长时程增强诱导和维持过程中海马CA1区神经细胞粘附分子蛋白水平与mRNA表达的变化

既往研究发现,神经细胞粘附分子(neural cell adhesion molecules,NCAM)对海马CA1区突触传递长时程增强(longterm potentiation,LTP)的诱导和维持极为关键。本文采用原位杂交法和Western blot法,观察了大鼠海马腑片LTP诱导和维持过程中NCAM mRNA和蛋白水平的动态变化过程。结果显示,强直刺激诱发fEPSP斜率升高10 min时,海马CA1区NCAM mRNA染色阳性神经元数量显著增加(76.6±11.5个),NCAM蛋白水平亦明显升高(7.190±0.64任意单位/50μg蛋白)。强直刺激诱发fEPSP斜率升高1 h时,NCAM mRNA染色阳性神经元数量为73.3±14.0个,NCAM蛋白量为9.031±0.71任意单位/50 μg蛋白;与强直刺激后10 min比较,NCAM mRNA表达无显著变化,而NCAM蛋白水平变化明显。NMDA受体特异阻断剂AP-5在损害LTP的同时,显著抑制NCAM mRNA和蛋白的增加。实验结果表明,在大鼠海马LTP诱导和维持过程中,NCAM mRNA增强的表达相对稳定,而NCAM蛋白水平呈现先低后高的变化。     

Chemical energetics of force development, force maintenance, and relaxation in mammalian smooth muscle

High-energy phosphate utilization (delta approximately P) associated with force development, force maintenance, and relaxation has been determined during single isometric tetani in the rabbit taenia coli. ATP resynthesis from glycolysis and respiration was stopped without deleterious effects on the muscle. At 18 degrees C and a muscle length of 95% l0, the resting rate of energy utilization is 1.8 +/- 0.2 nmol/g . s-1, or 0.85 +/- 0.2 mmol approximately P/mol of total creatine (Ct) . s-1, where Ct = 2.7 mumol/g wet wt. During the initial 25 s of stimulation when force is developed, the average rate of delta approximately P was -8.2 +/- 0.8 mmol/mol Ct . s-1, some four times greater than during the subsequent 35 s of force maintenance, when the rate was -2.0 +/- 0.6 mmol approximately P/mol Ct . s-1. The energy cost of force redevelopment (0 to 95% P0) after a quick release from the peak of a tetanus is very low compared with the initial force development. Therefore, the high rate of energy utilization during force development is not due only to internal work done against the series elasticity nor to any high rate of cross-bridge cycling inherently associated with force development. The high economy of force maintenance compared with other muscle types is undoubtedly due to a slower cross-bridge cycle time. The energy utilization during 45 s of relaxation was not statistically significant, and integral of Pdt/delta approximately P was higher during relaxation than during force maintenance in the stimulated muscle.     

Time-resolved changes in equatorial x-ray diffraction and stiffness during rise of tetanic tension in intact length-clamped single muscle fibers.

We report the first time-resolved x-ray diffraction studies on tetanized intact single muscle fibers of the frog. The 10, 11, 20, 21, 30, and Z equatorial reflections were clearly resolved in the relaxed fiber. The preparation readily withstood 100 1-s duration (0.4-s beam exposure) tetani at 4 degrees C (less than 4% decline of force and no deterioration in the 10, 11 equatorial intensity ratio at rest or during activation). Equatorial intensity changes (10 and 11) and fiber stiffness led tension (t1/2 lead 20 ms at 4 degrees C) during the tetanus rise and lagged during the isometric phase of relaxation. These findings support the existence of a low force cross-bridge state during the rise of tetanic tension and isometric relaxation that is not evident at the tetanus plateau. In "fixed end" tetani lattice expansion occurred with a time course similar to stiffness during the tetanus rise. During relaxation, lattice spacing increased slightly, while the sarcomere length remained isometric, but underwent large changes after the "shoulder" of tension. Under length clamp control, lattice expansion during the tetanus rise was reduced or abolished, and compression (2%) of the lattice was observed. A lattice compression is predicted by certain cross-bridge models of force generation (Schoenberg, M. 1980. Biophys. J. 30:51-68; Schoenberg, M. 1980. Biophys. J. 30:69-78).     

