Low-frequency Fatigue Research Articles

Changes in force, surface and motor unit EMG during post-exercise development of low frequency fatigue in vastus lateralis muscle

We investigated the effects of low frequency fatigue (LFF) on post-exercise changes in rectified surface EMG (rsEMG) and single motor unit EMG (smuEMG) in vastus lateralis muscle (n = 9). On two experimental days the knee extensors were fatigued with a 60-s-isometric contraction (exercise) at 50% maximal force capacity (MFC). On the first day post-exercise (15 s, 3, 9, 15, 21 and 27 min) rsEMG and electrically-induced (surface stimulation) forces were investigated. SmuEMG was obtained on day two. During short ramp and hold (5 s) contractions at 50% MFC, motor unit discharges of the same units were followed over time. Post-exercise MFC and tetanic force (100 Hz stimulation) recovered to about 90% of the pre-exercise values, but recovery with 20 Hz stimulation was less complete: the 20-100 Hz force ratio (mean +/- SD) decreased from 0.65+/-0.06 (pre-exercise) to 0.56+/-0.04 at 27 min post-exercise (P<0.05), indicative of LFF. At 50% MFC, pre-exercise rsEMG (% pre-exercise maximum) and motor unit discharge rate were 51.1 +/- 12.7% and 14.1 +/- 3.7 (pulses per second; pps) respectively, 15 s post-exercise the respective values were 61.4 +/- 15.4% (P<0.05) and 13.2 +/- 5.6 pps (P>0.05). Thereafter, rsEMG (at 50% MFC) remained stable but motor unit discharge rate significantly increased to 17.7 +/- 3.9 pps 27 min post-exercise. The recruitment threshold decreased (P<0.05) from 27.7 +/- 6.6% MFC before exercise to 25.2 +/- 6.7% 27 min post-exercise. The increase in discharge rate was significantly greater than could be expected from the decrease in recruitment threshold. Thus, post-exercise LFF was compensated by increased motor unit discharge rates which could only partly be accounted for by the small decrease in motor unit recruitment threshold.

Bench-to-bedside review: Weaning failure – should we rest the respiratory muscles with controlled mechanical ventilation?

The use of controlled mechanical ventilation (CMV) in patients who experience weaning failure after a spontaneous breathing trial or after extubation is a strategy based on the premise that respiratory muscle fatigue (requiring rest to recover) is the cause of weaning failure. Recent evidence, however, does not support the existence of low frequency fatigue (the type of fatigue that is long-lasting) in patients who fail to wean despite the excessive respiratory muscle load. This is because physicians have adopted criteria for the definition of spontaneous breathing trial failure and thus termination of unassisted breathing, which lead them to put patients back on the ventilator before the development of low frequency respiratory muscle fatigue. Thus, no reason exists to completely unload the respiratory muscles with CMV for low frequency fatigue reversal if weaning is terminated based on widely accepted predefined criteria. This is important, since experimental evidence suggests that CMV can induce dysfunction of the diaphragm, resulting in decreased diaphragmatic force generating capacity, which has been called ventilator-induced diaphragmatic dysfunction (VIDD). The mechanisms of VIDD are not fully elucidated, but include muscle atrophy, oxidative stress and structural injury. Partial modes of ventilatory support should be used whenever possible, since these modes attenuate the deleterious effects of mechanical ventilation on respiratory muscles. When CMV is used, concurrent administration of antioxidants (which decrease oxidative stress and thus attenuate VIDD) seems justified, since antioxidants may be beneficial (and are certainly not harmful) in critical care patients.

Low-frequency fatigue, post-tetanic potentiation and their interaction at different muscle lengths following eccentric exercise

