The training effect of repeated isometric muscle contractions

Conclusions A training program of repeated isometric contractions with submaximal tension of the elbow flexors, increased to a striking degree the ability to perform isometric contractions until fatigue, while the ability to perform dynamic contractions of the same muscle group remained almost unaltered.This is interpreted as a change in the function of the parts of the central nervous system involved, summarized in the term motor learning. It is stressed that motorlearning is a major factor when increase of the endurance is the aim of the training.Isometric contractions with submaximal tension do not seem to be an optimal stimulus if an increase of the isometric strength is required.     

The effect of varying the time of concentric and eccentric muscle actions during resistance training on skeletal muscle adaptations in women

This study investigated the effect of manipulating the time to complete both the concentric (CON) and eccentric (ECC) muscle actions during resistance training on strength, skeletal muscle properties and cortisol in women. Twenty-eight women (mean ± SE age = 24.3 ± 1.1 year) with strength training experience completed three training sessions per week for 9 weeks. Two sets of four lower body exercises (leg press, parallel squat, knee extension and knee flexion) were completed using 6–8 RM intensity. The long CON (LC) group performed the CON action for 6 s and the ECC action for 2 s, while the long ECC (LE) group completed the CON and ECC phases for 2 and 6 s, respectively. Both groups experienced significant increases in leg press CON only, ECC only and combined ECC and CON maximal strength (1 RM). Immunohistochemical analyses demonstrated that both types I and IIA vastus lateralis fibre areas significantly increased following LC training while only type I fibre area increased following LE training. There was a decrease in MHCIId(x) with a concomitant increase in MHCIIa (P < 0.05) in both groups. Twenty-four hour urinary cortisol significantly increased after LC training only. It was concluded that LC resistance training was more effective than LE for increasing both types I and IIA fibre area and cortisol when time under tension and intensity of muscle actions were matched between the two modes of resistance training in young healthy women.     

The effect of prolonged isometric contractions on muscle fluid balance

Ultrasound scanning was performed at three sites above the fossa supraspinata on nine healthy subjects and five patients with myofascial shoulder pain. This method produced a well-defined depiction of the soft tissue layers above the fossa supraspinata and reproducible muscle thickness measurements. In the healthy subjects the average distance from the skin surface to the trapezius muscle was 7.7 mm and the average thickness of the trapezius muscle was 5.3 mm, and the average thickness of supraspinatus muscle was 20.0 mm. The supraspinatus muscle was thinner at the medial measuring site than at the other two sites. In contrast, a tendency towards a larger distance was seen from the skin to trapezius muscle at the medial measuring site than at the other two sites. No statistical differences were found between the two groups of subjects either at rest or during brief shoulder abductions. All the subjects performed a 30° unilateral isometric shoulder abduction test to exhaustion. The median endurance time was 33 min for the healthy subjects and only 5 min for the patients. The ratings of perceived exertion (RPE) were in line with this, since the increment in RPE with time was larger for the patients than for the healthy group. The reduced shoulder abduction endurance time in the patient group may have been related to impaired muscle function and/or pain development. During the 33-min shoulder abduction in the healthy subjects, the thickness of supraspinatus muscle increased by 14%, indicating muscle swelling, whereas the thickness of trapezius muscle remained constant. The fluid imbalance in the supraspinatus muscle compartment may well play a role in the development of muscle fatigue and the disorders found in industry resulting from prolonged work with arms elevated.     

