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At least 433 records · Page 24Linked to original sources
PubMed1998

Effects of BAPTA on force and Ca2+ transient during isometric contraction of frog muscle fibers.

The effects of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) on force and intracellular Ca2+ transient were studied during isometric twitches and tetanuses in single frog muscle fibers. BAPTA was added to the bathing solution in its permeant AM form (50 and 100 microM). There was no clear correlation between the changes in force and the changes in Ca2+ transient. Thus during twitch stimulation BAPTA did not suppress the Ca2+ transient until the force had been reduced to <50% of its control value. At the same time, the peak myoplasmic free Ca2+ concentration reached during tetanic stimulation was markedly increased, whereas the force was slightly reduced by BAPTA. The effects of BAPTA were not duplicated by using another Ca2+ chelator, EGTA, indicating that BAPTA may act differently as a Ca2+ chelator. Stiffness measurements suggest that the decrease in mechanical performance in the presence of BAPTA is attributable to a reduced number of active cross bridges. The results could mean that BAPTA, under the conditions used, inhibits the binding of Ca2+ to troponin C resulting in a reduced state of activation of the contractile system.

Animals↗
PubMed1984

Redistribution of sarcomere length during isometric contraction of frog muscle fibres and its relation to tension creep.

Changes in length of successive 0.5-0.8 mm segments along single muscle fibres of Rana temporaria were recorded during 3 s isometric (fixed fibre ends) tetani at 2.15 and 2.60 micron sarcomere length. The measurements were performed by means of a photo-electric detector system which recorded the distance between opaque markers (ca. 60 microns in width) that were attached to the upper surface of the fibre. The segment length change had an initial rapid phase (1) which coincided with the steep rise of force and a subsequent slow phase (2) which coincided with the upper, rounded portion of the force myogram and the 'plateau' of the tetanus. At 2.15 micron sarcomere length the majority of the central segments (comprising approximately 90% of the fibre) shortened to various degrees during phase 1. A considerable redistribution of length occurred during phase 2 in that some segments shortened at the expense of others which were forcibly stretched. The central region, taken as a whole, shortened by 0.1-0.5% during phase 2. The end segments were consistently found to elongate during phase 1. However, they were able to hold the tension, without further elongation, during phase 2. The pattern of length changes within the central region of the fibre observed at 2.15 micron sarcomere spacing remained largely the same after increasing the sarcomere length to 2.60 micron. However, in contrast to the situation at 2.15 micron sarcomere length there was an over-all (0.4-1.5%) elongation of the central region of the fibre during phase 2 at the great fibre length. This elongation of the central region was associated with marked shortening of the end segments. The sarcomere length of the end segments (s.1.e) was compared to that of the central region of the fibre (s.l.c) at various fibre rest lengths. There was no significant difference between s.l.e and s.l.c when the fibre was just taut, i.e. at approximately 2.1 micron sarcomere length. The following relationship between s.l.e and s.l.c was found to apply for values of s.l.c ranging between 2.2 and 2.7 micron: s.l.e = 0.636 s.l.c + 0.744 (correlation coefficient, 0.93). The possibility was explored that redistribution of sarcomere length along the fibre causes the slow climb of force ('tension creep') that occurs during a tetanus at great (greater than 2.2 micron) sarcomere lengths. Tension creep could be reproduced, after peak force had been attained, during an isometric tetanus by releasing the fibre to shorten within the range 2.6-2.3 micron sarcomere length.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1989

Endurance and recovery from a sustained isometric contraction in human jaw-elevating muscles.

Ten males sustained maximal voluntary contractions (MVC) of the jaw elevators. Unilateral bite force and electromyographic (EMG) activity were recorded from the right masseter and temporalis. The experiment comprised three endurance trials, and each trial consisted of two sustained 100% MVC clenching tasks. Between the two tasks, the subjects took a randomly assigned rest of either 30, 120 or 300 s. Immediately after each task, they performed an additional brief 100% MVC to check for contractile failure. EMGs were taken from the beginning and end of each sustained 100% MVC and used to determine and compare the EMG centre frequency. The difference in endurance times between the first and second clenchings was greatest for the 30-s rest and progressively decreased as the rest period increased. The beginning EMG centre frequency was significantly greater than the end for each clenching. These findings strongly support the proposal that even though the jaw elevators are resistant to a contractile element failure, a contraction-induced pain is produced, which limits a sustained jaw-closing effort. Recovery from this effort pain is related to the post-contraction blood flow.

Adult↗
PubMed1997

A simulation study of the electromyographic volley at initiation of rapid isometric contractions in the first dorsal interosseous muscle.

