Ramp Shortening Research Articles

Can a rheological muscle model predict force depression/enhancement?

A new phenomenological model of activated muscle is presented. The model is based on a combination of a contractile element, an elastic element that engages upon activation, a linear dashpot and a linear spring. Analytical solutions for a few selected experiments are provided. This model is able to reproduce the response of cat soleus muscle to ramp shortening and stretching and, unlike standard Hill-type models, computations are stable on the descending limb of the force–length relation and force enhancement (depression) following stretching (shortening) is predicted correctly. In its linear version, the model is consistent with a linear force–velocity law, which in this model is a consequence rather than a fundamental characteristic of the material. Results show that the mechanical response of activated muscle can be mimicked by a viscoelastic system. Conceptual differences between this model and standard Hill-type models are analyzed and the advantages of the present model are discussed.

Effects of rapid shortening on rate of force regeneration and myoplasmic [Ca2+] in intact frog skeletal muscle fibres.

1. The effect of rapid shortening on rate of force regeneration (dF/dtR) was examined in single, intact frog (Rana temporaria) skeletal muscle fibres (3.0 C). Step releases leading to unloaded shortening were applied after 500 ms of stimulation, during the plateau of an isometric tetanus. Initial mean sarcomere length ranged from 2.05 to 2.35 micrometer; force regeneration after shortening was at 2.00 micrometer. 2. Values for dF/dtR following a 25 nm half-sarcomere-1 release were 3.17 +/- 0.17 (mean +/- s.e.m., n = 8) times greater than the initial rate of rise of force before release (dF/dtI). As release size was increased from 25 to 175 nm half-sarcomere-1, the relationship between release size and dF/dtR decreased sharply before attaining a plateau value that was 1.34 +/- 0.09 times greater than dF/dtI. Despite wide variations in dF/dtR, the velocity of unloaded shortening remained constant (2.92 +/- 0.08 micrometer half-sarcomere-1 s-1; n = 8) for the different release amplitudes used in this study. 3. To investigate its role in the attenuation of dF/dtR with increased shortening, the effects of rapid ramp (constant velocity) shortening on intracellular free Ca2+ concentration ([Ca2+]i) were monitored using the Ca2+-sensitive fluorescent dye furaptra. Compared with an isometric contraction, rapid fibre shortening was associated with a transient increase in [Ca2+]i while force regeneration after shortening was associated with a transient reduction in [Ca2+]i. The greatest reductions in [Ca2+]i were associated with the largest amplitude ramps. 4. Cross-bridge-mediated modifications of the Ca2+ affinity of troponin C (TnC) may explain the fluctuations in [Ca2+]i observed during and after ramps. Associated fluctuations in TnC Ca2+ occupancy could play a role in the reduction of dF/dtR with increasing release size.

Comparison of energy output during ramp and staircase shortening in frog muscle fibres.

1. We compared the rates of work and heat production during ramp shortening with those during staircase shortening (sequence of step releases of the same amplitude, separated by regular time intervals). Ramp or staircase shortening was applied to isolated muscle fibres (sarcomere length, 2.2 microns; temperature, approximately 1 degree C) at the plateau of an isometric tetanus. The total amount of shortening was no greater than 6% of the fibre length. 2. During ramp shortening the power output showed a maximum at about 0.8 fibre lengths per second (Lo s-1), which corresponds to 1/3 the maximum shortening velocity (Vo). For the same average shortening velocity during staircase shortening (step size, approximately 0.5% Lo) the power output was 40-60% lower. The rate of heat production for the same average shortening velocity was approximately 45% higher during staircase shortening than during ramp shortening. 3. The relation between rate of total energy output and shortening velocity was well described by a second order regression line in the range of velocities used (0.1-2.3 Lo s-1). For any shortening velocity the rate of total energy output (power plus heat rate) was not statistically different for staircase (step size, approximately 0.5% Lo) and ramp shortening. 4. The mechanical efficiency (the ratio of the power over the total energy rate) during ramp shortening had a maximum value of 0.36 at 1/5 Vo; during staircase shortening, for any given shortening velocity, the mechanical efficiency was reduced compared with ramp shortening: with a staircase step of about 0.5% Lo at 1/5 Vo the efficiency was approximately 0.2. 5. The results indicate that a cross-bridge is able to convert different quantities of energy into work depending on the different shortening protocol used. The fraction of energy dissipated as heat is larger during staircase shortening than during ramp shortening.

Cross-bridge attachment and stiffness during isotonic shortening of intact single muscle fibers

Equatorial x-ray diffraction pattern intensities (I10 and I11), fiber stiffness and sarcomere length were measured in single, intact muscle fibers under isometric conditions and during constant velocity (ramp) shortening. At the velocity of unloaded shortening (Vmax) the I10 change accompanying activation was reduced to 50.8% of its isometric value, I11 reduced to 60.7%. If the roughly linear relation between numbers of attached bridges and equatorial signals in the isometric state also applies during shortening, this would predict 51-61% attachment. Stiffness (measured using 4 kHz sinusoidal length oscillations), another putative measure of bridge attachment, was 30% of its isometric value at Vmax. When small step length changes were applied to the preparation (such as used for construction of T1 curves), no equatorial intensity changes could be detected with our present time resolution (5 ms). Therefore, unlike the isometric situation, stiffness and equatorial signals obtained during ramp shortening are not in agreement. This may be a result of a changed crossbridge spatial orientation during shortening, a different average stiffness per attached crossbridge, or a higher proportion of single headed crossbridges during shortening.

