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P. Connes1 S. Racinais1 F. Sara1,2 L. Marlin1 C. Hertogh1 C. Saint-Martin3 M. Etienne-Julan3 O. Hue1

Does the Pattern of Repeated Sprint Ability Differ between Sickle Cell Trait Carriers and Healthy Subjects?

Sickle cell trait (SCT) is a genetic abnormality affecting the synthesis of normal haemoglobin [Hb] and is the heterozygous form of sickle cell anaemia. The aim of the present study was to compare the ability to repeat maximal cycling sprints (RSA; repeated sprint ability) between SCT carriers (SCT group, n = 7) and a control group with normal haemoglobin [Hb] (n = 7). The two groups performed a 10-s maximal cycling sprint in order to determine the peak power output (Ppeak10). They then performed an RSA test that consisted of five 6-s maximal cycling sprints interspersed with 24 s of passive recovery. For each sprint, the peak power output (Ppeak6) and the work over the 6-s (W6) were calculated. The sum of each W6 developed during the test was considered

Introduction Sickle cell anaemia is a genetic abnormality of haemoglobin [Hb] caused by the substitution of a single amino acid, valine, for glutamic acid, causing the mutation of normal haemoglobin (HbA) into abnormal haemoglobin (HbS). Sickle cell trait (SCT) is the heterozygous form of sickle cell anaemia (HbS < 50 %) and is particularly frequent among people of African descent. The prevalence of SCT is around 20 ± 40 % in some areas of Sub-Saharan Africa and reaches 10% in several European metropolises [42]. In usual physiological conditions (normoxia and resting conditions), SCT is not associated with clinical symptoms. However,

to be the total work (Wtot). The decrements over the repeated sprints for Ppeak6 (P6dec) and W6 (W6dec) were also determined. We found no difference in Ppeak10, Wtot and W6dec between the two groups. However, the drop in Ppeak6 and W6 during the RSA test appeared earlier in the SCT group and the decrease in Ppeak6 over the RSA test was greater in the SCT group than in the control group (p < 0.05). In conclusion, we found that: 1) maximal anaerobic performance determined during a single sprint was not altered by SCT, but 2) repeated sprint ability was different in SCT carriers compared with sportsmen with normal Hb.

Physiology & Biochemistry

Abstract

Key words Maximal intermittent cycling exercise ´ haemoglobinopathy ´ exercise limitation ´ haemoglobin S

several studies have reported cases of health complications in SCT carriers when they were confronted with stressful physiological conditions, like hypoxia or strenuous exercise. Studies investigating aerobic exercise performance and exercise tolerance in sportsmen carrying SCT have reported surprising results. It is well known that red blood cells (RBCs) from SCT carriers have a diminished affinity for oxygen [30], which might lead to the assumption that aerobic performance is limited in these individuals. However, when SCT carriers and control subjects are closely matched in terms of physical activity level, no difference in maximal oxygen consumption (VÇO2max) or maximal

Affiliation Laboratoire ACTES (EA 3596), UniversitØ des Antilles et de la Guyane, Campus de Fouillole, Pointe-à-Pitre, Guadeloupe 2 UMR S 458 Inserm, UniversitØ des Antilles et de la Guyane, CHU de Pointe-à-Pitre, Guadeloupe 3 Centre IntØgrØ de la DrØpanocytose ªGuy MØraultº, CHU de Pointe-à-Pitre, Guadeloupe 1

Correspondence Philippe Connes, PhD ´ Laboratoire ACTES (EA 3596), UniversitØ des Antilles et de la Guyane ´ Campus de Fouillole ´ 97159 Pointe-à-Pitre ´ Guadeloupe (French West Indies) ´ E-mail: [email protected] Accepted after revision: December 5, 2005 Bibliography Int J Sports Med 2006; 27: 1 ± 6  Georg Thieme Verlag KG ´ Stuttgart ´ New York ´ DOI 10.1055/s-2006-923834 ´ ISSN 0172-4622

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Table 1 Subject characteristics and haematological parameters in the SCT and control groups Mass (kg)

BMI (kg ´ m±2)

Age (y)

Height (cm)

SCT (n = 7)

24.3  4.0

181.6  1.0

72.1  2.8

21.9  0.9

Control (n = 7)