The effect of hypoxia on shortening contractions in rat diaphragm muscle

Hypoxia is known to reduce isometric contractile properties of isolated rat diaphragm bundles. Its effect on isotonic contractile properties (i.e. force–velocity relationship and power output) has not been studied. We hypothesized that hypoxia reduces velocity of shortening and consequently power output of the unfatigued muscle, and shortens endurance time during isotonic contractions. Force–velocity relationship, power output, and fatigue resistance of rat diaphragm muscle bundles were measured during hypoxia (PO₂: 6.6 ± 0.2 kPa) and compared with hyperoxia (PO₂: 91.8 ± 0.7 kPa). Force was clamped from 1 to 100% of maximal tetanic force (P_o). Fatigue during isotonic contractions was induced by repeated stimulation every 2 s at a clamp level of 33% of P_o. Hypoxia did not affect isometric force generation compared with hyperoxia, nor contraction or relaxation time. In contrast, maximum shortening velocity decreased significantly (hypoxia: 4.2 ± 0.3, hyperoxia: 6.0 ± 0.2 L_o/s, P < 0.05). The force–velocity curve shifted downwards (P < 0.05). Hypoxia lowered power output at each load compared with hyperoxia (P < 0.05). The isotonic endurance time was shorter during hypoxia compared with hyperoxia (80 ± 2 vs. 130 ± 3 s, P < 0.05). These data show that hypoxia depresses isotonic contractile properties and power output, and reduces endurance time during repeated isotonic contractions.     

Effect of tubocurarine on static and dynamic muscle contractions in man

The effect of a bolus injection of tubocurarine (0.1 mg X kg-1 i.v.) was followed in six young subjects by registration of static, and slow (30 degrees X s-1) and fast (150 degrees X s-1) dynamic (isokinetic) maximal voluntary leg extensions. Mechanograms from both unblocked and curarized muscle contractions showed a "notch" after about 440 ms separating two relative maxima. The mechanograms were divided by an arbitrary straight line connecting the starting point of the contraction curve and the notch. The line separated an area (Nm X s) above and to the left (alpha-component) from an area below and to the right (beta-component) of the line. Tubocurarine affected the beta-component selectively until about 70% reduced in the static contractions. With further curarization the alpha-component was also reduced in size. The alpha-component was equally affected during the three types of contractions, while the faster the contraction the more the beta-component was reduced. The results suggest that static as well as dynamic human muscle contractions can be divided into two parts with a different sensitivity for tubocurarine, one of which seems to have a sensitivity which depends on the contraction velocity.     

The effect of hypoxia on shortening contractions in rat diaphragm muscle.

Hypoxia is known to reduce isometric contractile properties of isolated rat diaphragm bundles. Its effect on isotonic contractile properties (i.e. force-velocity relationship and power output) has not been studied. We hypothesized that hypoxia reduces velocity of shortening and consequently power output of the unfatigued muscle, and shortens endurance time during isotonic contractions. Force-velocity relationship, power output, and fatigue resistance of rat diaphragm muscle bundles were measured during hypoxia (PO2: 6.6 +/- 0.2 kPa) and compared with hyperoxia (PO2: 91.8 +/- 0.7 kPa). Force was clamped from 1 to 100% of maximal tetanic force (Po). Fatigue during isotonic contractions was induced by repeated stimulation every 2 s at a clamp level of 33% of Po. Hypoxia did not affect isometric force generation compared with hyperoxia, nor contraction or relaxation time. In contrast, maximum shortening velocity decreased significantly (hypoxia: 4.2 +/- 0.3, hyperoxia: 6.0 +/- 0.2 Lo/s, P < 0.05). The force-velocity curve shifted downwards (P < 0.05). Hypoxia lowered power output at each load compared with hyperoxia (P < 0.05). The isotonic endurance time was shorter during hypoxia compared with hyperoxia (80 +/- 2 vs. 130 +/- 3 s, P < 0.05). These data show that hypoxia depresses isotonic contractile properties and power output, and reduces endurance time during repeated isotonic contractions.     