Electron probe X-ray microanalysis of post-tetanic Ca2+ and Mg2+ movements across the sarcoplasmic reticulum in situ

Ca2+ and Mg2+ movements across the sarcoplasmic reticulum (SR) of frog skeletal muscle fibers were measured in situ by electron probe microanalysis of muscles rapidly frozen following a tetanus. At 400 ms following a 1.2-s tetanus at room temperature, the force had relaxed to base-line, and 0.3 mmol of Ca2+/liter of cytoplasmic H2O had been pumped by the SR, indicating that the in situ pumping of the SR Ca-ATPase is sufficiently high to account for the removal of Ca2+ from the Ca2+-specific sites of troponin (0.18 mmol of Ca2+-specific sites/liter of cytoplasmic H2O) and for the rate of relaxation from a tetanus at room temperature. The half-time of the return of the total 1.0 mmol of Ca2+/liter of cytoplasmic H2O released during a tetanus was 1.1 s, comparable to the slow Koff rate of Ca2+ from (carp) parvalbumin (1.0 s-1) and consistent with the hypothesis that the return of this Ca2+ to the terminal cisternae is rate-limited by the Ca2+ off-rate from parvalbumin. The return of the Mg2+ taken up by the terminal cisternae during a tetanus to resting levels was significantly slower than the time course of the Ca2+ movements, suggesting that the Mg2+ permeability of the SR in situ is low and may be transiently increased during tetanic stimulation.     

海马长时程增强与26S蛋白酶复合体活性变化的关系

实验观察了大鼠海马脑片上突触传递长时程增强(long term potentiation,LTP)的产生和维持中26S蛋白酶复合体活性的动态变化过程,初步分析了介导其变化的受体途径。结果显示:强直刺激前,26S蛋白酶复合体活性为190±14.3 cpm/(100 μg·2 h),强直刺激诱导fEPSP斜率增加10 min时,其活性升为273±18.3 epm/(100μg·2 h),强直刺激诱导fEPSP斜率增加60 min时,26S蛋白酶复合体活性又降为210±12.8 cpm/(100μg·2 h)。NMDA受体特异阻断剂AP-5在损害L1P产生的同时,抑制26S蛋白酶复合体活性升高。实验结果提示:大鼠海马LTP产生过程中,26S蛋白酶复合体活性存在一个短时间的,依赖于N-methyl-D-aspartate(NMDA)受体的升高过程。     

Effect of stretching training on the viscoelastic properties of human tendon structures in vivo.

The purpose of this study was to examine whether stretching training altered the viscoelastic properties of human tendon structures in vivo. Eight men performed the stretching training for 3 wk. Before and after the stretching training, the elongation of the tendon and aponeurosis of medial gastrocnemius muscle was directly measured by ultrasonography while the subjects performed ramp isometric plantar flexion up to the voluntary maximum, followed by a ramp relaxation. The relationship between the estimated muscle force (Fm) and tendon elongation (L) during the ascending phase was fitted to a linear regression, the slope of which was defined as stiffness of tendon structures. The percentage of the area within the Fm-L loop to the area beneath the curve during ascending phase was calculated as an index representing hysteresis. To assess the flexibility, the passive torque of the plantar flexor muscles was measured during the passive stretch from 0 degrees (anatomic position) to 25 degrees of dorsiflexion with a constant velocity of 5 degrees/s. The slope of the linear portion of the passive torque-angle curve during stretching was defined as flexibility index. Flexibility index decreased significantly after stretching training (-13.4 +/- 4.6%). On the other hand, the stretching training produced no significant change in stiffness but significantly decreased hysteresis from 19.9 +/- 11.7 to 12.5 +/- 9.5%. The present results suggested that stretching training affected the viscosity of tendon structures but not the elasticity.     

Changes of nucleotide contents and of energy charge induced by contraction, catch and relaxation in smooth molluscan muscle fibres. An analysis using reversed-phase ion-pair high-performance liquid chromatography

1. The content of 14 different nucleotides, including cAMP, in isolated muscle fibres of the anterior byssus retractor muscle of Mytilus edulis was analysed. The nucleotide levels were determined after the muscle fibres performed phasic, tonic, or tetanic contractions and after serotonin-induced relaxation of tonic contraction. 2. Isolated resting muscle fibres revealed a lower energy charge than freshly-dissected ones. 3. During active force development adenosine energy charge decreased to stay at the same low level during catch, tetanus and serotonin-induced relaxation of catch respectively. 4. The energy charges of the guanosine, uridine and cytidine systems did not show changes parallel to the adenosine system. 5. The levels of cyclic AMP were only changed under the influence of serotonin.     

Effect of small release on force during sarcomere-isometric tetani in frog muscle fibers.

