Muscle fatigue and fatigue-related biomechanical changes during a cyclic lifting task

STUDY DESIGN

Electromyographic and biomechanical methods were utilized to investigate correlations between indexes of localized muscle fatigue and changes in the kinematics and kinetics of motion during a cyclic lifting task.

SUMMARY OF BACKGROUND DATA

Recent advances in time-frequency analysis procedures for electromyographicic signal processing provide a new way of studying localized muscle fatigue during dynamic contractions. These methods provide a means to investigate fatigue-related functional impairments in patients with low back pain.

OBJECTIVES

To study the relationship between localized muscle fatigue and the biomechanics of lifting and lowering a weighted box. Fatigue-related changes in the electromyographicic signal of trunk and limb muscles were evaluated and compared to kinematic and kinetic measures in order to determine whether lifting strategy is modified with fatigue.

METHODS

A total of 14 healthy male subjects (26 +/- 5 years) cyclically lifted and lowered a 13 kg box (12 lifts/min) for 4.5 minutes. A 5-second static maximum lifting task was included immediately before and after the cyclic lifting task to measure changes in lifting strength and static electromyographicic fatigue indexes. Electromyographic signals from 14 muscle sites (including paravertebral and limb muscles) were measured. Changes in the electromyographicic Instantaneous Median Frequency, a fatigue index, were computed using time-frequency analysis methods. This index was compared with more standardized measures of fatigue, such as those based on electromyographicic median frequency acquired during a static trunk extension test, subjective fatigue measures, and maximal static lifting strength. Biomechanical measures were gathered using a motion analysis system to study kinematic and kinetic changes during the lifting task.

RESULTS

During the cyclic lifting task, the electromyographic Instantaneous Median Frequency significantly decreased over time in the paravertebral muscles, but not in the limb muscles. Paravertebral electromyographicic Instantaneous Median Frequency changes were consistent with self-reports of fatigue as well as decreases in trunk extension strength. The magnitude of muscle-specific changes in electromyographicic Instantaneous Median Frequency was not significantly correlated with electromyographicic median frequency changes from the static trunk extension task. The load of the box relative to the maximal static lifting strength significantly affected the electromyographicic Instantaneous Median Frequency changes of paravertebral back muscles. Significant changes with fatigue during the task were found in the angular displacements at the knee, hip, trunk, and elbow. These biomechanical changes were associated with increased peak torque and forces at the L4-L5 vertebral segment.

CONCLUSIONS

Our results demonstrate correlation between localized muscle fatigue and biomechanical adaptations that occur during a cyclic lifting task. This new technique may provide researchers and clinicians with a means to investigate fatigue-related effects of repetitive work tasks or assessment procedures that might be useful in improving education, lifting ergonomy, and back school programs. Although both the dynamic and static tasks resulted in spectral shifts in the electromyographicic data, the fact that these methods led to different muscle-specific findings indicates that they should not be considered as equivalent assessment procedures.

Impact of pneumococcal vaccination on morbidity and mortality of geriatric patients: a case-controlled study

BACKGROUND

Infections due to Streptococcus pneumoniae are the major cause of adult community-acquired pneumonia, especially in elderly persons with chronic medical conditions. Despite their well-documented efficacy against bacteraemic disease and deaths in the elderly population, pneumococcal polysaccharide vaccines are still very much underused.

OBJECTIVE

A retrospective, case-controlled study was performed to investigate the impact of vaccination with a polysaccharide pneumococcal vaccine, together with other risk factors, on the incidence of pneumonia and deaths in patients in a long-stay geriatric hospital.

METHODS

Subjects were 1,077 residents in a long-stay geriatric hospital in Vienna, Austria, including 359 patients diagnosed as having pneumonia during the period from July 1996 to October 1998 and 718 control subjects. Two controls resident in the hospital during the same time period were matched for each case according to demographic characteristics, chronic illness, and duration of stay in the geriatric ward. The vaccination status was established for all subjects. A logistic regression analysis was performed to assess the efficacy of vaccination and the impact of other cofactors on disease and death.