19.4  0.6

182.4  2.3

73.4  3.0

22.0  0.8

TV (h ´ wk±1)

[Hb] (g ´ dL±1)

HbS (%)

9.0  2.0

15.2  0.6

39.3  0.7

11.0  2.0

15.3  0.3

±

Values are means  SEM. BMI: body mass index; TV: training volume; [Hb]: haemoglobin concentration; HbS: haemoglobin S. No significant difference between the two groups

Physiology & Biochemistry 2

aerobic power output (MAP) has been observed between these two populations [5, 21, 33, 37 ± 39]. Thus, it seems that SCT does not limit maximal aerobic exercise performance. Nevertheless, when SCT carriers performed prolonged submaximal exercise to assess exercise tolerance, endurance and aerobic capacity, they had lower aerobic capacity than healthy subjects [14]. For heavy prolonged exercise, VÇO2 may reach a delayed steady state that is higher than the VÇO2 requirement estimated by extrapolating the relationship between VÇO2 and work rate during incremental exercise [3, 47], or it may rise continuously until VÇO2max is attained and/or exercise is terminated [35]. This socalled slow component has been suggested to be an important determinant of exercise tolerance and aerobic capacity [46]. When the magnitude of the slow component is high, the tolerance of exercise is decreased. We recently investigated the kinetics of oxygen uptake during heavy submaximal exercise in SCT carriers (with or without alpha-thalassaemia) and we observed a higher magnitude of the slow component of VÇO2 in SCT carriers in comparison with a control group [14]. These data indicate that SCT carriers are prone to limited exercise tolerance and lower aerobic capacity than control subjects, although they have the same MAP or VÇO2max as controls. These results are supported by previous epidemiological studies that suggested impaired exercise tolerance in SCT carriers during submaximal aerobic exercise [26, 27]. Many team sports require the participants to repeatedly produce maximal or near-maximal sprints of short duration (< 6 s) interspersed with brief recovery periods, over an extended period of time (60 ± 90 min) [10]. The ability to recover quickly in order to reproduce subsequent sprints is an important fitness requirement that has been termed repeated sprint ability (RSA) [10]. Most of the energy required to resynthesise ATP for a single 6-s maximal sprint is provided by phosphocreatine (PCr) degradation and anaerobic glycolysis [20]. Anaerobic glycolysis leads to the intracellular accumulation of hydrogen ions (H+), which is considered to contribute to muscular fatigue [41], although a recent study also supports the concept that exercise-induced lactic acidosis might counterbalance the fatigue arising from a rundown of Na+ and K+ gradients at the muscular level [23]. In addition, Westerblad et al. [45] demonstrated that the direct inhibition of force production by acidification is not a major factor in muscular fatigue at physiological temperatures. Nevertheless, during repeated sprint exercise, the recovery of power output between the sprints has been suggested to depend on the ability to resynthesise PCr [11] and to buffer acidosis [9], both of which primarily depend on aerobic physical fitness and the muscle ox-

idative capacity [9, 31]. Furthermore, RSA and aerobic physical fitness are usually well correlated [9,15, 32]. As SCT carriers have an impaired aerobic capacity and exercise tolerance [14], despite normal VÇO2max or MAP, and because RSA depends on aerobic capacity, SCT carriers should have a lower RSA than subjects with normal Hb. The aim of this study was to compare RSA between sportsmen with SCT and sportsmen without SCT.

Materials and Methods Subjects Fourteen sportsmen participated in the study after giving informed written consent. Seven were SCT carriers and composed the SCT group and seven had normal Hb and composed the control group. The two groups were matched in terms of sports specialities, training volume, height and weight (Table 1). All had been recruited from the Faculty of Sport of the University of the French West Indies and Guyana. None were specifically trained in cycling to avoid any sport-specific adaptations. The sports specialities were: volleyball (2 SCT carriers and 2 controls), handball (1 SCT carrier and 1 control), sprinting (1 SCT carrier and 1 control), basketball (1 SCT carrier and 1 control), running/soccer (1 SCT carrier and 1 control) and martial arts (1 SCT carrier and 1 control). Each subject underwent clinical examinations before admission to the study. Subjects with anaemia and/or alphathalassaemia were excluded from the study, as were those who were unwilling to follow the study protocol. Haematological data are listed in Table 1. Experimental protocol This study was approved by the local Ethics Committee of the University of the French West Indies and Guyana and was in accordance with the guidelines set by the Declaration of Helsinki for human subjects. The subjects came to the laboratory and, after a resting period, blood was collected to analyse haematological parameters, detect the presence of SCT and determine blood lactate concentration ([La]b). The subjects were asked to avoid training the day before the exercise test. The subjects then performed an exercise test to evaluate RSA, following the recommendations of Bishop et al. [10]. Blood was sampled at the end of the test to analyse [La]b. Exercise test As recommended by Bishop et al. [9,10], the exercise test started with a 5-min warm-up that consisted of cycling at 84 W on a standard friction-loaded cycle ergometer (Monark 824E, Stock-