We investigated the effect of small shortening imposed on frog muscle fibers during sarcomere-isometric tetani. Sarcomere length was initially kept constant, then slightly shortened (1%-5% of initial length) and clamped again for the remainder of the tetanus. Force level after the shortening was higher than the force level preceding the release. The size of the increase was larger than that predicted by the descending limb of the linear force-length relation. The difference between measured and predicted force levels increased with sarcomere length. At a sarcomere length of 3.2 microns, the force level after the shortening was higher by 50% than the force level expected from the linear descending limb. Dispersion of sarcomere-length within the sampled region was measured by two independent methods: striation imaging and analysis of the intensity profile of the first diffraction order. Sarcomere-length inhomogeneity in the sampled region was too small (standard deviation from the average sarcomere-length was +/- 0.03 microns) to account for the size of the increase in force. We studied the dependence of increase in tetanic force level after small sarcomere-length release on the size, velocity and timing of the release, as well as on initial sarcomere-length. Release size was the major determinant of the amount of increase in force. Release of 20 nm per half sarcomere was sufficient to produce an almost full force increase. Larger releases increased the force only moderately. Over the range studied, release velocity and timing had little or no effect.     

Stiffness, force, and sarcomere shortening during a twitch in frog semitendinosus muscle bundles

The time course of force and stiffness during a twitch was determined at 6 and 26 degrees C in frog semitendinosus muscle bundles using the transmission time technique of Schoenberg, M., J.B. Wells, and R.J. Podolsky, 1974, J. Gen. Physiol. 64:623-642. Sarcomere shortening due to series compliance was also measured using a laser light diffraction technique. Following stimulation, stiffness developed more rapidly than force, but had a slower time course than published Ca2+ transients determined from light signals using Ca2+ sensitive dyes (Baylor, S.M., W.K. Chandler, and M.W. Marshall, 1982, J. Physiol. (Lond.). 331:139-177). Stiffness (S) did not reach its tetanic value during a twitch at 6 or 26 degrees C, although at 6 degrees C, it approached close to this value with S-twitch/S-tetanus = 0.82 +/- 0.07 (+/- SEM). During relaxation, force fell more rapidly than stiffness both for a twitch and also a tetanus. Also in this paper, several of the assumptions inherent in using the transmission time technique for the measurement of stiffness are considered in detail.     

Diazepam reveals different rate-limiting processes in rat skeletal muscle contraction

The action of the tranquilizer diazepam on rat skeletal muscle showed that relaxation of isometric twitches is controlled by different processes in extensor digitorum longus (fast-twitch) and soleus (slow-twitch) muscles. Diazepam caused an increase in the amplitude of twitches in fibres from both muscles but increased the twitch duration only in soleus. The amplitude of fused tetani were reduced in both muscles and the rate of relaxation after the tetanus slowed by as much as 34% when the amplitude of the tetanus was reduced by only 11%. The slower tetanic relaxation indicated that calcium uptake by the sarcoplasmic reticulum was slower than normal in slow- and fast-twitch fibres. We conclude therefore that calcium uptake by the sarcoplasmic reticulum is rate limiting for twitch relaxation in slow-twitch but not fast-twitch fibres and suggest that calcium binding to parvalbumin controls relaxation in the fast fibres.     

Electromyographic amplitude ratio of serratus anterior and upper trapezius muscles during modified push-ups and bench press exercises

Imbalance and weakness of the serratus anterior and upper trapezius force couple have been described in patients with shoulder dysfunction. There is interest in identifying exercises that selectively activate these muscles and including it in rehabilitation protocols. This study aims to verify the UT/SA electromyographic (EMG) amplitude ratio, performed in different upper limb exercises and on two bases of support. Twelve healthy men were tested (average age = 22.8 +/- 3.1 years), and surface EMG was recorded from the upper trapezius and serratus anterior using single differential surface electrodes. Volunteers performed isometric contractions over a stable base of support and on a Swiss ball during the wall push-up (WP), bench press (BP), and push-up (PU) exercises. All SEMG data are reported as a percentage of root mean square or integral of linear envelope from the maximal value obtained in one of three maximal voluntary contractions for each muscle studied. A linear mixed-effect model was performed to compare UT/SA ratio values. The WP, BP, and PU exercises showed UT/SA ratio mean +/- SD values of 0.69 +/- 0.72, 0.14 +/- 0.12, and 0.39 +/- 0.37 for stable surfaces, respectively, whereas for unstable surfaces, the values were 0.73 +/- 0.67, 0.43 +/- 0.39, and 0.32 +/- 0.30. The results demonstrate that UT/SA ratio was influenced by the exercises and by the upper limb base of support. The practical application is to show that BP on a stable surface is the exercise preferred over WP and PU on either surfaces for serratus anterior muscle training in patients with imbalance between the UT/SA force couple or serratus anterior weakness.     