Low-frequency fatigue (LFF) and post-tetanic potentiation (PTP) were quantified at different muscle lengths in rat medial gastrocnemius (GM) muscle. In situ experiments were performed on GM muscle-tendon complexes of anaesthetised (urethane, 1.5 g kg(-1) i.p.) Wistar rats (N=8). Force-length characteristics were determined at maximal (200 Hz) and submaximal (60 Hz) stimulation. Data for submaximally stimulated muscle were obtained in a non-potentiated and in a potentiated condition. LFF was induced by a series of 40 eccentric contractions. Post-exercise (40-80 min), data for the force-length relationships were obtained once more. Whereas force loss at 200 Hz-stimulation was least at optimum muscle length, L(0,200 Hz), (17.0+/-1.4%, mean +/-S.E.M.), force loss at 60 Hz-stimulation was maximal near L(0,200 Hz) (55.1+/-4.3% at L(0,200 Hz)-1 mm). When the muscle was potentiated, force loss at 60 Hz-stimulation was maximal at short muscle length: L(0,200 Hz)-4 mm (53.5+/-3.8%). The extent of LFF, quantified by a decrease in the 60:200 Hz force ratio, varied with muscle length: LFF increased with decreasing muscle lengths when muscles were potentiated. However, in the non-potentiated condition, LFF was maximal at a length just below L(0,200 Hz); the 60:200 Hz force ratio had decreased to 54.6+/-5.9% of the pre-exercise ratio at L(0,200 Hz)-1 mm. Compared with the non-potentiated condition, LFF was less pronounced in the potentiated condition. PTP counteracted LFF particularly at long muscle lengths. However, at short muscle lengths, LFF was still observed in potentiated muscles.

Voluntary low-force contraction elicits prolonged low-frequency fatigue and changes in surface electromyography and mechanomyography

Controversies exist regarding objective documentation of fatigue development with low-force contractions. We hypothesized that non-exhaustive, low-force muscle contraction may induce prolonged low-frequency fatigue (LFF) that in the subsequent recovery period is detectable by electromyography (EMG) and in particular mechanomyography (MMG) during low-force rather than high-force test contractions. Seven subjects performed static wrist extension at 10% maximal voluntary contraction (MVC) for 10 min (10%MVC 10min). Wrist force response to electrical stimulation of extensor carpi radialis muscle (ECR) quantified LFF. EMG and MMG were recorded from ECR during static test contractions at 5% and 80% MVC. Electrical stimulation, MVC, and test contractions were performed before 10%MVC 10min and at 10, 30, 90 and 150 min recovery. In spite of no changes in MVC, LFF persisted up to 150 min recovery but did not develop in a control experiment omitting 10%MVC 10min. In 5% MVC tests significant increase was found in time domain of EMG from 0.067 ± 0.028 mV before 10%MVC 10min to 0.107 ± 0.049 and 0.087 ± 0.05 mV at 10 and 30 min recovery, respectively, and of the MMG from 0.054 ± 0.039 m s −2 to 0.133 ± 0.104 and 0.127 ± 0.099 m s −2, respectively. No consistent changes were found in 80% MVC tests. In conclusion, non-exhaustive low-force muscle contraction resulted in prolonged LFF that in part was identified by the EMG and MMG signals.

The Sarcoplasmic Reticulum in Muscle Fatigue and Disease: Role of the Sarco(endo)plasmic Reticulum Ca2+-ATPase

Skeletal muscles induced to contract repeatedly respond with a progressive loss in their ability to generate a target force or power. This condition is known simply as fatigue. Commonly, fatigue may persist for prolonged periods of time, particularly at low activation frequencies, which is called low-frequency fatigue. Failure to activate the contractile apparatus with the appropriate intracellular free calcium ([Ca2+]f) signal contributes to fatigue but the precise mechanisms involved are unknown. The sarcoplasmic reticulum (SR) is the major organelle in muscle that is responsible for the regulation of [Ca2+]f, and numerous studies have shown that SR function, both Ca2+ release and Ca2+ uptake, is impaired following fatiguing contractile activity. The major aim of this review is to provide insight into the various cellular mechanisms underlying the alterations in SR Ca2+ cycling and cytosolic [Ca2+]f that are associated both with the development of fatigue during repeated muscle contraction and with low-frequency or long-lasting fatigue. The primary focus will be on the role of the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) in normal muscle function, fatigue, and disease.

Coexistence of potentiation and low-frequency fatigue during voluntary exercise in human skeletal muscle.

