is substantially smaller than in whole muscles, are in agreement with the results on whole muscles. The results are interpreted to mean that shortening heat is produced by the interaction of thick and thin filaments in contracting muscle.