Linear relationship between VO2max and VO2max decrement during exposure to acute hypoxia

The purpose of these experiments is to test the hypothesis that exercise-induced hypoxemia at sea level in highly trained athletes might be exacerbated during acute hypoxia and therefore result in correspondingly larger decrements in maximal O2 uptake (VO2max) compared with less trained individuals. Thirteen healthy male volunteers were divided into two groups according to their level of fitness: 1) trained endurance athletes (T) (n = 7), with a VO2max range of 56-75 ml.kg-1.min-1 and 2) untrained individuals (UT) (n = 6), with a VO2max range of 33-49 ml.kg-1.min-1. Subjects performed two incremental cycle ergometry tests to determine VO2max under hypoxic conditions [14% O2-86% N2, barometric pressure (PB) = 760 Torr] and normoxic conditions (21% O2-79% N2, PB = 760 Torr). Tests were single blind, randomly administered, and separated by at least 72 h. Mean percent oxyhemoglobin saturation (%SaO2) during maximal exercise under hypoxic conditions was significantly (P less than 0.05) lower in the T group (77%) compared with the UT group (86%). Furthermore, the T group exhibited larger decrements (P less than 0.05) in VO2max (normoxic-hypoxic) compared with the UT group. Finally, a significant linear correlation (r = 0.94) existed between normoxic VO2max (ml.kg-1.min-1) and delta VO2max (normoxic-hypoxic). These data suggest that highly T endurance athletes suffer more severe gas exchange impairments during acute exposure to hypoxia than UT individuals, and this may explain a portion of the observed variance in delta VO2max among individuals during acute altitude or hypoxia exposure.     

Modeling of summation of individual twitches into unfused tetanus for various types of rat motor units.

Repeated stimulation of motor units (MUs) causes an increase of the force output that cannot be explained by linear summation of equal twitches evoked by the same stimulation pattern. To explain this phenomenon, an algorithm for reconstructing the individual twitches, that summate into an unfused tetanus is described in the paper. The algorithm is based on an analytical function for the twitch course modeling. The input parameters of this twitch model are lead time, contraction and half-relaxation times and maximal force. The measured individual twitches and unfused tetani at 10, 20, 30 and 40 Hz stimulation frequency of three rat motor units (slow, fast resistant to fatigue and fast fatigable) are processed. It is concluded that: (1) the analytical function describes precisely the course of individual twitches; (2) the summation of equal twitches does not follow the results from the experimentally measured unfused tetani, the differences depend on the type of the MU and are bigger for higher values of stimulation frequency and fusion index; (3) the reconstruction of individual twitches from experimental tetanic records can be successful if the tetanus is feebly fused (fusion index up to 0.7); (4) both the maximal forces and time parameters of individual twitches subtracted from unfused tetani change and influence the course of each tetanus. A discrepancy with respect to the relaxation phase was observed between experimental results and model prediction for tetani with fusion index exceeding 0.7. This phase was predicted longer than the experimental one for better fused tetani. Therefore, a separate series of physiological experiments and then, more complex model are necessary for explanation of this distinction.     

Calcium transients and the effect of a photolytically released calcium chelator during electrically induced contractions in rabbit rectococcygeus smooth muscle.

Intracellular Ca2+ was determined with the fura-2 technique during electrically induced contractions in the rabbit rectococcygeus smooth muscle at 22 degreesC. The muscles were electrically activated to give short, reproducible contractions. Intracellular [Ca2+] increased during activation; the increase in [Ca2+] preceded force development by approximately 2 s. After cessation of stimulation Ca2+ fell, preceding the fall in force by approximately 4 s. The fluorescence properties of fura-2 were determined with time-resolved spectroscopy using synchrotron light at the MAX-storage ring, Lund, Sweden. The fluorescence decay of free fura-2 was best described by two exponential decays (time constants approximately 0.5 and 1.5 ns) at low Ca2+ (pCa 9). At high Ca2+ (pCa 4.5), fluorescence decay became slower and could be fitted by one exponential decay (1.9 ns). Time-resolved anisotropy of free fura-2 was characteristic of free rotational motion (correlation time 0.3 ns). Motion of fura-2 could be markedly inhibited by high concentrations of creatine kinase. Time-resolved spectroscopy measurements of muscle fibers loaded with fura-2 showed that the fluorescence lifetime of the probe was longer, suggesting an influence of the chemical environment. Anisotropy measurements revealed, however, that the probe was mobile in the cells. The Ca2+-dependence of contraction and relaxation was studied using a photolabile calcium chelator, diazo-2, which could be loaded into the muscle cells in a similar manner as fura-2. Photolysis of diazo-2 leads to an increase in its Ca2+-affinity and a fall in free Ca2+. When muscles that had been loaded with diazo-2 were illuminated with UV light flashes during the rising phase of contraction, the rate of contraction became slower, suggesting a close relation between intracellular Ca2+ and the cross-bridge interaction. In contrast, photolysis during relaxation did not influence the rate of force decay, suggesting that relaxation of these contractions is not determined by the rate of Ca2+ removal or due to an increased Ca2+ sensitivity, but instead is limited by other processes such as deactivation by dephosphorylation or detachment of tension-bearing cross-bridges, possibly regulated by thin filament systems.     

