[Changes in the H reflex induced by isometric training with respect to isotonic training].
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Adolescent↗
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Cortical excitability is not depressed in movement-modulated stretch response of human thumb flexor.
There is strong evidence that the predominant pathway of the long-latency stretch reflex for flexor pollicis longus crosses the motor cortex. This reflex response is diminished during active thumb movements. We tested the hypothesis that this could be due to a decrease in the excitability of the transcortical component during movement. During isometric, concentric and eccentric thumb movements, transcranial magnetic stimulation (TMS) of the motor cortex was given at a time when the reflex signal was traversing the motor cortex. TMS was also given earlier in separate runs when the signal was traversing the spinal cord under each of the three contractile conditions. The electromyogram was analysed for non-linear summation between stretch responses and the potential evoked by the cortical stimulus. The response to TMS alone was uniform across the three types of contraction, and the lack of cortical involvement in the short-latency reflex was confirmed. The TMS-evoked response summed in a non-linear manner with the long-latency reflex response, confirming that the excitability of the motor cortex was increased as the reflex signal passed through it. The long-latency response was markedly depressed during isotonic compared with isometric contractions. However, the non-linear summation was not greater during the isometric contractions. Thus, the depressed reflex responses during isotonic movements do not stem from reduced motor cortical responsiveness or afferent input to the transcortical pathway, and may instead reflect modulation of cutaneous reflexes during isotonic contractions.
Adolescent↗
Therapeutic exercise: types and indications.
In the intact subject, strength is determined by the physiologic cross-sectional area of all the muscle fibers as influenced by the activity of the neural elements. Passive range-of-motion exercises prevent or help correct contractures but do not build strength. High-resistance, low-repetition exercises simultaneously develop strength and endurance. Isotonic exercises are generally preferable to isometric exercises.
Exercise Therapy↗
Bronchial smooth muscle mechanics of a canine model of allergic airway hyperresponsiveness.
Although we have reported that tracheal smooth muscle from sensitized dogs shows altered mechanical properties, we did not know, because of technical difficulties with the preparation, whether similar changes occur in the properties of sensitized central bronchial smooth muscle (BSM), the site at which the acute asthmatic response is believed to develop. We have now succeeded in developing a cartilage-free BSM preparation that retains optimal mechanical properties. Such strips were obtained from mongrel dogs that had been sensitized to ragweed pollen. Controls were littermates injected with adjuvant alone. Length-tension relationships were obtained for both control and sensitized BSM strips (CBSM and SBSM, respectively). The maximal active stresses were the same (P greater than 0.05) when normalized to muscle fraction in total tissue cross-sectional area [6.2 +/- 0.6 x 10(4) and 5.9 +/- 0.6 x 10(4) (SE) for SBSM and CBSM, respectively]. This suggests that optimal tension is an insensitive indicator of bronchial hyperresponsiveness and that isotonic studies might be more revealing. The maximal shortening velocity (Vo) for SBSM at 2 s [0.35 +/- 0.017 (SE) lo/s, where lo signifies optimal muscle length], in the course of a 10-s contraction, was significantly greater (P less than 0.05) than Vo measured for CBSM (0.27 +/- 0.015 lo/s). However, Vo did not differ at the 8-s point of contraction. The sensitized group demonstrated a statistically significantly greater maximal shortening capacity (0.67 +/- 0.04 lo) than the control group (0.51 +/- 0.04 lo). At 2 s of contraction, 80% of maximal SBSM shortening had been completed and was significantly greater than for CBSM.(ABSTRACT TRUNCATED AT 250 WORDS)
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Task-dependent effects on motor-evoked potentials and on the following silent period.