RESULTS

In cases and controls, 514 (47.7%) had received a pneumococcal vaccine within 2 years prior to the study. There were no differences in demographic characteristics, underlying medical conditions, or duration of stay in the hospital between vaccinated and non-vaccinated patients. In patients diagnosed with pneumonia, 66% were unvaccinated. Logistic regression analysis showed that vaccination significantly decreased the risk of pneumonia (odds ratio 0.279; p < 0.0001). Of the cofactors tested, only gender (lower risk in males) and diabetes mellitus (higher risk) had any impact on disease risk and vaccine efficacy. There appeared to be a highly significant effect of vaccination, reducing the risk of all deaths (odds ratio 0.269; p < 0.0001) and deaths due to pneumonia (odds ratio 0.331; p < 0.0001).

CONCLUSION

This study showed that vaccination with a polysaccharide pneumococcal vaccine is effective in all groups of geriatric subjects and has a consequential value for health and well-being of elderly institutionalized patients.

Issues in designing flexible trials

We outline the general framework of adaptive combination tests and discuss their relationship to flexible group sequential designs. An important field of applications is sample size reassessment. We discuss reassessment rules based on conditional power arguments using either the observed or the prefixed effect size. These rules tend to lead to large expected sample sizes for small actual effects. However, the application of a maximal bound for the second stage sample size leads to more favourable properties. Additionally, we consider an optimized reassessment rule in terms of expected sample sizes. Since the adaptive design does not use the classical test statistics for some types of sample size reassessments, the adaptive test may reject the null hypothesis while the classical one-sample test does not. We characterize sample size reassessment rules, where such inconsistencies are avoided. Finally, the extension of flexibility to the number of stages is explored. In the first interim analysis a second interim analysis is only planned if the chance to achieve a decision there is high. This leads to savings in the average number of interim analysis performed, without paying a noticeable price in terms of expected sample size.

Sequential tests for noninferiority and superiority

The problem of simultaneous sequential tests for noninferiority and superiority of a treatment, as compared to an active control, is considered in terms of continuous hierarchical families of one-sided null hypotheses, in the framework of group sequential and adaptive two-stage designs. The crucial point is that the decision boundaries for the individual null hypotheses may vary over the parameter space. This allows one to construct designs where, e.g., a rigid stopping criterion is chosen, rejecting or accepting all individual null hypotheses simultaneously. Another possibility is to use monitoring type stopping boundaries, which leave some flexibility to the experimenter: he can decide, at the interim analysis, whether he is satisfied with the noninferiority margin achieved at this stage, or wants to go for more at the second stage. In the case where he proceeds to the second stage, he may perform midtrial design modifications (e.g., reassess the sample size). The proposed approach allows one to "spend," e.g., less of alpha for an early proof of noninferiority than for an early proof of superiority, and is illustrated by typical examples.

Factors that influence the duration of splint wear in peripheral nerve lesions

OBJECTIVE

To assess the weekly frequency and overall duration of wear for splints used to treat peripheral nerve lesions and factors that possibly influence splint wear.

DESIGN

A total of 78 patients (23 women, 55 men) who had been treated with a hand splint for peripheral nerve palsy were interviewed by telephone, based on a preformulated questionnaire. The duration of wear, reasons for terminating use, the effect of the splint, the patients' assessment of splint treatment, the patients' instruction, and the patients' level of education were recorded. Survival analysis for the entire period of wear and logistic regressions to determine factors that influence splint wear were performed. Comparison of effect scores between lesions in the dominant those in the nondominant hand were carried out with a test.

RESULTS

The median duration of wear was 6 mo for daytime and 4 mo for nighttime splints. Daytime splints were worn by 85% and nighttime splints by 84% of patients for 5-7 days per week. The reported effect of the splint was the only significant factor that influenced the frequency of wear. The splint was reported to be significantly more effective in the dominant hand than in the nondominant hand. Numerically, radial nerve lesions and cock-up wrist splints were most common.

CONCLUSION

Regular splint wear was registered in the majority of patients and was positively influenced by a good effect of the splint, which was reported to be better in the dominant hand.

Reliability of EMG time-frequency measures of fatigue during repetitive lifting

PURPOSE

To test the short-term and long-term reliability of time-frequency electromyographic (EMG) measures of fatigue during repetitive dynamic lifting and compare it with the reliability of median frequency (MF) EMG measures of fatigue during static lifting.

METHODS

Fourteen' healthy male subjects (26 +/- 5 years) repetitively (12 lifts/min) lifted and lowered a box (29 x 25 x 23 cm, 13 kg) for 4.5 min during 3 different tests on 2 different days. EMG data and the biomechanics of motion were recorded. Before and after dynamic lifting, static maximum lifting tests were performed. At the end of each of the two sessions, subjects performed a static lift at 80% of their maximum lifting force for 30 s.

