Swimming Program Research Articles

Swimming Programs for Infants and Toddlers

Infant and toddler aquatic programs provide an opportunity to introduce young children to the joy and risks of being in or around water. Generally, children are not developmentally ready for swimming lessons until after their fourth birthday. Aquatic programs for infants and toddlers have not been shown to decrease the risk of drowning, and parents should not feel secure that their child is safe in water or safe from drowning after participating in such programs. Young children should receive constant, close supervision by an adult while in and around water.

Effects of swimming training on three superoxide dismutase isoenzymes in mouse tissues.

The purpose of the present study was to investigate the effects of swimming training on the changes in three superoxide dismutase (SOD) isoenzymes in mice. The trained mice underwent a 6-wk swimming program (1 h/day, 5 days/wk) in water at 35-36 degrees C. Immunoreactive extracellular SOD (EC-SOD), copper- and zinc-containing SOD (CuZn-SOD), and manganese-containing SOD (Mn-SOD) contents and their mRNA abundance were determined in serum, heart, lung, liver, kidney, and gastrocnemius muscle. EC-SOD content in liver and kidney was significantly increased with training. After training, CuZn-SOD content rose significantly only in kidney but decreased significantly in heart, lung, and liver. Mn-SOD content showed a significant increase in lung, kidney, and skeletal muscle but a significant decrease in liver. In most tissues, however, the changes in SOD isoenzyme contents were not concomitant with those in their mRNA levels. The results obtained thus suggest that, except for kidney, the responses in mouse tissues of three SOD isoenzymes (protein levels and mRNA abundance) to swimming training are different and that kidney may be one of the most sensitive organs to adapt to oxidative stress during physical training, although the mechanism remains vague.

REDUCING NONPOINT SOURCE POLLUTANT LOADS TO TAMPA BAY USING CHEMICAL TREATMENT

REDUCING NONPOINT SOURCE POLLUTANT LOADS TO TAMPA BAY USING CHEMICAL TREATMENTTampa Bay, located adjacent to the City of Tampa on the west coast of Florida, is an estuary of national significance included in the National Estuary Program. One of the primary goals of the Southwest Florida Water Management District's (SWFWMD) SWIM Program is to work cooperatively with local governments to construct retrofit projects to reduce pollutant loads from nonpoint sources to Tampa Bay....Author(s)Jeffrey L. HerrHarvey H. HarperSourceProceedings of the Water Environment FederationSubjectSession 9 - Surface Water Quality and Ecology Symposium I: Wet Weather - Stormwater IssuesDocument typeConference PaperPublisherWater Environment FederationPrint publication date Jan, 2000ISSN1938-6478SICI1938-6478(20000101)2000:14L.633;1-DOI10.2175/193864700784607479Volume / Issue2000 / 14Content sourceWEFTECFirst / last page(s)633 - 653Copyright2000Word count696

Effects of a movement and swimming program on water orientation skills and self-concept of kindergarten children with cerebral palsy.

In this study the effect of an experimental movement and swimming program of six months on motor function in the water measured by means of the Water Orientation Score and self-perception measured by means of the Martinek-Zaichkowsky Self-concept Scale was investigated. 23 children participated in the program, completing both tests prior to and after the intervention. An age-, sex-, and disability-matched control group of 23 children completed only the Self-concept Scale at pre- and posttest. Analysis indicated a significant improvement in Water Orientation Score of children in the trained group, but no effect on scores of the Self-concept Scale.