The silent period (SP) after transcranial stimulation is used as a diagnostic tool in various central nervous system disorders although no standardized experimental setup has been established. The aim of this study was to demonstrate the influence of an isotonic compared to an isometric experimental condition. The SP after transcranial magnetic brain stimulation in the biceps brachii and brachioradialis muscle was up to 130% longer when elicited during a maintain-position (isotonic) task as compared to a maintain-force task (isometric) when stimulus intensities of 5% to 25% above threshold were used. The mean SP duration in these muscles was positively correlated to the mean contraction time in both tasks. However, no such relationship was observed for the trials within the individual subjects. We speculate that the invariably longer SP of the maintain-position task was due to the different "motor set" which predictively determined the muscle behavior after the stimulus. In the maintain-position trials, the stimulus-induced long-lasting flexion movement is counteracted by a motor set aiming to relax the elbow flexors immediately after the stimulus. In the maintain-force task the contraction twitch is short and a force drop below the preset level must be prevented by a motor set aiming to contract the elbow flexors immediately after the stimulus. The latter may increase the synaptic input to the motoneuron pool and facilitate the reoccurrence of the electromyogram terminating the SP. At high-stimulus intensities the SP duration increased in both tasks, and the task-dependent differences disappeared. Therefore, when using the SP duration for diagnostic purposes, isometric conditions and high-stimulus intensities should be used.
Adult↗
Transient length-related mechanical states in smooth muscle.
The isotonic shortening of electrically stimulated ovarian ligament smooth muscle strips from rabbits was studied by briefly applying sudden increases in afterload (force steps, 0.6 to 3.0 s long) just sufficient to halt the shortening. Upon removal of the extra afterload, the isotonic shortening velocity significantly increased compared with prestep velocities measured at the same muscle lengths. The degree of potentiation depended upon the duration of the force step. Muscles yielded initially when the step was applied, and their stiffness decreased. During the zero-velocity portion of the force step there was a steady increase in stiffness back to levels appropriate to that force. Complete force-velocity curves were made following short (0.6 s) and long (3.0 s) force steps. The values for Vmax (and all intermediate velocities) were significantly greater following a long force step. In a final experimental series, muscles were held isometric immediately after removal of the force step. Force rose monotonically, with a more rapid redevelopment following a long force step. A mathematical model is presented, according to which the effect of the duration of the force step on the poststep mechanical properties may be due to either the alteration of an internal resistance to shortening or a change in the kinetic properties of the cross-bridge array. A hypothesis is proposed relating the steady decline in isotonic shortening velocity to a partial local depletion of energy-yielding substrates.
Animals↗
Force-velocity relation of frog skeletal muscle fibres shortening under continuously changing load.
1. The force-velocity (P-V) relation of single frog skeletal muscle fibres was examined in non-steady-state conditions in which the load on the fibre changed continuously with time. Two such types of condition were used; in one type, the load was increased from zero to the maximum isometric force (P0) (auxotonic condition), while in the other type the load was decreased form P0 to zero at constant rates (ramp decrease in load). 2. The P-V curves obtained in the auxotonic condition were convex upwards and always below the hyperbolic P-V curve obtained in the isotonic condition. Different curves were obtained depending on the compliance of auxotonic load. The shortening velocity for a given amount of load increased with increasing compliance. 3. Qualitatively similar P-V relations were obtained irrespective of whether the fibre was made to shorten auxotonically at the onset of stimulation or after the development of P0. 4. If the force at any time after the onset of auxotonic shortening was normalized relative to the isometric force at the same time after the onset of isometric force development, the normalized force versus velocity curves were found to fit well to the hyperbolic P-V curve in the isotonic condition except for the low-force region. 5. The P-V curves obtained during the ramp decrease in load were hyperbolic in shape except for the humps at the high-force region and always above the P-V curve in the isotonic condition. Different curves were obtained depending on the rate of load decrease. The maximum shortening velocity increased with increasing rate of load decrease. 6. The above features of the P-V relations could well be simulated by a simplified Huxley contraction model, indicating that the kinetic properties of the cross-bridges are the same in both steady- and non-steady-state conditions.
Animals↗
Plasma lactic dehydrogenase activities in men during bed rest with exercise training.