RESULTS

Significant fatigue-related changes were observed during the lifting exercise via EMG time-frequency analysis at the paravertebral L5, L2, T10, and vastus lateralis (VL) electrode sites. Two parameters for assessing fatigue during dynamic contractions [i.e., the Instantaneous Median Frequency (IMDF) and its time dependent change] were shown to be reproducible both in the short-term (2 h) and long-term (2 wk). The corresponding ICCs reflecting the reproducibility of values between sessions were 96.9% (L5), 98.1% (L2), 90.1% (T10), 96.4% (UT), 98.0% (GM), 89.5% (VL), and 99.0% (BF), respectively. For most EMG recording sites, the reliability of the IMDF measures was not dependent upon the postural strategy that the subject used to accomplish the lifting task or on the subject's strength as measured via the static maximum lifting test. A comparison between the ICC values of the IMDF measures and those of the parameters utilized to assess fatigue during static sustained lifts [i.e., the Median Frequency (MDF) and its change during the test] revealed equally good reproducibility for most EMG recording sites. The respective ICC values that took into account time dependent trends for the IMDF parameter were 87.1% (L5), 62.4% (L2), 90.1% (T10), 0% (UT), 72.7% (GM), 45.4% (VL), and 100% (BF), and for the MDF parameter 94.9% (L5), 73.0% (L2), 80.9% (T10), 100% (UT), 89% (GM), 91.7% (VL), and 90.9% (BF), respectively.

CONCLUSIONS

The time-frequency approach allows one to derive EMG spectral parameters that can be used to monitor muscle fatigue during dynamic real-world tasks such as lifting.

[Optimizing electrotherapy--a comparative study of 3 different currents]

Neuromuscular electrical stimulation can be used to increase strength of skeletal muscle. In neuromuscular electrical stimulation of innervated skeletal muscle, the application of the highest possible intensities of electricity is an important determinant of therapeutic success with regard to strength training. The therapeutic goal of "strengthening muscles" is opposed to the side effect of discomfort through the application of electricity. The aim of the present study was to compare three forms of electric current with respect to subjective tolerance and maximum achievable muscular strength. Twenty-nine healthy male volunteers were included in a single-blind study in which three forms of electric current were applied in a randomized fashion. The following types were compared: a short monophasic form, a biphasic form that was twice as long in terms of impulse duration, and a long monophasic form with an equal impulse duration than tht of the biphasic form and with an impulse form that corresponded to the short monophasic current. Stimulation was administered via surface electrodes placed on the knee extensors in the lower extremity on the right side. The intensity of the current was increased to the individual limit of tolerance or to a maximum of 100 mA. The main target parameter used to determine the success of treatment was the maximum electrically induced strength as a percentage of maximal voluntary contraction. The short monophasic form of electricity was associated with much less discomfort than the long monophasic and biphasic forms (p < 0.0001, p = 0.0062). Furthermore, the biphasic form was better tolerated (and therefore had a larger therapeutic range of application) than the long monophasic form (p = 0.041). The biphasic and long monophasic forms produced higher values for maximum electrically induced strength than did the short monophasic form (p = 0.0001, p = 0.0010). To summarize: the biphasic form had a larger range of therapeutic application than the long monophasic form. Furthermore, the biphasic form produced 40% more electrically induced strength than the short monophasic one. It may be concluded that, in terms of therapeutic application, the biphasic form of electric current is superior to the monophasic forms described in the present study.

The SCARF osteotomy for the correction of hallux valgus deformities

The authors report their experience with a modified SCARF osteotomy with three years follow-up. Correction of moderate to severe hallux valgus deformities was achieved using a Z step osteotomy cut to realign the first metatarsal bone. A retrospective analysis was undertaken in 89 consecutive patients (111 feet). Results were analyzed by clinical examination, a questionnaire including the AOFAS forefoot score, and plain roentgenograms. Hallux valgus and intermetatarsal angle improved at mean 19.1 degrees and 6.6 degrees, respectively. Mean forefoot score improved from 50.1 to 91 points out of 100 possible points. Satisfactory healing time was expressed by an average return back to work of 5.8 weeks and back to sport of 8.3 weeks. Persistence or recurrence of hallux valgus was seen in seven patients (6%). The complication rate was 5.4% including superficial wound infection, traumatic dislocation of the distal fragment, and hallux limitus. The presented technique provides predictable correction of moderate to severe hallux valgus deformities.