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Sickle cell trait detection and haematological measurements Blood samples were drawn from the antecubital vein at rest in EDTA tubes. A part of the sampled blood was screened by isoelectric focusing to test for the haemoglobin type. The results were confirmed by citrate agar electrophoresis. The various haemoglobins were isolated and quantified by high performance liquid chromatography (HPLC, haemoglobin Testing System Bio-RAD, Bio-RAD Laboratories, GmbH, München, Germany). The presence of HbS was confirmed by a solubility test. Positive test results for SCT were determined by the presence of HbA1 at a value of more than 50 %, HbS at a value of less than 50% and a normal percentage of HbA2. The other part of the sampled blood was used to de-

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termine Hb concentration ([Hb]) and mean cell volume (MCV, data not shown). MCV, HbS percentage and [Hb] were used for the diagnosis of anaemia and indirect detection of alpha-thalassaemia, which usually result in haematological modifications [17]. Blood lactate measurements Fingertip arterialised blood micro-samples were taken with a lancet (B ± D micro-Fine +, Becton Dickinson, NJ, USA) for lactate analysis at rest and immediately at the end of the fifth sprint of the RSA test. The [La]b was determined with an instrument for the resolution of lactate (Accusport, Boeringer Mannheim, Mannheim, Germany) and a testing strip (BM-Lactate, Roche Diagnostics, Mannheim, Germany). This instrument has been demonstrated to be valid and reliable [8]. Statistical analysis Values are presented as mean  SEM. Subject characteristics, haematological data, W10, Ppeak10, P6dec, W6dec, and Wtot were compared between the two groups using Students t-test. The [La]b were compared between the two groups and between rest and the end of the RSA test using a two-way analysis of variance (ANOVA) with repeated measures. The time courses of Ppeak6 and W6 during the test were compared between the two groups using a two-way ANOVA with repeated measures. Pair-wise contrasts were used when necessary to determine where the significant differences had occurred. Statistical significance was established at p < 0.05.

Physiology & Biochemistry

holm, Sweden) with a moment of inertia of 0.42 kg ´ m±2 for the flywheel (calculated with Lakomys method [25]). The warm-up was followed by a 3-min resting period in the seated position. Then, each subject performed a 10-s maximal sprint on the cycle ergometer against a braking resistive force applied on the flywheel set at 50 g ´ kg±1 body mass. A load between 50 and 75 g ´ kg±1 of body mass is relevant for the determination of maximal power output in young adults [16]. The subjects were instructed to accelerate as fast as possible and to maintain this effort for the 10 s. A disk and a photoelectric cell were fixed on the flywheel and the frame, respectively, in order to obtain 12 speed readings per flywheel revolution. The force was calculated by summing the braking resistive force and the inertial force. The latter was calculated from the acceleration using the method of Martin et al. [29]. The power output was calculated by multiplication of the velocity by the force per pedal revolution. The data were collected by an acquisition card (DAQPad 6020E, National Instruments, n(city), TX, USA) and analysed by software using a LabVIEW interface (LabVIEW, National Instruments, n(city) TX, USA). Then, the peak power output (Ppeak10) and the work (W10) were determined. The work was also determined in the first 6 s of the 10-s sprint (data not shown) and this was used as a criterion score for the subsequent test. Upon completion of the 10-s test, subjects rested for 5 min before performing supramaximal intermittent exercises that consisted of five 6-s maximal cycling sprints beginning every 30 s (RSA test). During the first sprint, subjects were required to achieve at least 95% of their criterion score. If this was not achieved, the subject rested for a further 5 min and then recommenced the RSA test (note that this scenario never occurred). During the 24-s recovery between sprints, subjects rested completely in the seated position. A few seconds before beginning each sprint, subjects were asked to assume the starting position (always the same leg ready to push on the pedal) and await the start signal. Strong verbal encouragement was given to each subject during all sprints. This test has been reported to be both valid [10] and reliable [19] for assessing RSA. Several parameters were calculated, in line with previous studies investigating RSA [7,10, 20]. For each sprint, the peak power output (Ppeak6) and the work over the 6 s (W6) were calculated. The sum of each W6 developed during the RSA test corresponded to the total work (Wtot). The Ppeak6 (P6dec) and W6 (W6dec) decrements over the repeated sprints were also calculated, based on the method of FitzSimons et al. [19]. The data for power and work were only expressed in absolute values (watts and kJ, respectively) because the subjects of the two groups had been matched for anthropometric characteristics. The laboratory was air-conditioned (22 ± 23 8C) during all testing.