Assessing Change in Swimming Skills Using the Hierarchical Linear Model

The purpose of this study was to apply the hierarchical linear model (HLM) to assess children's changes in swimming skills. A total of 20 children from a preschool swimming program, 10 boys and 10 girls, were tested by a swimming testing battery at the beginning and at the 4th, 9th, 12th, and 16th months of the program. The swimming-skill scores were analyzed using the HLM at two levels. At Level 1, children's skill levels were defined as a function of a linear growth curve. At Level 2, mean growth trajectory, individual variation in growth trajectory, and reliability of initial status and growth rates were examined first. Then, to determine the impact of the between-participant variables of gender, entry age, and previous swimming experience, parameters (initial status and growth rates) of modeling at Level 1 were defined as a function of these between-participant variables. Entry age was the only determinant found to significantly affect initial status and change in swimming kicking skill. There were, however, other unidentified factors that also contributed to the change in swimming skills. The HLM provided a useful and flexible means for studying changes in motor skills.

EFFECT OF ONE-YEAR SWIMMING PROGRAM ON PLASMA LIPOPROTEIN CONCENTRATIONS IN POSTMENOPAUSAL WOMEN 739

Swimming has been accepted as a useful exercise to increase of aerobic capacity in middle-aged and elderly individuals. However, little information is available to evaluate the effect of swimming program on lipid metabolism for postmenopausal women. Therefore, this study was conducted to test effect of one-year swimming program on plasma lipid and lipoprotein concentrations in postmenopausal women. Trained (n=8, frequency @ 2 days/week, age 60±6 yr. BMI 22.5±1.8 kg/m2. PeakVo2 29.7±2.5 ml/kg/min at initial.mean±SD) and control women (n=11, 59±4 yr. 22.9±3.2 kg/m2, 28.4±3.4 ml/kg/min) were tested after one year. No changes in BMI were observed in the two groups. At the one-year follow-up, trained women increased PeakVo2 (31.2±2.3 ml/kg/min, P<0.05), whereas no change was observed in untrained group (27.3±4.1 ml/kg/min, P>0.10). No changes were occurred in plasma triglycerides after the follow-up (Trained:90±32→84±24 mg/dl. P>0.10. Untrained: 79±54→86±51 mg/dl. P>0.10). Plasma total cholesterol (Ch) in untrained group significantly increased from 215±36 to 231±37 mg/dl (P<0.05) after the follow-up, whereas no change was observed in trained group (231±37→230±41 mg/dl. P>0.10). HDL-Ch in the trained group was not changed after the program(60±14→57±12 mg/dl. P>0.10). Although LDL-Ch in untrained group remarkably increased at the follow-up(126±29→144±33 mg/dl. P<0.05), no change was observed in trained (153±31→156±36 mg/dl. P>0.10). These results suggest that swimming program performed @ 2 days a week protects the change of plasma lipoprotein profile for postmenopausal women.

EFFECTS OF SWIMMING TRAINING ON ANTIOXIDANT ENZYMES IN KIDNEY OF YOUNG AND OLD MICE 1310

The current study was undertaken to investigate the effect of swimming training on the antioxidant enzyme system in kindney of young and old mice. Both young and old mice (n = 24), aged 2 and 26 months old, respectively, were divided into the sedentary and swimming-trained groups. The trained mice underwent a 6-w swimming program (1 h/day, 5 days/w) in water at 35 °C. Cu,Zn-superoxide dismutase (Cu.Zn-SOD) activity was significantly decreased with aging but was not influenced by swimming training, such changes being similar to those noted for catalase activity rather than for glutathione peroxidase activity. After swimming training Mn-SOD activity increased significantly only in old mice but was unaffected by aging. Although neither aging nor swimming training had overt effect on the expression of Cu,Zn-SOD mRNA, the immunoreactive Cu,Zn-SOD content in young mice decreased significantly after the training. Meanwhile, as shown in thefigure, the Mn-SOD mRNA expression in old mice was reduced by half after swimming training, accompanied by a significant decrease in its immunoreactive content; unexpectedly, however, the Mn-SOD content in young mice did not parallel its mRNA expression. These findings suggest that the antioxidant enzyme system in mouse kidney trends to be downregulated with aging, and that swimming training fails to attenuate such reduced levels of the antioxidant enzymes.