Past and future exceedances of nitrogen critical loads in Europe

Critical loads of acidity and nutrient nitrogen--simple measures of the sensitivity of ecosystems to deposition--have been widely used for setting emission reduction targets in Europe. In contrast to sulfur, the emissions of nitrogen compounds remain high in the future. This is also true for the exceedances of critical loads until 2010. Looking further into the future, climate change is likely to influence ecosystem sensitivity, and thus critical loads. It is shown that higher temperatures, changed precipitation patterns, and modified net primary production mainly increase critical loads, except in mountainous and arid regions. Using consistent scenarios of climate change and air pollution from a recently completed European study (AIR-CLIM), it is shown that the exceedances in 2100 of the critical loads are declining in comparison to 2010. However, exceedances of critical loads of nutrient nitrogen remain substantial, even under the most stringent scenario. This confirms the increasing role nitrogen plays in environmental problems in comparison to sulfur. Thus research should focus on the effects of nitrogen in the environment, especially under conditions of climate change, to support nitrogen-emission mitigating policies. This not only reduces acidification and eutrophication, but also helps curb the formation of tropospheric ozone.

Short segment stimulation of the anterior transposed ulnar nerve at the elbow

OBJECTIVE

To determine whether short segment stimulation after anterior subcutaneous transposition of the ulnar nerve reaches normal values and correlates with postoperative clinical findings.

DESIGN

Comparative cross-sectional study.

SETTING

Outpatient clinic of a university department of physical medicine and rehabilitation.

PATIENTS

Nineteen patients (15 men, 4 women) with 21 surgically treated ulnar neuropathies at the elbow; and 19 healthy controls (11 men, 8 women) with 24 measured nerves.

INTERVENTIONS

Assessed motor function of ulnar innervated muscles and staged into 4 categories; used questionnaire to assess clinical course of the nerve lesion and graded into 5 categories; took electrophysiologic recordings to measure motor conduction velocity and compound muscle action potentials; and studied short segment stimulation across elbow and lower arm.

MAIN OUTCOME MEASURES

Mean +/- standard deviation of ulnar short segment conduction time across the elbow, amplitude and motor conduction velocity; grading of ulnar nerve lesions; grading of the course of disease after surgery; and logistic regression and correlation (Spearman's correlation coefficient) for electrophysiologic and clinical parameters.

RESULTS

Sixteen nerves showed focal conduction slowing in patients. No significant correlation between the course of disease and electrophysiologic parameters was seen. For stepwise logistic regression, there was a significant effect between grade of nerve lesion and amplitude, but no significant effect between the course of disease and electrophysiologic parameters.

CONCLUSION

A focal conduction slowing across the elbow after anterior subcutaneous transposition does not correlate with postoperative clinical findings.

Cross-cultural adaptation of the Minnesota Living with Heart Failure Questionnaire for German-speaking patients

We performed a cross-cultural adaptation of the "Minnesota Living with Heart Failure Questionnaire" (LHFQ) for use in German-speaking chronic heart failure patients. The instrument was translated and back translated, pre-tested and reviewed by a committee. The German version was tested in 114 patients with chronic heart failure. Reliability was assessed by a test-retest procedure and Cronbach's coefficient alpha of internal consistency (0.94). To assess concurrent validity, we compared the LHFQ sum scores with the New York Heart Association classification rating (r = 0.53; p < 0.0001), the 6-minute walk (r = -0.39; p < 0.0001), the left ventricular ejection fraction (r = -0.24; p = 0.011) and big-endothelin (r = 0.27; p = 0.004). Construct validity on the LHFQ scores in comparison with the Medical Outcomes Study SF-36 Health Survey (MOS SF-36) was significant (-0.41 to -0.74; all p < 0.0001). The reliability and validity of the German version of the LHFQ was proved; the questionnaire can be recommended for use in future clinical trials.

Flexible two-stage designs: an overview

In this overview we introduce the basic ideas behind a new flexible approach in sequential designs. The different concepts based on two-stage combination tests and conditional error functions are brought together. We sketch the construction of p-values, confidence intervals, and median unbiased estimates. Finally, recursive combination tests are introduced which extend the flexibility to the choice of the number of interim analyses.