Peak oxygen uptake (peak VO2) and basal total activity of lactic dehydrogenase (LDH-T) and its five isoenzymes were measured in seven men, 19-21 years old, before, during, and after three 2-week bed rest (BR) periods, each separated by 3-week recovery periods. For 1 h X d-1 during BR, they performed isometric (250 kcal X -1) leg exercise, or no exercise. LDH-T was reduced (p less than 0.05) with all three regimens by day 10 of BR, but the decrease occurred at different rates. The earliest significant (p less than 0.05) reduction in LDH-T with the no-exercise regimen was associated with the greatest decrease in peak VO2 of 12.3%; isotonic exercise was next with a peak VO2 decrease of 9.2%; finally, isometric exercise had the least decrease in peak VO2 at 4.8%. Irrespective of the exercise regimen during BR, the increases in isoenzyme activities occurred mainly in LDH-1 and LDH-2 (heart subunits). Isometric (aerobic) muscular strength training appears to maintain skeletal muscle integrity and, perhaps, oxygen uptake capacity better during bed rest than isotonic exercise training. Reduced hydrostatic pressure during bed rest, however, ultimately overrides effects of both moderate isometric and isotonic exercise training (metabolism) in the resulting decrease in LDH-T.
Adult↗
Gender differences in skeletal muscle fatigability are related to contraction type and EMG spectral compression.
The purposes of this study were 1) to evaluate gender differences in back extensor endurance capacity during isometric and isotonic muscular contractions, 2) to determine the relation between absolute load and endurance time, and 3) to compare men [n = 10, age 22.4 +/- 0.69 (SE) yr] and women (n = 10, age 21.7 +/- 1.07 yr) in terms of neuromuscular activation patterns and median frequency (MF) shifts in the electromyogram (EMG) power spectrum of the lumbar and hip extensor muscles during fatiguing submaximal isometric trunk extension exercise. Subjects performed isotonic and isometric trunk extension exercise to muscular failure at 50% of maximum voluntary contraction force. Women exhibited a longer endurance time than men during the isometric task (146.0 +/- 10.9 vs. 105.4 +/- 7.9 s), but there was no difference in endurance performance during the isotonic exercise (24.3 +/- 3.4 vs. 24.0 +/- 2.8 repetitions). Absolute load was significantly related to isometric endurance time in the pooled sample (R(2) = 0.34) but not when men and women were analyzed separately (R(2) = 0.05 and 0.04, respectively). EMG data showed no differences in neuromuscular activation patterns; however, gender differences in MF shifts were observed. Women demonstrated a similar fatigability in the biceps femoris and lumbar extensors, whereas in men, the fatigability was more pronounced in the lumbar musculature than in the biceps femoris. Additionally, the MF of the lumbar extensors demonstrated a greater association with endurance time in men than in women (R(2) = 0.45 vs. 0.19). These findings suggest that gender differences in muscle fatigue are influenced by muscle contraction type and frequency shifts in the EMG signal but not by alterations in the synergistic activation patterns.
Adult↗
Isotonic (dynamic) and isometric (static) effort in the assessment and evaluation of diastolic hypertension: correlation and clinical use.
We compared the effect of two exercise stress tests on blood pressure in normal and borderline populations. The aim of the trial was to determine if isometric exercise testing by handgrip can replace the isotonic exercise test in population screening for the detection of mild and latent hypertension. The study involved 150 subjects; 62 were normotensive and 88 were borderline hypertensive. No significant statistical difference was found in diastolic pressure between the tests. In normotensive subjects, the diastolic response after isotonic effort was 79.3 +/- 9.6 mm Hg and 89.0 +/- 9.7 mm Hg after the isometric test (p less than 0.05). However, both results did not pass 100 mm Hg. In borderline hypertensive patients the diastolic response to the isotonic test was 105.6 +/- 8.8 mm Hg and after the isometric test 107.7 +/- 10.6 mm Hg (NS). The results show that the handgrip isometric test can replace the complicated isotonic test for the screening detection and evaluation of hypertensives in the population.
Adult↗
Differences between VO2 maxima of twitch and tetanic contractions are related to blood flow.