Monopolar surface electromyograms (EMGs) of rapid isometric abduction of the first dorsal interosseous muscle (FDI) were initiated from an EMG volley that was characterized by a negative potential lasting over several tens of milliseconds. An EMG model was developed to study how the EMG volley was generated. EMGs were defined as the linear summations of surface-recorded action potential trains originating from single motor units (MUs). All action potential trains had the same discharge pattern but different recruitment thresholds, depending on the potential amplitude. Real action potentials in single MUs in FDI were recorded with a monopolar surface electrode, one of which was used as a prototype wave in simulation. The model predicted an initial negative potential comparable to that of the EMG volley observed in rapid contractions of FDI. Results from our simulation studies suggest that the EMG volley is caused by at least two independent factors: (1) the negative phase of the action potential is greater in area than the positive one, in which the effect is enhanced by the high discharge rate of many MUs; (2) many MUs are recruited within a short time in an orderly fashion starting from those with small action potentials to those with large ones.

Action Potentials↗
PubMed2001

Effects of different duration isometric contractions on tendon elasticity in human quadriceps muscles.

1. The present study aimed to investigate the influence of isometric training protocols with long- and short-duration contractions on the elasticity of human tendon structures in vivo. The elasticity was assessed through in vivo determination of the elongation (L) of the tendons and aponeuroses using ultrasonography, while the subjects performed ramp isometric exercise up to maximum voluntary contraction (MVC). 2. Eight young males completed 12 weeks (4 days per week) of a unilateral isometric training programme on knee extensors, which consisted of two different combinations of contraction and relaxation times at 70 % MVC: one leg was trained using a short-duration protocol (3 sets of 50 repetitions of contraction for 1 s and relaxation for 2 s), and the other leg was trained using a long-duration protocol (4 sets of a combination of contraction for 20 s and relaxation for 1 min). The training volume per session, expressed as the integrated torque, was the same for the two protocols. 3. Both protocols resulted in a significant increase in MVC: 31.8 +/- 17.2 % for the short-duration protocol and 33.9 +/- 14.4 % for the long-duration protocol. Moreover, the training produced significant increases in the muscle volume of the constituents of the quadriceps femoris, with similar relative gains for the two protocols: 7.4 +/- 3.9 % for the short-duration protocol and 7.6 +/- 4.3 % for the long-duration protocol. 4. The short-duration protocol produced no significant change in L values at any of the force production levels. For the long-duration protocol, however, the L values above 550 N were significantly shorter after training. Analysis revealed that the group x test time interaction effect on tendon stiffness was significant. Stiffness increased significantly for the long-duration protocol, but not for the short-duration protocol. 5. The present study demonstrates a greater increase in stiffness of human tendon structures following isometric training using longer duration contractions compared to shorter contractions. This suggests that the changes in the elasticity of the tendon structures after resistance training may be affected by the duration of muscle contraction.

Adult↗
PubMed1990

Influence of simultaneous bilateral exertion on muscle strength during voluntary submaximal isometric contraction.

The influence of simultaneous bilateral exertion on muscle strength was tested under the conditions in which the same or different levels of strength were exerted by the right and left arm (or hand). Isometric muscle strength of elbow flexion, elbow extension and hand grip was studied. Subjects voluntarily exerted 25%, 50%, and 75% of maximal strength based on their subjective judgement without the feedback of the strength actually exerted. Involuntary decrements of muscle strength were caused by the bilateral exertion. Muscle strength of both sides decreased under the condition where the same level of strength was exerted by the right and the left arm (or hand). When different levels of strength were exerted by each arm (or hand), the strength of the weaker side considerably decreased, while the strength of the stronger side did not decrease.

Adolescent↗
PubMed2002

Effects of volatile anesthetics on elastic stiffness in isometrically contracting ferret ventricular myocardium.

The effects of halothane, isoflurane, and sevoflurane on elastic stiffness, which reflects the degree of cross-bridge attachment, were studied in intact cardiac muscle. Electrically stimulated (0.25 Hz, 25 degrees C), isometrically twitching right ventricular ferret papillary muscles (n = 15) at optimal length (L(max)) were subjected to sinusoidal length oscillations (40 Hz, 0.25- 0.50% of L(max) peak to peak). The amplitude and phase relationship with the resulting force oscillations was decomposed into elastic and viscous components of total stiffness in real time. Increasing extracellular Ca(2+) concentration in the presence of anesthetics to produce peak force equal to control increased elastic stiffness during relaxation, which suggests a direct effect of halothane and sevoflurane on cross bridges.

Anesthetics, Inhalation↗