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Results Subjects characteristics and haematological parameters As shown in Table 1, there was no significant difference between the two groups for age, mass, height, body mass index (BMI), training volume or [Hb]. The 10-s sprint test Ppeak10 (954.9  43.2 W and 853.0  55.1 W in the SCT and control groups, respectively) and W10 (6.9  0.2 kJ and 6.8  0.3 kJ in the SCT and control groups, respectively) were not significantly different between the two groups. The RSA test Figs. 1 and 2 show the time courses for Ppeak6 and W6, which decreased during the RSA test in both groups (p < 0.05). Ppeak6 and W6 did not significantly differ between groups, whatever the sprint considered. The time course for Ppeak6 was significantly different between the two groups (p < 0.05). Ppeak6 significantly decreased in the SCT group between sprint 1 and sprint 2 and the following sprints, whereas it was unchanged in the control group until the fourth sprint. Moreover, Ppeak6 significantly decreased between sprint 2 and sprint 3 in the SCT group, whereas it was unchanged in the control group. Then, Ppeak6 significantly decreased in both groups between sprints 3 and 4 and decreased only in the SCT group between sprint 4 and the last sprint. The time course for W6 also differed between the two groups (p < 0.05). W6 significantly decreased between sprint 1 and sprint 2 in the SCT group, whereas it remained unchanged in the other group until the third sprint. Then, W6 significantly decreased in both groups between each successive sprint.

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Fig. 1 Time course of the peak power output (Ppeak6) determined for each sprint during the RSA test in the SCT (black line) and the control (dotted line) groups. ² Significant difference in the SCT group (p < 0.05), * Significant difference in the control group (p < 0.05).

Fig. 2 Time course of the work (W6) determined for each sprint during the RSA test in the SCT (black line) and the control (dotted line) groups. ² Significant difference in the SCT group (p < 0.05), * Significant difference in the control group (p < 0.05).

The sum of each W6 (i.e., Wtot) did not differ between the two groups. Wtot was 20.6  0.3 kJ and 21.3  1.3 kJ in the SCT group and control group, respectively. P6dec was significantly higher in the SCT group compared with the control group (14.6  2.4 % and 8.1  1.6 % in the SCT and control groups, respectively; p < 0.05) and W6dec was not statistically different between the two groups (15.5  2.5 % and 11.1  2.0% in the SCT and control groups, respectively).

The present study also examined the responses to exercise consisting of five sprints of 6 s, interspersed with 24 s of passive recovery. The peak anaerobic power (Ppeak6) and the work (W6) decreased over the RSA test in both groups, as previously demonstrated [9], but the decrease began earlier in the SCT group. Moreover, the decrease in Ppeak6 between the first and the last sprint was less marked in the control group. These results indicate that the SCT carriers showed a different ability to recover and reproduce performance in subsequent sprints than the subjects with normal Hb, even though the total amount of work (Wtot) performed during the RSA test was not significantly different between the two groups.

The [La]b significantly increased in the SCT group (2.0  0.2 mM and 12.0  1.4 mM at rest and at the end of the RSA test, respectively) and in the control group (1.8  0.2 mM and 13.4  2.5 mM at rest and at the end of the RSA test, respectively) in response to exercise (p < 0.05), but no difference was observed between the two groups.