The role of muscle potentiation in overcoming low-frequency fatigue (LFF) as it developed during submaximal voluntary exercise was investigated in eight males (age 26.4 +/- 0.7 years, mean +/- SE) performing isometric leg extension at approximately 30% of maximal voluntary contraction for 60 min using a 0.5-duty cycle (1 s contraction, 1 s rest). At 5, 20, 40, and 60 min, exercise was interrupted for 3 min, and the maximum positive rate of force development (+dF/dtmax) and maximal twitch force (Pt) were measured in maximal twitch contractions at 0, 1, 2, and 3 min of rest (R0, R1, R2, R3); they were also measured at 15 min of recovery following the entire 60-min exercise period. These measures were compared with pre-exercise (PRE) as an indicator of potentiation. Force at low frequency (10 Hz) was also measured at R0, R1, R2, and R3, and at 15 min of recovery, while force at high frequency (100 Hz) was measured only at R0 and R3 and in recovery. Voluntary exercise increased twitch +dF/dtmax at R0 following 5, 20, 40, and 60 min of exercise, from 2553 +/- 150 N/s at PRE to 39%, 41%, 42%, and 36% above PRE, respectively (P<0.005). Twitch +dF/dtmax decayed at brief rest (R3) following 20, 40, and 60 min of exercise (P<0.05). Pt at R0 following 5 and 20 min of exercise was above that at PRE (P<0.05), indicating that during the early phase of moderate-intensity repetitive exercise, potentiation occurs in the relative absence of LFF. At 40 and 60 min of exercise, Pt at R0 was unchanged from PRE. The LFF (10 Hz) induced by the protocol was evident at 40 and 60 min (R0-R3; P<0.05) and at 15 min following exercise (P<0.05). High-frequency force was not significantly compromised by the protocol. Since twitch force was maintained, these results suggest that as exercise progresses, LFF develops, which can be compensated for by potentiation.

Fatigue in the Forearm Resulting From Low-Level Repetitive Ulnar Deviation

This study measured low-frequency fatigue (LFF) in the extensor carpi ulnaris (ECU) muscle while workers completed a repetitive ulnar deviation task. Using a repeated measures design, 10 healthy women participated in three conditions, each lasting 2 consecutive days: a control condition in which subjects remained inactive, and two repetitive work conditions involving repeated ulnar deviation of the wrist at 20 and 25 repetitions per minute at individual workloads deemed acceptable for 8 hours through a psychophysical protocol. LFF of the ECU muscle and self-reported levels of fatigue were recorded eight times throughout the control and workdays before (time 0), during (2, 4.25, 6.75, 8 hours), and after (9, 10, and 11 hours) exposure. The ratio of the isometric force produced by electrical stimulus at 20 pulses per second (pps) to the isometric force produced by 50 pps provided the measure of LFF. The ratios were lower on workdays compared with the control days, indicating the presence of LFF during repetitive work. During repetitive work the ratios decreased during the day, indicating the muscles fatigued as the day progressed. The psychophysically determined workloads, although not creating noticeable discomfort to the subjects, were high enough to create low levels of muscle fatigue.

Low-frequency fatigue is fibre type related and most pronounced after eccentric activity in rat medial gastrocnemius muscle.

Effects of fibre type composition and type of contraction on low-frequency fatigue (LFF) were investigated in isolated rat medial gastrocnemius (GM) muscle. Fast oxidative or fast glycolytic GM muscle parts of anaesthetised male Wistar rats (n=18) were activated selectively by maximal electrical stimulation of the nerve after selective cutting of sub-branches. LFF was induced by a series of 40 isometric, concentric or eccentric contractions. Post exercise (55 min), the force-frequency curves differed significantly from the pre-exercise curves. Decreased forces were exerted mainly at the lower frequencies. This effect was significantly greater for glycolytic than oxidative muscle parts and following eccentric compared to isometric and concentric exercise. Seventy minutes following eccentric exercise, the relative values of the 60:200 Hz force ratios for the oxidative compared to the glycolytic parts were 65.6+/-2.2% and 43.6+/-4.6% (mean+/-SE) of the pre-fatigue values (=100%), respectively. In conclusion, for conditions of identical activation, eccentric exercise led to significantly more LFF than isometric and concentric exercise. In addition, and independent of the exercise type, fast glycolytic muscle parts were more susceptible to LFF than fast oxidative muscle parts.

Tension-time index, fatigue, and energetics in isolated rat diaphragm: a new experimental model.