Analysis of the unfused tetanus course in fast motor units of the rat medial gastrocnemius muscle

The course of unfused tetani with the sag effect in fast motor units of rat medial gastrocnemius was studied. The analysis of the course of successive contractions within these tetani showed that the high peak force at the beginning of tetanus before the sag resulted from temporary, very efficient sum mation of contractions at this phase, both in FF (fast fatigable) and FR (fast resistant to fatigue) units. The process of summation developed in spite of parallel shortening of the contraction and relaxation. The peak of tetanus force was visible on the average at the 2nd contraction in FF units and at the 5th contraction in FR units. After the tetanus peak the process of the efficient summation was completed and the force decreased what was visible as a sag. In the following part of the tetanus, mainly in FF units, the potentiation occurred and the force of successive contractions increased. The rise of force was visible in spite of shortening of the contraction time and was due to prolongation of the relaxation in this part of the tetanus. These observations indicated that the processes of the summation of successive contractions before sag and during the potentiation underwent different mechanisms discussed in this paper. Considerable release of Ca2+ ions from the sarcoplasmic reticulum was proposed as a possible mechanism responsible for a very efficient summation at the beginning of the tetanus whereas phosphorylation of regulatory light chain of myosin (RLC) in muscle fibers was considered as the reason of potentiation. Moreover, the present analysis revealed that previously found differences in tetani profiles between FF and FR units resulted from faster development of described changes in the course of contractions summating into the tetanus in FF motor units.     

A non-cross-bridge stiffness in activated frog muscle fibers

Force responses to fast ramp stretches of various amplitude and velocity, applied during tetanic contractions, were measured in single intact fibers from frog tibialis anterior muscle. Experiments were performed at 14 degrees C at approximately 2.1 microm sarcomere length on fibers bathed in Ringer's solution containing various concentrations of 2,3-butanedione monoxime (BDM) to greatly reduce the isometric tension. The fast tension transient produced by the stretch was followed by a period, lasting until relaxation, during which the tension remained constant to a value that greatly exceeded the isometric tension. The excess of tension was termed "static tension," and the ratio between the force and the accompanying sarcomere length change was termed "static stiffness." The static stiffness was independent of the active tension developed by the fiber, and independent of stretch amplitude and stretching velocity in the whole range tested; it increased with sarcomere length in the range 2.1-2.8 microm, to decrease again at longer lengths. Static stiffness increased well ahead of tension during the tetanus rise, and fell ahead of tension during relaxation. These results suggest that activation increased the stiffness of some sarcomeric structure(s) outside the cross-bridges.     

Muscle activities of the lower limb during level and uphill running

This study aimed to compare the muscle activities of the lower limb during overground level running (LR) and uphill running (UR) by using a musculoskeletal model. Six male distance runners ran at three running speeds (slow: 3.3 m/s; medium: 4.2 m/s; and high: 5.0 m/s) on a level runway and a slope of 9.1% grade in which force platforms were mounted. A musculoskeletal leg model and optimization were used to estimate the muscle activation and muscle torque from the joint torque of the lower limb calculated by the inverse dynamics approach. At high speed, the activation and muscle torque of the muscle groups surrounding the hip joints, such as the hamstrings and iliopsoas, during the recovery phase were significantly greater during UR than during LR. At all the running speeds, the knee extension torque by the vasti during the support phase was significantly smaller during UR. Further, the hip flexion and knee extension torques by the rectus femoris during UR were significantly greater than those during LR at all the speeds; this would play a role in compensating for the decrease in the knee extension torque by the vasti and in maintaining the trunk in a forward-leaning position. These results revealed that the activation and muscle torque of the hip extensors and flexors were augmented during UR at the high speed.     