The purpose of this investigation was to compare oxygen uptake (VO2) and fatigue characteristics of isotonic tetanic contractions with those observed during isotonic twitches in dog gastrocnemius-plantaris muscle. Tetanic contractions (1/s, 200-ms trains of 50 impulses/s) elicited a peak VO2 of 9.01 +/- 0.42 mumol.g-1.min-1, which declined 29% in 30 min. The peak was significantly lower during 4/s twitches (6.23 +/- 0.36 mumol.g-1.min-1), but the rate of decline was similar. Peak blood flow (Q) was 37% higher and decreased more slowly during tetanic than twitch contractions. VO2/Q and VO2/venous PO2 were similar in both groups at peak VO2 and later declined or remained constant over time. Power was significantly greater with tetanic contractions with the relative decline between 3 and 30 min similar in both groups (32 and 37%). In conclusion, tetanic contractions result in significantly higher VO2 and power than do twitch contractions. This was derived primarily from increased Q because the arteriovenous O2 difference was similar. A significant determinant of the difference in Q between twitch and tetanic contractions is mechanical hindrance of Q. There is relatively more time for unhindered flow in the tetanic contractions. In electrically stimulated muscles, maximal VO2 is related to Q and reflects mainly Q through the muscle rather than the VO2 capacity of the muscle.
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Isotonic shortening parameters but not isometric force development are altered in ragweed pollen sensitized canine bronchial smooth muscle.
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Animals↗
Developmental differences in vascular smooth muscle mechanics in pulmonary and systemic circulations.
To evaluate the maturational changes in vascular muscle mechanics we studied and compared the isotonic half-time relaxation (t1/2P,CE) and maximal load-bearing capacity normalized to stress of pulmonary and systemic arterial muscle from perinatal and adult sheep. For the pulmonary and systemic vessels t1/2P,CE was significantly shorter in adult than in perinatal sheep (P < 0.01). In newborns t1/2P,CE of the pulmonary vessels was 185 +/- 31 (SE) s, longer than that of the systemic vessels (64 +/- 10 s; P < 0.01). In adults t1/2P,CE of the pulmonary vessels (101 +/- 14 s) was longer than that of the systemic vessels (37 +/- 5 s; P < 0.01). Maximal load-bearing capacities normalized to stress of pulmonary vessels of fetal and newborn sheep were twofold greater than those of adult sheep and of the systemic vessels of newborns and adults (P < 0.01). In conclusion, significant maturational changes in the isotonic and isometric mechanical properties of vascular pulmonary and systemic smooth muscle were observed in sheep.
Aging↗
Stimulus-response relationships for isotonic shortening and isometric tension generation in rabbit trachealis.
Nonspecific bronchial hyperresponsiveness in asthma is characterized by increased maximal airway narrowing (reactivity) and increased sensitivity of the airways. A decreased load on airway smooth muscle (ASM) has been suggested as a mechanism of increased reactivity. We hypothesized that decreased ASM load can also cause a leftward shift in the dose-response curve and explain increased sensitivity. We tested this hypothesis using rabbit tracheal smooth muscle strips in vitro by measuring isotonic shortening and isometric force during electrical field stimulation (1-100 Hz) at the length at which maximal active tension developed (Lmax), 90% Lmax, and 110% Lmax The frequency-response relationships expressed as frequency vs. percent maximal shortening or tension were not different at Lmax or 110% Lmax, but at 90% Lmax the frequency vs. shortening relationship was significantly shifted leftward relative to the frequency vs. tension relationship (P < 0.05). The electrical field stimulation frequencies that produced 50% maximal response for isometric tension and for isotonic shortening, respectively, were 6.7 +/- 1.9 and 3.9 +/- 0.7 Hz at 90% Lmax, 9.2 +/- 2.1 and 7.5 +/- 1.9 Hz at 100% Lmax, and 2.8 +/- 1.0 and 1.2 +/- 0.5 Hz at 110% Lmax. We conclude that, at lengths below Lmax, isotonic shortening is facilitated compared with isometric tension and therefore decreased ASM load in vivo may result in increased sensitivity.
Animals↗
Blood pressure behaviour during physical activity.