Homo-oligomerisation and nuclear localisation of mouse histone deacetylase 1

Reversible histone acetylation changes the chromatin structure and can modulate gene transcription. Mammalian histone deacetylase 1 (HDAC1) is a nuclear protein that belongs to a growing family of evolutionarily conserved enzymes catalysing the removal of acetyl residues from core histones and other proteins. Previously, we have identified murine HDAC1 as a growth factor-inducible protein in murine T-cells. Here, we characterise the molecular function of mouse HDAC1 in more detail. Co-immunoprecipitation experiments with epitope-tagged HDAC1 protein reveal the association with endogenous HDAC1 enzyme. We show that HDAC1 can homo-oligomerise and that this interaction is dependent on the N-terminal HDAC association domain of the protein. Furthermore, the same HDAC1 domain is also necessary for in vitro binding of HDAC2 and HDAC3, association with RbAp48 and for catalytic activity of the enzyme. A lysine-rich sequence within the carboxy terminus of HDAC1 is crucial for nuclear localisation of the enzyme. We identify a C-terminal nuclear localisation domain, which is sufficient for the transport of HDAC1 and of reporter fusion proteins into the nucleus. Alternatively, HDAC1 can be shuttled into the nucleus by association with another HDAC1 molecule via its N-terminal HDAC association domain. Our results define two domains, which are essential for the oligomerisation and nuclear localisation of mouse HDAC1.

Interim analysis and sample size reassessment

This article deals with sample size reassessment for adaptive two-stage designs based on conditional power arguments utilizing the variability observed at the first stage. Fisher's product test for the p-values from the disjoint samples at the two stages is considered in detail for the comparison of the means of two normal populations. We show that stopping rules allowing for the early acceptance of the null hypothesis that are optimal with respect to the average sample size may lead to a severe decrease of the overall power if the sample size is a priori underestimated. This problem can be overcome by choosing designs with low probabilities of early acceptance or by midtrial adaptations of the early acceptance boundary using the variability observed in the first stage. This modified procedure is negligibly anticonservative and preserves the power.

Phenological weighting of ozone exposures in the calculation of critical levels for wheat, bean and plantain

This paper presents phenological weighting factors to be applied to AOT40 (accumulated ozone exposure above a threshold of 40 nl l(-1)) ozone exposure-response relationships for crops at different growth stages. The quantification of such factors represents a step-forward in the derivation of Level II critical levels for ozone. The weighting factors presented are derived from published literature on the sensitivity of wheat (Triticum aestivum), bean (Phaseolus vulgaris) and plantain (Plantago major) to ozone at different growth stages. Weighting functions were calculated using either multiple linear regression or the reciprocal residual mean square (RMS(-1)). The resulting weights were transformed into multiplication factors to be applied to the monthly AOT40 during the 3-month assessment period of critical level exceedance. Interspecific differences were too large to allow for the development of a unified weighting function for the three species considered. For wheat grain yield, the derived multiplication factors varied by almost four-fold (0.40, 1.06, 1.54), while those for bean pod yield varied by only about 25% (0.85, 1.01, 1.14). The available data for plantain were restricted to short-term studies conducted under controlled conditions. These data were not suitable for the derivation of weighting factors comparable to those derived for bean and wheat. Based on known differences in wheat development and phenology across Europe, the need for a geographic differentiation of the time period for the calculation of the critical level exceedances is also discussed and examples provided of the adoption of the derived weightings in the mapping of critical level exceedances. Differences between critical level exceedance maps using weighted and unweighted AOT40 calculations are discussed.

Substrate binding is a prerequisite for stabilisation of mouse thymidine kinase in proliferating fibroblasts