Muscle contraction and relaxation-response time in response to on or off status of visual stimulus

Background

It is unclear whether response time is affected by a stimulus cue, such as a light turned on or off, or if there are differences in response to these cues during a muscle contraction task compared with a muscle relaxation task. The objective of this study was to assess the response time of a relaxation task, including the contraction portion of the task, to a stimulus of a light turned on or off. In addition, we investigated the effect of the pre-contraction level on the relaxation task.

Results

Contraction response time was significantly shorter during the light-on status than during the light-off status (P <0.01), and relaxation response time in each maximum voluntary contraction was significantly longer during the light-on status than during the light-off status (P <0.01). The relaxation response time became longer in order of 25% to 75% maximum voluntary contraction regardless of light-on or -off status, and was significantly longer than the contraction response time (P <0.05-0.01).

Conclusions

This study found that as the contraction level increased, the relaxation response time became longer than the contraction response time regardless of light status. However, contraction response time or relaxation response time findings were opposite to this during the light-on status and light-off status: contraction response time became shorter in the light-on status than in the light-off status and relaxation response time became longer in the light-on status than in the light-off status. These results suggest that the length of each response time is affected by motor control in the higher order brain and involves specific processing in the visual system.     

Interpreting muscle function from EMG: lessons learned from direct measurements of muscle force

Electromyography is often used to infer the pattern of productionof force by skeletal muscles. The interpretation of muscle functionfrom the electromyogram (EMG) is challenged by the fact thatfactors such as type of muscle fiber, muscle length, and musclevelocity can all influence the relationship between electricaland mechanical activity of a muscle. Simultaneous measurementsof EMG, muscle force, and fascicle length in hindlimb musclesof wild turkeys allow us to probe the quantitative link betweenforce and EMG. We examined two features of the force–EMGrelationship. First, we measured the relaxation electromechanicaldelay (r-EMD) as the time from the end of the EMG signal totime of the end of force. This delay varied with locomotor speedin the lateral gastrocnemius (LG); it was longer at slow walkingspeeds than for running. This variation in r-EMD was not explainedby differences in muscle length trajectory, as the magnitudeof r-EMD was not correlated with the velocity of shorteningof the muscle during relaxation. We speculate that the longerrelaxation times at slow walking speeds compared with runningmay reflect the longer time course of relaxation in slower musclesfibers. We also examined the relationship between magnitudeof force and EMG across a range of walking and running speeds.We analyzed the force–EMG relationship during the swingphase separately from the force–EMG relationship duringstance phase. During stance, force amplitude (average force)was linearly related to mean EMG amplitude (average EMG). Forcesduring swing phase were lower than predicted from the stancephase force–EMG relationship. The different force–EMGrelationships during the stance and swing phases may reflectthe contribution of passive structures to the development offorce, or a nonlinear force–EMG relationship at low levelsof muscle activity. Together the results suggest that any inferenceof force from EMG must be done cautiously when a broad rangeof activities is considered.     

Fatigue and recovery contractile properties of young and elderly men

The 24 h recovery pattern of contractile properties of the triceps surae muscle, following a period of muscle fatigue, was compared in physically active young (25 years, n = 10) and elderly (66 years, n = 7) men. The fatigue test protocol consisted of 10 min of intermittent submaximal 20 Hz tetani. The maximal twitch (Pt) and tetanic force at 3 frequencies (10, 20 and 50 Hz) were determined at baseline and at 15 min, 1, 4 and 24 h after fatiguing the muscle. Maximal voluntary contraction (MVC) and vertical jump (MVJ) were also assessed. The loss of force during the fatigue test was not significantly different between the young (18 +/- 13%) and elderly (22 +/- 15%). Both groups showed similar and significant reductions of Pt (15%), tetanic force (10 to 35%) and rate of force development (dp/dt) (20%) 15 min and 1 h into recovery. The loss of force was greater at the lower stimulation frequencies of 10 and 20 Hz. Time-to-peak tension was unchanged from baseline during recovery in either group. The average rate of relaxation of twitch force (-dPt/dt) was decreased (p less than 0.05) and half-relaxation time significantly increased at 15 min and 1 h in the elderly but not the young. The findings indicate that after fatiguing contractions, elderly muscle demonstrates a slower return to resting levels of the rate and time course of twitch relaxation compared to the young.