The effect of gamma-aminobutyric acid on the potassium and acetylcholine contractions of skeletal muscle

Summary Gamma-aminobutyric acid depressed the potassium contraction of the skeletal muscles and its sensitization to potassium ions caused by guanidine, aminopyridine, veratrine, and had no material effect on the aceytlcholine muscle contractions.(Presented by Active Member of the Akad. Med. Nauk SSSR V. M. Karasik) Translated from Byulleten Eksperimental'noi Biologii i Meditsiny, Vol. 55, No. 8, pp. 75–76, August, 1963     

Electromechanical delay in the vastus lateralis muscle during dynamic isometric contractions

Summary Electromechanical delay (EMD) values were obtained using a cross-correlation technique for a series of 14 repetitive submaximal dynamic isometric contractions of the vastus lateralis performed by five subjects. To avoid a phase lag, which is introduced with one-way filtering, the EMG was processed with a bi-directional application of a second-order Butterworth filter. A mean EMD value of 86 ms (SD=5.1 ms) was found. Moreover, contraction and relaxation delays were computed and compared. There was a significant difference between the contraction and relaxation delays (P<0.005). The mean contraction delay was 81.9 ms and the mean relaxation delay was 88.8 ms. Despite this significant difference, the computed contraction and relaxation delay values lie in the same range as the total phase lag, calculated with the cross-correlation technique. The magnitude of EMD values found supports the need to account for this delay when interpreting temporal aspects of patterns of intermuscular coordination.     

The training effect of dynamic maximal resistence exercises

Summary 7 physiotherapists participated in a training program of 10 maximal resistance dynamic exercises a day. The training period was 5 weeks. The left elbow flexors was trained.Before and after the training period the dynamic strength, the isometric strength, the dynamic endurance and the isometric endurance were tested.The training resulted primarily in an increase of the dynamic and isometric strength, while the dynamic endurance (= working capacity) remained nearly unaltered compared with the increase seen in previous experiments. The isometric endurance too remained unaltered.This work is supported by a grant from the P. Carl Petersens Foundation.     

训练轨迹对上肢肌肉协同的影响

目的通过分析康复机器人辅助训练过程中沿不同轨迹运动的上肢肌肉协同特性,探究运动轨迹对上肢肌肉骨骼特性的影响,为康复机器人训练轨迹优化设计提供基础实验数据与指导。方法首先在末端牵引式康复机器人系统中设计3种上肢运动训练轨迹(L1为直线,L2为弧线,L3为半圆),然后采集12名健康志愿者在沿3种训练轨迹上肢运动过程中的表面肌电信号,并使用非负矩阵分解算法进行肌肉协同特性的获取,对不同轨迹组间肌肉协同结构的相似系数、屈肌占比以及募集模式积分系数进行对比分析,探讨康复机器人不同训练轨迹对上肢肌肉骨骼特性的影响。结果同一训练轨迹中各志愿者的肌肉协同结构具有较高相似性(平均SI>0.81)。各轨迹组的肌肉协同结构中的屈肌占比随运动进程逐渐增加。各轨迹组的肌肉协同募集模式均具有时序特性,前期伸肌起主要作用,后期屈肌起主要作用,中期屈肌占比随轨迹曲率增加而增加。轨迹L1与L2、L2与L3协同结构非常相似(SI>0.90),而L1与L3协同结构较为相似(SI>0.75)。结论康复机器人辅助上肢的训练轨迹对上肢肌肉特性有一定影响,不同训练轨迹带来的肌肉协同结构较为相似,但是运动过程中屈肌群占比及协同贡献度会为了协调动作而发生变化。由此可推测不同训练轨迹对不同肌群的训练强度可能会有所不同。康复机器人训练轨迹设计需根据康复需求进行优化设计。     

Structural muscle damage and muscle strength after incremental number of isometric and forced lengthening contractions

Summary Exercise-induced muscle damage is characterized by histological changes, like Z-line streaming, inflammatory response and decreased muscle function reflected in a prolonged decline in maximal isometric muscle strength after eccentric work. It is assumed that force decrement is mainly related to the amount of structural damage. However, the relationship between number of eccentric contractions, magnitude of structural damage and force decrement is not very well documented. Therefore we studied the effect of an increasing number of both isometric and eccentric (forced lengthening) contractions on histological parameters of muscle damage and maximal isometric force in an experimental in situ rat model. Tibialis anterior muscles of male Wistar rats were subjected to an increasing number of either isometric or eccentric contractions and were examined for histological markers of muscle damage. The present study shows that muscle damage increases progressively with the number of forced lengthening contractions. Maximal isometric torque was found to decline after both types of exercise. However, the decline after forced lengthening exercise was more pronounced. Only a weak relationship between percentage of histological muscle damage and isometric torque after forced lengthening contractions was found. The findings of the present study suggest that the decline in muscle force after eccentric exercise may partly be attributed to other factors than structural damage.     