The tension-time index (TTI) has been used to estimate mechanical load, energy utilization, blood flow, and susceptibility to fatigue in contracting muscle. The TTI can be defined, for a rhythmically contracting muscle, as the product of average force development divided by maximum tetanic force times duty cycle [contraction time / (contraction + relaxation time)]. In this study, the TTI concept was applied to isolated diaphragm via a method that allowed TTI to be clamped at a predetermined value. The hypothesis tested was that, at constant TTI, muscle energetics and the extent of fatigue would vary with stimulation frequency. Isolated diaphragm strips were stimulated at 25, 50, 75, or 100 Hz for 4 min, one per second. Duty cycle was continuously adjusted to maintain TTI at 0.07, which was near the highest TTI tolerated for 4 min, at 20-Hz stimulation. At the end of the fatigue run, muscles were either immediately frozen for determination ATP, creatine, and creatine phosphate concentrations (n = 6) or stimulated for evaluation of low- and high-frequency fatigue (n = 5). Results demonstrated no difference in the extent of fatigue or in the final ATP and creatine phosphate concentrations between groups. Large within-run increases in duty cycle were required at low stimulation frequencies, but only small increases were required at the highest frequencies. The results demonstrate that, at a constant TTI, similar fatigue properties predominate at all stimulation frequencies with no clear distinction between high- and low-frequency fatigue. The method of clamping TTI during fatigue may be useful for evaluating energetics and contractile function between treatment groups in isolated muscle when treatment influences baseline contractile characteristics.

Symptoms and quadriceps fatigability after walking and cycling in chronic obstructive pulmonary disease.

Patients with chronic obstructive pulmonary disease (COPD) may stop cycling due to leg effort rather than breathlessness. However, cycling is not relevant to many patients, although walking may be more familiar. A total of 84 patients with COPD were asked to name the predominant symptom limiting incremental shuttle walking, endurance shuttle walking, incremental cycle ergometry, and endurance cycle ergometry, performed to exhaustion on four separate days. Furthermore, quadriceps fatigability was evaluated in 12 patients by measuring unpotentiated and potentiated twitch quadriceps tensions before and 30 minutes after incremental walking and cycling. Breathlessness alone was a more commonly cited limiting symptom after incremental walking compared with incremental cycling (81 vs. 34%; p < 0.001) and after endurance walking compared with endurance cycling (75 vs. 29%; p < 0.001). In addition, there was no significant change in mean pre- and postwalking twitch quadriceps tensions. However, cycling produced a significant reduction (unpotentiated 7.42 +/- 2.22 vs. 6.48 +/- 2.09 kg [p < 0.001]; potentiated 10.19 +/- 3.99 vs. 8.45 +/- 3.18 kg [p < 0.001]). Pre- to postexercise changes were significantly greater during cycling compared with walking (unpotentiated p = 0.01; potentiated p = 0.003). Leg effort is an infrequent symptom after walking in COPD, and low frequency fatigue of the quadriceps is an infrequent feature of incremental walking.

LOW-FREQUENCY FATIGUE INCREASES WITH THE NUMBER OF STRETCHES OF ACTIVATED RAT SKELETAL MUSCLES

PURPOSE To examine the relationship between stretch number of activated plantar flexor muscles and low-frequency fatigue. METHODS In female Sprague Dawley rats (age 3–5 mo), nerve stimulation was used to produce muscle contractions. Stretches [3 stretches (S), 10S, 60S or 240S (n = 6 in each group)] were imposed on isometric contractions [80 Hz, 0.2 ms pulse duration, 5.3±0.3 V (mean±SE)] by ankle rotation from 90° to 40° (velocity: 600 °·s−1, simulation time: 0.9 s, inter-stretch rest time: 40 s). Forty seconds after the last stretch, an isometric contraction (80 Hz at 90°) was performed. Force-frequency relationships with isometric contractions at 90° were measured before and 1 hour after the stretch protocols. An index for low-frequency fatigue was calculated by the change in the 20:100 Hz isometric force ratio. RESULTS All stretch protocols resulted in isometric force deficits (3S: 37.0±2.4%, 10S: 44.6±1.5%, 60S: 74.1±1.9%, 240S: 83.4±2.0%). After 1 hour, substantial deficits remained (3S: 33.3±1.7%, 10S: 37.2±2.3%, 60S: 67.6±1.5%, 240S: 77.7±1.2%) and low-frequency fatigue was present in all groups. Force deficits with contractions at 20 Hz were significantly larger than at 100 Hz (3S: 45.9±2.4% vs. 30.8±2.0%, 10S: 57.1±3.0% vs. 34.2±2.1%, 60S: 78.9±0.4% vs. 63.0±1.7%, 240S: 88.5±1.3% vs. 73.9±1.4%). Low-frequency fatigue became significantly larger with an increase in the number of stretches (P = 0.0009). CONCLUSIONS The amount of low-frequency fatigue was related to the number of stretches of activated skeletal muscles providing indirect evidence for an increased failure of excitation-contraction coupling with stretch number. Supported by NIOSH R01-OH-02918.