Thymidine kinase (TK) expression in mammalian cells is strictly growth regulated, with high levels of the enzyme present in proliferating cells and low levels in resting cells. We have shown that mouse TK expressed from a constitutive promoter is still subject to this regulation. The drastic decline in TK enzyme levels in resting cells is largely due to a pronounced reduction in the half-life of the protein. Deletion of the 30 C-terminal amino acid residues from TK abrogates growth regulation, rendering the enzyme very stable. Moreover, the substrate thymidine was sufficient to stabilise the labile TK protein in quiescent cells. Here, we report that the ability of TK to bind substrates is essential for both growth-dependent regulation and stabilisation by the substrate. By mutation or elimination of the binding sites for either of the two substrates, ATP and thymidine, we expressed TK proteins lacking enzymatic activity which abolished growth-regulated expression in both cases. Mutant TK proteins impaired in substrate binding were subject to rapid degradation in exponentially growing cells and thymidine was no longer sufficient to inhibit this rapid decay. A C-terminal truncation known to stabilise the TK wild-type protein in resting cells did not affect the rapid turnover of enzymatically inactive TK proteins. Proteasome inhibitors also failed to stabilise these substrate-binding mutants. By cross-linking experiments, we show that TK proteins with mutated substrate-binding sites exist only as monomers, whereas active TK enzyme forms dimers and tetramers. Our data indicate that, In addition to the C terminus intact substrate-binding sites are required for growth-dependent regulation of TK protein stability.

Load-dependence of fatigue related changes in tremor around 10 Hz

OBJECTIVES

The aim of the study was to investigate the effects of different loads on tremor around 10 Hz during fatiguing contractions.

METHODS

Eighteen healthy volunteers performed sustained isometric knee extensions at 30%, 50% and 70% maximum voluntary contraction (MVC). During the fatiguing contractions, mechanical recordings were made with a high-resolution force sensor. Tremor-power was calculated for the 6-20 Hz frequency window as a function of time normalized to endurance time.

RESULTS

Initial tremor power was different between the high and low load tasks. Changes of tremor with contraction time differed between the three tasks, in that tremor of the 30% MVC contraction showed the least decrease throughout the sustained contraction, whilst that of the 50% and 70% MVC showed progressively higher decreases. At failure, all 3 contractions merged to the same tremor level.

CONCLUSION

Load-dependent, fatigue-related 6-20 Hz tremor changes during sustained submaximum voluntary contractions seem mainly the consequence of recruitment of new units and fatigue-related properties of the high threshold motor units of muscles.

Characterization of the translation-dependent step during iron-regulated decay of transferrin receptor mRNA

Iron regulates the stability of the mRNA encoding the transferrin receptor (TfR). When iron is scarce, iron regulatory proteins (IRPs) stabilize TfR mRNA by binding to the 3'-untranslated region. High levels of iron induce degradation of TfR mRNA; the translation inhibitor cycloheximide prevents this. To distinguish between cotranslational mRNA decay and a trans effect of translation inhibitors, we designed a reporter system exploiting the properties of the selectable marker gene thymidine kinase (TK). The 3'-untranslated region of human transferrin receptor, which contains all elements necessary for iron-dependent regulation of mRNA stability, was fused to the TK cDNA. In stably transfected mouse fibroblasts, the expression of the reporter gene was perfectly regulated by iron. Introduction of stop codons in the TK coding sequence or insertion of stable stem-loop structures in the leader sequence did not affect on the iron-dependent regulation of the reporter mRNA. This implies that global translation inhibitors stabilize TfR mRNA in trans. Cycloheximide prevented the destabilization of TfR mRNA only in the presence of active IRPs. Inhibition of IRP inactivation by cycloheximide or by the specific proteasome inhibitor MG132 correlated with the stabilization of TfR mRNA. These observations suggest that inhibition of translation by cycloheximide interferes with the rate-limiting step of iron-induced TfR mRNA decay in a trans-acting mechanism by blocking IRP inactivation.

Win-stay, lose-shift strategies for repeated games-memory length, aspiration levels and noise

Win-stay, lose-shift, the principle to retain a successful action is a simple and general learning rule that can be applied to all types of repeated decision problems. In this paper I consider win-stay, lose-shift strategies with diverse memory sizes and strategies that adapt their aspiration levels, i.e. the payoff level considered as "success". I study their evolution for the Prisoner's Dilemma, as well as in a rapidly changing environment, where a randomly selected game is assigned to the players. For win-stay, lose-shift strategies with memory one the average payoffs are computed and their evolutionary stability is discussed. Using computer simulations I show that the win-stay, lose-shift strategies with longer memory are very successful both for the Prisoner's Dilemma, where cooperation dominates even for high noise levels, and the randomly assigned games, where the players achieve nearly the expected Pareto optimal payoffs. I discuss the impact of noise and show that the memory length of the players increases with the noise level. These results indicate that the win-stay, lose-shift principle is a very successful strategy in repeated games with noise.