Experimental muscle pain changes motor control strategies in dynamic contractions

This study investigated the effect of muscle pain on muscle activation strategies during dynamic exercises. Ten healthy volunteers performed cyclic elbow flexion/extension movements at maximum speed for 2 min after injection of (1) hypertonic (painful) saline in the biceps brachii, (2) hypertonic saline in both biceps brachii and triceps brachii, and (3) isotonic (nonpainful) saline in the biceps brachii muscle. Surface electromyographic (EMG) signals were collected from the upper trapezius, biceps brachii, triceps brachii, and brachioradialis muscles (to estimate EMG amplitude) and with an electrode arrays from biceps brachii (to estimate muscle fiber conduction velocity [CV]). In all conditions, the acceleration of the movement decreased throughout the exercise, and kinematic parameters were not altered by pain. With respect to the control condition, pain induced a decrease of the biceps brachii (mean ± SE, –23±4%) and brachioradialis (–10±0.4%) integrated EMG (IEMG) in the beginning of the exercise, and an increase (45±3.5%) of the upper trapezius IEMG at all time points during the exercise. The biceps brachii IEMG decreased over time during the nonpainful exercises (–11±0.6%) while it remained constant in the painful condition. Biceps brachii CV decreased during painful conditions (–12.8±2.2%) while it remained constant during the nonpainful condition. In conclusion, muscle pain changes the motor control strategy to sustain the required dynamic task both in the relative contribution between synergistic muscles and in the motor unit activation within the painful muscle. Such a changed motor strategy may be highly relevant in models of occupational musculoskeletal pain conditions.     

Effects of real and imagined training on voluntary muscle activation during maximal isometric contractions

In this study we directly tested the hypothesis that isometric strength training increases voluntary drive to muscles. In addition, it was attempted to replicate the findings of an earlier study that showed imagined training increases voluntary strength as much as actual training, as this finding provides key support for the hypothesis that training increases voluntary drive (Yue & Cole 1992). Fifty-four subjects were randomly allocated to groups that performed 8 weeks of isometric training of the elbow flexor muscles, imagined isometric training, or a control task involving the lower limbs. Voluntary isometric strength and activation of the elbow flexor muscles were measured before and after training. Voluntary activation was measured with a sensitive form of twitch interpolation. Training, imagined training and control groups increased voluntary isometric elbow flexor strength by means of 17.8% (±3.1 SEM), 6.8% (±2.6) and 6.5% (±3.0), respectively. The training group increased in strength significantly more than imagined training and control groups (P = 0.01 for both comparisons), but the small difference between imagined training and control groups was not significant (P = 0.31). Prior to training, voluntary activation of all subjects was high (96.2 ± 0.5%). This did not change significantly with training and there were no significant differences between groups. These data challenge the hypothesis that training of the elbow flexor muscles increases isometric strength by inducing adaptations of the central nervous system, because they show that training does not increase voluntary activation and imagined training does not increase strength.     