Fatigue in the forearm resulting from low-level repetitive ulnar deviation.

This study measured low-frequency fatigue (LFF) in the extensor carpi ulnaris (ECU) muscle while workers completed a repetitive ulnar deviation task. Using a repeated measures design, 10 healthy women participated in three conditions, each lasting 2 consecutive days: a control condition in which subjects remained inactive, and two repetitive work conditions involving repeated ulnar deviation of the wrist at 20 and 25 repetitions per minute at individual workloads deemed acceptable for 8 hours through a psychophysical protocol. LFF of the ECU muscle and self-reported levels of fatigue were recorded eight times throughout the control and workdays before (time 0), during (2, 4.25, 6.75, 8 hours), and after (9, 10, and 11 hours) exposure. The ratio of the isometric force produced by electrical stimulus at 20 pulses per second (pps) to the isometric force produced by 50 pps provided the measure of LFF. The ratios were lower on workdays compared with the control days, indicating the presence of LFF during repetitive work. During repetitive work the ratios decreased during the day, indicating the muscles fatigued as the day progressed. The psychophysically determined workloads, although not creating noticeable discomfort to the subjects, were high enough to create low levels of muscle fatigue.

Low cycle fatigue and fatigue crack growth behaviour of Sn–Ag eutectic solder

Low cycle fatigue tests on as‐cast Sn–Ag eutectic solder (96.5Sn–3.5Ag) were carried out using a non‐contact strain controlled system at 20°C with different frequencies (10−3–1 Hz). Steps at the boundaries of Sn‐dendrites were found to be the initiation sites for microcracks in the case of low frequency fatigue tests, while for high frequency tests, cracks predominantly initiated at the boundaries of subgrains formed within Sn‐dendrites. The link up of these cracks and the propagation of cracks inside the specimen occurred both transgranularly through Sn–Ag eutectic phases, and intergranularly along Sn‐dendrite boundaries and/or subgrain boundaries. Propagation of stage II cracks for various frequencies could be characterized by theC*‐parameter.

Recent advances in the understanding of skeletal muscle fatigue.

Prolonged or repeated contractions of skeletal muscles lead to impaired muscle function, fatigue develops. Fatigue may be caused by factors within the muscle cells (peripheral fatigue) and diminished activation from the central nervous system (central fatigue). The relative importance of peripheral central fatigue depends on the type of physical activity. Central fatigue may be more prominent in elderly subjects. Increased concentration of inorganic phosphate seems to be of major importance for acute peripheral fatigue. There is frequently a long-lasting depression of force production after fatiguing muscle activity, especially at low stimulation frequencies. This low-frequency fatigue seems to be due to "structural" changes in proteins involved in intracellular Ca handling. Contractions in which the muscle is stretched (eccentric contractions) cause muscle weakness and damage. The initial defect induced by eccentric contractions is overstretched sarcomeres, but these appear to cause localized membrane tears that subsequently contribute to muscle weakness and damage.

Changes in surface EMG signal under the influence of peripheral fatigue

The purpose of this study was to find out if surface EMG signal enables a differentiation between high- and low-frequency peripheral fatigue that was caused with two different sports activities. Ten students (age, 24.2 ± 3.39 years; weight, 79.32 ± 12.95 kg) performed consecutive hops for 60 s (to induce high-frequency fatigue) and cycling at maximal intensity for 30 s (to induce low-frequency fatigue). Before and after both activities, the m. vastus lateralis was stimulated with 2 consecutive trains at 20 Hz and 100 Hz and, at the same time, the torque at the knee joint was measured. The torque ratio between 100 Hz and 20Hz was calculated to define the type of peripheral fatigue. Additionally, a response of m. vastus lateralis to 2 supramaximal electrical impulses delivered to the femoral nerve at 50 Hz was measured. Hopping resulted in high-frequency fatigue, the ratio between forces at 100 Hz and 20 Hz decreased for 10.7 ± 8.3% (p

Influence of frequency on low cycle fatigue behavior of Pb-free solder 96.5Sn–3.5Ag

Low cycle fatigue (LCF) tests on as-cast Sn–Ag eutectic solder (96.5Sn–3.5Ag) were carried out using the non-contact strain controlled system at 20 °C with different frequencies (10 −3 –1 Hz). The LCF behavior in this frequency range followed the Coffin–Manson equation. The fatigue ductility coefficient was found to be influenced by frequency. Frequency-modified Coffin–Manson model successfully described frequency effects on the fatigue behavior. Steps at the boundaries of Sn-dendrites were the initiation sites for microcracks in the case of low frequency fatigue tests, while for high frequency tests, cracks predominantly initiated at the boundaries of subgrains formed in Sn-dendrites. The link up of these cracks and the propagation of cracks inside the specimen occurred both transgranularly through Sn–Ag eutectic phases, and intergranularly along Sn-dendrite boundaries and/or subgrain boundaries. The dislocation rearrangement (polygonization) is considered to be the cause of this small grain formation.