Discussion The present study showed that: 1. maximal performance determined during a single sprint was not altered by the presence of SCT and 2. repeated sprint performance was different in SCT carriers compared with subjects with normal Hb. Although the total amount of work (Wtot) performed during the RSA test was not different between the two groups, the drop in Ppeak6 and W6 appeared earlier in the SCT group and the decrease in Ppeak6 over the RSA test was greater in the SCT group than in the control group. As already observed in an earlier study by BilØ et al. [4], we found no difference in the anaerobic exercise performance between SCT carriers and subjects with normal Hb. BilØ et al. [4] used the force-velocity test to determine the peak anaerobic power [44], even though this classic test has been shown to underestimate it [12, 29]. In the present study, peak anaerobic power was determined from a single, 10-s, cycling sprint to allow the calculation of power integrating flywheel inertia [25]. The present study also reported that the work developed during the 10-s sprint test (i.e., W10) did not differ between the two groups. Together, these results reinforce the conclusion of BilØ et al. [4], who observed no impairment of anaerobic exercise performance in SCT carriers.

Gozal et al. [21] have previously examined the exercise performance of SCT carriers and control subjects during a series of four, 2 min, maximal cycle exercise tests separated by 20 min of passive or active recovery, but they failed to observe differences. They concluded that the presence of HbAS in trained subjects does not affect repeated maximal exercise performance requiring predominantly anaerobic contribution, but also some aerobic contribution, and that the integrity of the recovery process is preserved in SCT carriers [21]. The lack of difference in their study might be explained by the long recovery period, which allowed good recovery processes despite the presence of SCT in some subjects. Balsom et al. [2] reported that recovery interval affects recovery of RSA. The present study used an exercise protocol that mimics the often encountered type of exercise in team sports that requires participants to repeatedly produce maximal or near-maximal sprints of short duration (< 6 s) [10]. Indeed, we assume that the protocol used in our study was more adapted to assess the ability to repeat maximal sprint performance than the protocol used by Gozal et al. [21]. Gaitanos et al. [20] reported that a single, 6-s maximal sprint, led to a 57% decrease in PCr and a 13% reduction in ATP. The majority of the energy required to resynthesise ATP for such a sprint is provided by PCr degradation and the anaerobic pathway (glycogenolysis), which leads to lactate production [20]. The intracellular accumulation of H+ has been suggested to contribute to the appearance of muscular fatigue [18, 41], although some evidence suggests that intracellular acidosis of muscle may also have protective effects during muscular fatigue [1, 23]. During repeated

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Although Bishop et al. [9,10] and Racinais et al. [36] observed a decrease in performance immediately after the first sprint of the RSA test, the results from the studies of Gaitanos et al. [20] and Billaut et al. [7] suggested that a 30-s rest period is enough to restore the peak power output for at least three or four 6-s sprints or for two 8-s sprints. This was the case for our control subjects, who maintained their Ppeak6 from the first sprint to the third sprint, although the resting period between each sprint was 24 s instead of the 30 s in the studies from Billaut et al. [7] and Gaitanos et al. [20]. This might raise doubt about the intensity level at which the control subjects performed the first sprint. During the first sprint, the subjects were required to achieve at least 95% of their criterion score. If this was not achieved, the subject rested for a further 5 min and then recommenced the RSA test (in fact, this scenario never occurred). Thus, one may assume that the first sprint of the RSA test was performed maximally in the two groups. In contrast, the SCT group failed to maintain the Ppeak6 reached during the first sprint and Ppeak6 declined during the second and following sprints. It has been reported that the athletes with the highest glycolytic rate during the first sprint have the greatest decrement in performance over ten, 6-s sprints [20]. We did not measure [La]b at the end of the first sprint but, by measuring [La]b at rest and immediately at the end of the RSA test, we observed an increase in both groups without a significant difference between them. This lack of difference in [La]b between the two groups could suggest that the participation of the glycolytic pathway to RSA test performance was of the same magnitude in the two populations. However, Sara et al. [37] recently demonstrated that red blood cells from SCT carriers had a faster lactate transport activity via the monocarboxylate transporter MCT-1, and Connes et al. [13] observed that a high level of RBC lactate transport activity might compensate the rise in blood lactate concentration induced by exercise. The lack of difference in [La]b between the two groups could thus simply be the result of greater MCT-1 activity in the SCT carriers. Further studies are required to determine whether the glycolytic contribution is higher in SCT carriers, because the use of [La]b alone is insufficient to draw conclusions. Although the behaviour of the two groups during the RSA test was different, the general parameters relative to RSA (i.e., Ppeak6, and W6, Wtot, and W6dec) were not (except for P6dec). This indicates that the repeated sprint ability of the SCT carriers was not truly impaired. Our results instead suggest that the two groups per-