Power and efficiency: how to get the most out of striated muscle.

White muscle fibres from dogfish were stimulated during sinusoidal cycles of shortening and lengthening that mimic the in vivo pattern of movement. The results show that the timing of the stimulation relative to movement and also the frequency of the movement affect mechanical power output and efficiency. Maximum mechanical power is produced at a higher frequency of movement than maximum efficiency. The value of maximum power for a cycle of movement is less than half that produced during ramp shortening of fully active fibres. Higher efficiency is achieved during cyclic movement than during ramps, probably because work can be done with lower "overhead" costs for activation and because little ATP is used during lengthening.

Shift of activity from slow to fast muscle during voluntary lengthening contractions of the triceps surae muscles in humans.

1. Raw or rectified and integrated electromyograms (integrated EMGs) of the leg muscles were recorded during (a) isotonic ramp shortening or lengthening contractions consisting of foot plantar flexions against a constant load, or dorsal flexions accomplished by braking the load and yielding to it, respectively, and (b) isometric increasing or decreasing plantar torques accomplished by graded contractions or relaxations of the triceps muscles. 2. During plantar flexions or increasing torques, the EMG of soleus, gastrocnemius lateralis, medialis, and peroneus increased in parallel. During decreasing torques, motor unit derecruitment took place gradually and simultaneously. The tibialis anterior was silent. During dorsal flexions, one of two characteristic patterns was observed in different subjects: (a) soleus was abruptly derecruited at the beginning of the task, while gastrocnemius lateralis (or medialis) exhibited a large recruitment lasting throughout the lengthening contraction; (b) soleus remained active during the task, showing large motor unit potentials, while the gastrocnemius lateralis recruitment was of a lesser extent than in (a). Peroneus derecruitment was gradual and tibialis anterior activity was absent in both cases. 3. The EMG patterns observed during plantar flexions or in increasing and decreasing torques, and the two patterns observed during shortening or lengthening contractions, were closely reproduced during sinusoidal oscillations of the foot or in isometric contractions and relaxations. 4. When recruitment of the gastrocnemius lateralis was present during dorsal flexion, the slope of its integrated EMG envelope was steeper, the higher the velocity of lengthening contraction. The most rapid and the slowest tasks, however, did not require its activation. Gastrocnemius lateralis integrated EMGs of an amplitude similar to those occurring during lengthening contractions were observed only during ballistic plantar flexions. 5. The two patterns of triceps activation occurring during lengthening contraction could be traced to different mechanical characteristics of the soleus muscles, the gastrocnemius lateralis being activated preferentially in subjects with long soleus half-relaxation times, and the soleus in subjects with short soleus half-relaxation times. 6. The soleus and gastrocnemius lateralis H reflexes were tested during shortening and lengthening contractions.(ABSTRACT TRUNCATED AT 400 WORDS)

Tension responses of frog skeletal muscle to ramp and step length changes.

Tension responses to ramp shortening of varying speed in whole muscle or single fibres from the plateau of an isometric tetanus, revealed at least two distinct phases. There was a fast initial drop in tension followed by a change of slope and a definite inflexion on the tension record. As the velocity of the imposed length change was increased, the inflexion point appeared at a lower tension. Similar inflexions were not observed during ramp releases to an elastic band or a segment of semitendinosus tendon. The tension records obtained with moderately fast ramp length changes to contracting muscle reflect the T1 and T2 phases of the tension transients.

Behavior of single motor units of human tibialis anterior muscle during voluntary shortening contraction under constant load torque

The recruitment and discharge frequency of motor units during voluntary shortening contraction with constant load torque were studied in tibialis anterior muscle of 10 human subjects trained to dorsiflex the tibiopedal joint as linearly as possible from 30° plantar flexion from the neutral position (tibiopedal joint angle, 90°) by 20° in 1, 2, 3, 5, 8, and 10 s. A constant load torque of 0, 5, 10, 20, and 25% of maximum voluntary torque of the muscle was applied. When speed of shortening contraction was constant there was considerable degree of constancy in the muscle length at which individual motor units were recruited. However, the recruitment order was not rigidly fixed among the units whose length range of recruitment overlapped. When the contraction was performed faster the units were recruited earlier. The discharge frequencies of the motor units changed little with a given ramp shortening of the muscle. There were also few changes in frequency among different contraction speeds. With increasing load torque to the muscle, the discharge frequency increased slightly at a rate of 1 to 2 Hz by increase of the load. The results suggest that the speed of shortening was coded by recruitment of motor units and that discharge frequency did not play any major role when a muscle was shortened under constant load torque.

Decrease in stiffness during shortening in calcium activated skinned muscle fibers.

Single fibers from frog sartorius or semitendinosus muscle were mechanically skinned and activated in ATP salt solution containing 10 micro M Ca2+ (7 degrees C). After development of an isometric contraction, fibers were released at constant speed (0.03-2.4 s-1). During ramp shortening, stiffness was determined from the slope of the tension-length diagram obtained during superimposed quick stretches. Both force and stiffness decreased, as the ramp shortening proceeded and approached a steady value after about 60 ms. An increase in speed of shortening caused a decrease in fiber tension and stiffness and an increase in the ratio of stiffness to tension, suggesting a decrease in both the number of attached crossbridges and in the average force per crossbridge.