Development of mechanical tests for stress corrosion and fatigue corrosion studies

Two specific examples are presented to describe the experience gained in the use of mechanical tests for the study of stress corrosion cracking and corrosion-fatigue: 1) the design of tension test platforms and low-frequency fatigue machines, having cross-head displacement rates of the order of 1 μm/h; 2) the use of curved tension test pieces to take into account heterogeneities of the material microstructure, in the case of cracking phenomena in burried pipelines.

Vibration-induced muscle fatigue, a possible contribution to musculoskeletal injury.

Localized muscle fatigue resulting from 30-min sustained and intermittent grip exertions of 5% maximal voluntary contraction (MVC) with and without hand-vibration exposure (10 Hz, 7 mm displacement amplitude) was investigated. Muscle fatigue was quantified by the magnitude of the twitch force elicited in the right flexor digitorum superficialis muscle of the long finger using the low-frequency fatigue (LFF) method. The influence of vibration in the sustained grip exertion condition exacerbates fatigue as seen with the reduction in twitch force 30-60 min post-work task. Intermittent low grip force exertion conditions with and without vibration exposure show negligible fatigue, suggesting the benefit of rest in the work cycle. Perception of muscle fatigue was dissociated from the objective measure of twitch force, suggesting that LFF was not perceived. The presence of LFF and the lack of perception of LFF may increase the risk for the development of musculoskeletal disorders. The findings of this study may apply to the design of the work cycles and tasks that require the use of vibratory tools.

Factors in fatigue during intermittent electrical stimulation of human skeletal muscle.

During an electrically elicited isometric contraction, the metabolic cost of attaining is greater than of maintaining force. Thus fatigue produced during such stimulation may not simply be a function of the force-time integral (FTI), as previously suggested. The goal of the present study was to evaluate fatigue produced in human medial gastrocnemius by intermittent, isometric electrical stimulation with trains of different frequencies (20, 40, or 80 Hz) and durations (300, 600, or 1,200 ms) that produced different peak forces and FTIs. Each subject (n = 10) participated in a total of six sessions. During each session, subjects received a pre- and postfatigue testing protocol and a different, 150-train fatiguing protocol. Each fatiguing protocol used only a single frequency and duration. The fatigue produced by the different protocols was correlated to the initial peak force of the fatiguing protocols (r2= 0.74-0.85) but not to the initial or total FTI. All of the protocols tested produced a proportionately greater impairment of force in response to low- vs. high-frequency stimulation (i.e., low-frequency fatigue). There was no effect of protocol on low-frequency fatigue, suggesting that all the protocols produced comparable levels of impairment in excitation-contraction coupling. These results suggest that, for brief stimulated contractions, peak force is a better predictor of fatigue than FTI, possibly because of the different metabolic demands of attaining and maintaining force.

Parent material and weldments degradation on SASOL reduction reactors due to combined effect of thermal fatigue, vibration and hydrogen attack

All six installed reduction reactors showed the same failure pattern, which can be attributed to inadequate original design, material degradation due to service conditions and improper maintenance activities. Service is typically low frequency fatigue load conditions with changes of pressure, temperature and batch load. All revealed defects severely affected the integrity of the pressure envelope and vessels were classified for major repair work with subsequent in situ monitoring program. The justification for such extensive and costly repair required a positive evaluation of the current condition of equipment. After complete study, a report was prepared which included the fitness for purpose evaluation, remaining life assessment and recommendation for future field inspections. All reactors were found to be suitable for repair. Within the scope of repair a number of critical elements were redesigned and replaced. The repair was successful and all reactors were put back into operation. Unique expertise was obtained regarding setting the parameters of dehydrogenation in reference to the heavy wall items. Taking replicas and surface sampling appeared to be very reliable and simple tools in monitoring the condition of pressure envelope. For the purpose of future inspection shutdowns specific inspection requirements were put in place to ensure continuous monitoring of the vessel integrity.