Aerobic exercise is currently being recommended in addition to pharmacological therapy for lowering blood pressure levels in hypertensive patients, i.e. in subjects whose resting blood pressure levels exceed 145/90 mm Hg. On the other hand competitive sports are generally contraindicated in hypertensives, who are thought to be at increased risk of morbidity or mortality from their blood pressure levels. The present knowledge of blood pressure behaviour during isotonic physical activity is almost wholly based on the results obtained by means of the ergometric tests. Several maximal and submaximal exercise protocols have been introduced, but none has proved to be superior for diagnostic purposes. There is general agreement that the systolic blood pressure increase determined by isotonic exercise usually ranges from 50 to 70 mm Hg in both normotensive or hypertensive subjects. Diastolic blood pressure shows only minor changes in the normotensives, while in the hypertensives it tends to substantially increase because of their inability to adequately reduce their peripheral resistance. This mechanism may also explain the delay shown by the hypertensives in reaching pre-exercise blood pressure values during the recovery. On average diastolic blood pressure increases to a greater extent during bicycle ergometry than during treadmill, while no differences in exertional systolic blood pressure have been observed between the 2 tests. The results of several studies indicate that the blood pressure response to isotonic exercise is a marker for detection of hypertension earlier in the course of the disease, while resting blood pressure is still normal. According to some authors it is also of value in predicting future hypertension in individuals with borderline pressure levels. There are no conclusive data on the effect of training on blood pressure response to exercise. The majority of the published studies report small exertional pressure reductions after conditioning, which would merely reflect the reduction in resting blood pressure. Vasodilation greatly influences the exercise-induced rise in blood pressure; in fact the exertional pressor increase is blunted when the test is preceded by an adequate warm-up session. Isometric effort is thought to be contraindicated in hypertensive subjects, as it causes a pronounced increase not only of systolic but also of diastolic pressure. Mean blood pressure is, however, increased to the same extent by isotonic and isometric exercise, even though minor discrepancies have been reported by some authors.(ABSTRACT TRUNCATED AT 400 WORDS)
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Effects of short-term isotonic & isometric training on cardiovascular & pulmonary function.
A randomised control trial of short-term exercises on specific cardiovascular and respiratory parameters was undertaken in normal male college students. The effects of isotonic training (5BX programme) and isometric training (a programme of isometric exercises working all major groups of muscles) were compared with a control group with no specific workout. Both isotonic and isometric training resulted in significant cardiovascular improvement but seemed inadequate to improve vital capacity and flow rates. Isotonic training in addition, improved ventilatory efficiency. It is concluded that such isotonic or isometric training of thrice a week for ten weeks, requiring no equipment, less time and space can be promoted to improve physical fitness.
Adolescent↗
Reliability of clinical muscular endurance tests in patients with chronic heart failure.
BACKGROUND: In clinical praxis it is important to be able to assess the effect of rehabilitation in patients with chronic heart failure (CHF). There is therefore a need for simple clinical tests that are objective, reliable and valid measures. AIM: The aim of this study was to examine the reliability of three muscle endurance tests for patients with CHF and to compare the results to a healthy control group. METHOD: The study included 20 patients (7 women) with CHF and 20 healthy persons. Dynamic endurance of the shoulder and calf muscle was measured as well as static shoulder endurance. All three tests were performed twice, within 5-10 days. RESULTS: The reliability (rs) of the tests for the patients with CHF was between 0.90-0.99 and for the healthy persons between 0.79-0.98. There was also a significant difference in dynamic endurance in shoulder flexion and in heel-rise left leg test between patients with CHF and healthy persons. CONCLUSION: Tests of dynamic and static endurance of the shoulder muscle and dynamic calf muscle endurance are reliable assessment tools for use in clinical practice. Patients with CHF have reduced isotonic muscular endurance compared with healthy persons.
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Early effects of denervation on isometric and isotonic contractile properties of rat skeletal muscles.
The contractile properties of the fast-twitch m. extensor digitorum longus (EDL) and the slow-twitch m. soleus were examined after 5 days of denervation. An increase in twitch time to peak, twitch half relaxation time and twitch/tetanus ratio was observed. The EDL also showed a moderate decrease in maximal isotonic shortening velocity, which is taken as an indication that the contractile proteins are affected. A fatigue test showed that the EDL became less fatigable and the soleus less fatigue resistant after denervation.
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