ABSTRACT The active isometric force produced by muscles varies with muscle length in accordance with the force–length relationship. Compared with isometric contractions at the same final length, force increases after active lengthening (force enhancement) and decreases after active shortening (force depression). In addition to cross-bridges, titin has been suggested to contribute to force enhancement and depression. Although titin is too compliant in passive muscles to contribute to active tension at short sarcomere lengths on the ascending limb and plateau of the force–length relationship, recent evidence suggests that activation increases titin stiffness. To test the hypothesis that titin plays a role in force enhancement and depression, we investigated isovelocity stretching and shortening in active and passive wild-type and mdm (muscular dystrophy with myositis) soleus muscles. Skeletal muscles from mdm mice have a small deletion in the N2A region of titin and show no increase in titin stiffness during active stretch. We found that: (1) force enhancement and depression were reduced in mdm soleus compared with wild-type muscles relative to passive force after stretch or shortening to the same final length; (2) force enhancement and force depression increased with amplitude of stretch across all activation levels in wild-type muscles; and (3) maximum shortening velocity of wild-type and mdm muscles estimated from isovelocity experiments was similar, although active stress was reduced in mdm compared with wild-type muscles. The results of this study suggest a role for titin in force enhancement and depression, which contribute importantly to muscle force during natural movements.

ABSTRACT Laser diffraction (LD) is a valuable tool for measuring sarcomere length ( L s ), a major determinant of muscle function. However, this method relies on few measurements per sample that are often extrapolated to whole muscle properties. Currently it is not possible to measure L s throughout an entire muscle and determine how L s varies at this scale. To address this issue, we developed an actuated LD scanner for sampling large numbers of sarcomeres in thick whole muscle longitudinal sections. Sections of high optical quality and fixation were produced from tibialis anterior and extensor digitorum longus muscles of Sprague–Dawley rats ( N =6). Scans produced two-dimensional L s maps, capturing >85% of the muscle area per section. Individual L s measures generated by automatic LD and bright-field microscopy showed excellent agreement over a large L s range (ICC>0.93). Two-dimensional maps also revealed prominent regional L s variations across muscles.

SUMMARY The mechanisms causing the steady-state force enhancement following active skeletal muscle stretching are not well understood. Recently, we found direct evidence that part of the force enhancement is associated with the engagement of a passive component. In this study, we reproduced the conditions that give consistent passive force enhancement and evaluated the mechanical properties of this passive force enhancement so as to gain insight into its source. The three primary results were that (1) the passive force enhancement is long lasting (>25 s), (2) passive force enhancement was reduced in a dose-dependent manner by the amount of shortening preceding active muscle stretching, and (3) passive force enhancement could be abolished`instantaneously' by shortening-stretching the passive muscle by an amount equivalent to the active stretch magnitude. Together with the remaining results, we conclude that the source of the passive force enhancement must be arranged in parallel with the contractile force, it must consist of a viscoelastic molecular spring whose stiffness characteristic can be reset by shortening, and it must have a characteristic length that is governed by the length of the contractile components, possibly the sarcomeres. Based on these results, the molecular spring titin emerges as a possible candidate for the passive component of the steady-state force enhancement observed in this and previous studies.

SUMMARY We investigated force enhancement following stretching in the in situ cat soleus muscle on the ascending and descending limb of the force-length relationship by varying the amount and speed of stretching and the frequency of activation (5 Hz, 30 Hz). There was a small but consistent( P <0.05) amount of force enhancement following muscle stretching on the ascending limb of the force—length relationship for both stimulation frequencies. The steady-state active isometric forces following stretches of 9 mm on the descending limb of the force—length relationship were always equal to or greater than the corresponding forces from the purely isometric contractions at the length at which the stretch was started. Therefore, force production for these trials showed positive stiffness and was associated with stable behavior. Following active stretching of cat soleus on the descending limb of the force—length relationship,the passive forces at the end of the test were significantly greater than the corresponding passive forces for purely isometric contractions, or the passive forces following stretching of the passive muscle. This passive force enhancement following active stretching increased with increasing magnitude of stretch, was not associated with structural damage, and only disappeared once the muscle was shortened. For stretches of 6 mm and 9 mm, the passive force enhancement accounted for more than 50 % of the total force enhancement,reaching a peak contribution of 83.7 % for the stretches of 9 mm at a speed of 3 mm s -1 . The results of this study suggest that a passive structural element provides a great part of the force enhancement on the descending limb of the force—length relationship of the cat soleus. Furthermore, the results indicate that mechanisms other than sarcomere length non-uniformity alone are operative.

The aim of this study was to evaluate how fish locomote at different muscle temperatures. Sarcomere length excursion and muscle shortening velocity, V , were determined from high-speed motion pictures of carp, Cyprinus carpio (11–14 cm), swimming steadily at various sustained speeds at 10, 15 and 20°C. In the middle and posterior regions of the carp, sarcomeres of the lateral red muscle underwent cyclical excursions of 0.31 μm, centred around the resting length of 2.06 μm (i.e. from 1.91 to 2.22 μm). The amplitudes of the sarcomere length excursions were essentially independent of swimming speed and temperature. As tail-beat frequency increased linearly with swimming speed regardless of temperature, the sarcomeres underwent the same length changes in a shorter time. Thus, V increased in a linear and temperature-independent manner with swimming speed. Neither temperature nor swimming speed had an influence on tail-beat amplitude or tail height. Our findings indicate that muscle fibres are used only over a narrow, temperature-independent range of V / V max (0.17-0.36) where power and efficiency are maximal. Carp start to recruit their white muscles at swimming speeds where the red muscle V / V max becomes too high (and thus power output declines). When the V / V max of the active muscle falls too low during steady swimming, carp switch to ‘burst-and-coast’ swimming, apparently to keep V / V max high. Because V max (maximum velocity of shortening) of carp red muscle has a Q 10 of 1.63, the transition speeds between swimming styles are lower at lower temperatures. Thus, carp recruit their white anaerobic muscle at a lower swimming speed at lower temperatures (verified by electromyography), resulting in a lower maximum sustainable swimming speed. The present findings also indicate that, to generate the same total force and power to swim at a given speed, carp at 10°C must recruit about 50% greater fibre cross-sectional area than they do at 20°C. Note: Present address: Department of Plant Molecular Biology, University of Tennessee, Knoxville, TN 37996, USA. Present address: Department of Pure and Applied Biology, University of Leeds, Leeds LS2 9JT, England.