Effects of dynamic ischaemic training on human skeletal muscle dimensions

The effect of training under conditions of local leg ischaemia on muscle area and fibre dimensions was studied in nine males. Leg ischaemia was induced by enclosing the legs in a pressure chamber and sealing the opening with a rubber membrane at the level of the crotch. Air pressure over the legs was 50 mmHg. The subjects performed 16 sessions (45 min) of one-legged supine strenuous ischaemic training during 4 weeks. Exercise intensity was maintained as high as possible during the whole session. The contralateral leg served as a control leg and remained passive during exercise. Before and after the training period, muscle fibre dimensions were determined from biopsy samples taken from the m. vastus lateralis, and leg muscle dimensions were assessed by magnetic resonance imaging (MRI). In the trained leg, mean fibre area increased by 12% (P < 0.05). The MRI-assessed cross-sectional area of the vastus group increased by 4% (P=0.01). In the control leg, mean fibre area and the cross-sectional area of the vastus group were unchanged, while those of the adductor muscle group decreased by 4% (P < 0.05). It is concluded that a short period of strenuous ischaemic endurance training increases the cross-sectional area of the ischaemically trained muscle group, as measured both by MRI and from muscle biopsy samples. In contrast, the adductor muscles in the contralateral thigh showed a decreased cross-sectional area (as assessed by MRI), possibly due to the effects of the strenuous contralateral training, by mechanisms that have yet to be identified. Accepted: 26 December 1999     

The effect of training and detraining on muscle composition in the horse.

1. Percutaneous needle biopsies were obtained from six limb muscles in six horses before and during a training programme of 10 or 15 weeks designed to involve both aerobic and anaerobic work. In a subsequent detraining period, biopsies were also taken after 5 and 10 weeks. 2. Samples were analysed biochemically for enzyme activity of lactic dehydrogenase (LDH), creatine phosphokinase (CPK), aldolase (ALD), citrate synthase (CS), aspartate aminotransferase (AST), and alanine aminotransferase (ALT) and for glycogen content. Fibre typing was carried out histochemically before and 10 weeks after commencement of training. 3. There was a significant increase in the percentage of high myosin ATPase activity pH 9-4/high oxidative (FTH) fibres with a corresponding decrease in high myosin ATPase activity pH 9-4/low oxidative (FT) fibres and low myosin ATPase activity pH 9-4/high oxidative (ST) fibres after 10 weeks training. 4. During training, enzyme activities increased progressively but at different rates with an approximate twofold increase in all of the enzymes except CPK by the end of the training period. Changes in all the muscles studied were similar. Glycogen content increased by approximately 33% which was significant when all the muscles were considered together. 5. A decrease in enzyme activity occurred after 5 weeks detraining. However at 10 weeks a consistent but inexplicable increase in all enzyme levels, except CS again occurred. 6. It is concluded that training increased greatly the activity of enzymes involved in both aerobic and anaerobic metabolism.     

The mechanical effects of contractions on blood flow to the muscle

To determine whether muscle contractions can increase muscle blood flow independently from metabolic factors, we isolated the vasculature of the left diaphragm or gastrocnemius muscle of anesthetized and mechanically ventilated dogs. Arterial blood flow was controlled with a constant pressure source and the arterial pressure (P _a) was decreased in steps to obtain pressure-flow relationships (P- ) . The local vasculatures were maximally dilated with nitroprusside [mean (SD)114.0 (32.0) g·min^–1], adenosine [1.43(0.41) mmol·l^–1·min^–1], and acetylcholine [l.43(0.41) mmol·l^–1·min^–1] and theP- with and without spontaneous contractions (n = 6) , stimulated twitches (n = 12, 2–4 Hz), or tetanic trains (n = 7, 25 Hz) in the diaphragm and stimulated twitches (n = 6, 2–4 Hz), or tetanic contractions (n = 6, 12–16 trains) in the gastrocnemius were compared. The pressure axis intercept decreased (P < 0.5) with spontaneous contractions in the diaphragm and the slope did not change. AtP _a of 13.3 kPa, flow increased from 36.2 (34.9) to 43.9 (38.2) ml·min^–1·100 g^–1 (P < 0.05). During twitch contractions, the slope and intercept of theP- were not significantly different from vasodilatation alone, but the flow at a pressure of 13.3 kPa increased slightly. In the gastrocnemius (n = 6), continuous and intermittent tetanic contractions did not affectP- or flow atP _a of 100 mmHg (n = 6). Furthermore, increasing venous pressure to 6.7 kPa did not affect flow in this muscle. We conclude that the muscle pump has only a small direct effect on muscle blood flow and its main effect is to reduce venous pressures.