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formed the RSA test with different metabolic contributions and that the pattern of RSA thus differed between the two groups. Although the Ppeak6 measured during the first sprint of the RSA test was not different between the two groups, the mean value was slightly higher in the SCT group (982  46 W and 915  54 W in the SCT carriers and control group, respectively). As discussed above, the early decrease in W6 and Ppeak6 found in the SCT carriers could simply be an anaerobic advantage to compensate for the low aerobic capacity due to the presence of HbS [4, 24]. Besides, it has been hypothesised that anaerobic metabolism, and particularly the alactic anaerobic metabolism, could be enhanced by the presence of HbS [6, 24, 28]. Several authors have suggested a significant shift toward aerobic metabolism during intermittent exercise [20, 22], and RSA and aerobic physical fitness are usually well correlated [9,15, 32]. Although HbS has an impaired O2 affinity [30], it has been demonstrated that when SCT carriers and subjects with normal Hb are well matched for physical activity, they have the same VÇO2max [5, 21, 38, 39]. Thirteen of the 14 subjects in the present study participated in another study only one month later. In this second study, we demonstrated that the VÇO2max of the SCT carriers was not statistically different from that of the control group (see [37]). Although VÇO2max is the gold standard for assessing aerobic physical fitness, other indices are of interest [46, 47]. We recently observed slower oxygen uptake kinetics at the onset of submaximal exercise and a higher magnitude of the slow component of VÇO2 in SCT carriers in comparison with a control group, although the two groups had the same maximal aerobic power output [14]. The finding of a higher slow component in SCT carriers was possibly due to the loss in O2 availability to muscles or additional fibre recruitment. These data indicate that, although maximal aerobic physical fitness does not differ between SCT carriers and control sportsmen [5,14, 21, 37 ± 39], SCT carriers are prone to limited exercise tolerance and lower aerobic capacity, particularly during prolonged and submaximal exercise [14]. An impaired aerobic capacity thus might explain why our SCT carriers were unable to maintain their Ppeak6 and W6 over the RSA test. In conclusion, this study demonstrated that the repeated sprint ability (assessed with the RSA test) is not impaired in SCT carriers in comparison with control subjects but that the pattern of repeated maximal sprint performance differs between the two groups. We observed that the SCT carriers were not able to maintain the same maximal performance during successive sprints interspersed by short periods of recovery. However, the overall work performed by the SCT carriers during the RSA test was not different from that of the healthy subjects. Although the pattern of RSA differed between the two groups, this study suggests that SCT carriers are able to perform normally in team sports requiring this type of physical fitness. Further research is needed to clarify why the pattern of RSA differed between the SCT carriers and the subjects with normal Hb.

Acknowledgments The authors especially thank MD Hardy-Dessources for her pertinent remarks and all the athletes who participated in the present study.

Connes P et al. Intermittent Exercise in Sickle Cell Trait ¼ Int J Sports Med 2006; 27: 1 ± 6

Physiology & Biochemistry

sprint exercise, the recovery of power output between the sprints has been associated with the ability to resynthesise PCr [11] and to buffer acidosis [9], and with aerobic physical fitness and the muscle oxidative capacity [9, 31, 43]. No study has yet investigated the muscle and blood buffer capacities in SCT carriers. However, because lactic acidosis is known to promote red blood cell dehydration and the sickling process in SCT carriers [40], one may hypothesise that some (certainly few) of the red blood cells containing HbS become sickled and lose their deformability during the RSA test. In addition, SCT carriers usually have decreased RBC deformability at rest and in response to exercise compared with healthy subjects [33], which may lead to decreased oxygen availability to tissues [34]. This might explain why the ability to perform exercise consisting of repeated maximal sprints was different in this population compared with the control group.

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Satzbetrieb Ziegler + Müller Verlag Thieme/Hentze Datum 31.01.2006

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