EFFECTS OF TRAINING AND POST-EXERCISE DIET ON MUSCLE GLYCOGEN SUPERCOMPENSATION IN RATS 1293

One goal of this study was to test the hypothesis that the rate and extent of glycogen supercompensation are higher in endurance exercise trained, than in untrained, muscle. A second purpose was to test the possibility that the lower glycogen levels in rat than in human muscles are due to limited substrate availability rather than to intrinsic differences in the ability to store glycogen. Rats were trained using a 5wk swimming program in which duration of swimming was gradually increased to 6h/day over 3wk. Glycogen repletion was measured in trained and untrained rats after a glycogen depleting bout of exercise. There were remarkable differences in the rates of glycogen accumulation, and in the glycogen concentrations attained, between the two groups. In rats fed chow, epitrochlearis muscle glycogen concentration 24h post-exercise was 55±5 μmol/g muscle in the untrained, and 140±12 μmol/g in the trained, group. In rats given chow plus 5% sucrose in their drinking water, epitrochlearis glycogen was 73±5μmol/g in the untrained, and 176 μmol/g in the trained group 24h post-exercise. These differences could not be explained by a training effect on glycogen synthase. The training induced -50% increases in muscle GLUT4 glucose transporter protein and in hexokinase activity in epitrochlearis muscles. We conclude that training results in increases in both the rate and magnitude of muscle glycogen supercompensation, and that substrate availability limits glycogen repletion in rats fed a chow diet.

Contractile properties of clenbuterol-treated mdx muscle are enhanced by low-intensity swimming.

The beta 2-agonist clenbuterol has potent anabolic properties in normal and denervated muscle and, as such, may be of use in muscle wasting diseases such as muscular dystrophy. However, potential side effects such as the transformation of the fiber type pool toward increased proportions of fast-twitch fibers must be balanced with the beneficial anabolic properties. In the present report, we clearly show that extensor digitorum longus and soleus muscles from dystrophic mdx mice exposed to a combination of clenbuterol and low-intensity endurance swimming exercise did not undergo the slow- fast-twitch fiber transformations caused by clenbuterol administration alone, yet increases in the force-generating capacity of the soleus (30-40%) that resulted from the clenbuterol treatment were maintained after the swimming program. The increased sensitivity of dystrophin-deficient dystrophic muscle to clenbuterol and low-intensity exercise that is evident in this study may have therapeutic implications in the treatment of muscle wasting diseases.

Effect of endurance exercise training on muscle glycogen supercompensation in rats.

The purpose of this study was to test the hypothesis that the rate and extent of glycogen supercompensation in skeletal muscle are increased by endurance exercise training. Rats were trained by using a 5-wk-long swimming program in which the duration of swimming was gradually increased to 6 h/day over 3 wk and then maintained at 6 h/day for an additional 2 wk. Glycogen repletion was measured in trained and untrained rats after a glycogen-depleting bout of exercise. The rats were given a rodent chow diet plus 5% sucrose in their drinking water and libitum during the recovery period. There were remarkable differences in both the rates of glycogen accumulation and the glycogen concentrations attained in the two groups. The concentration of glycogen in epitrochlearis muscle averaged 13.1 +/- 0.9 mg/g wet wt in the untrained group and 31.7 +/- 2.7 mg/g in the trained group (P < 0.001) 24 h after the exercise. This difference could not be explained by a training effect on glycogen synthase. The training induced approximately 50% increases in muscle GLUT-4 glucose transporter protein and in hexokinase activity in epitrochlearis muscles. We conclude that endurance exercise training results in increases in both the rate and magnitude of muscle glycogen supercompensation in rats.

Angiotensin receptor 1 blockade does not prevent physiological cardiac hypertrophy in the adult rat

The renin-angiotensin system has been implicated in the hypertrophic adaptation of the heart to exogenous pathological loads, such as hypertension and aortic stenosis; however, the role of this hormonal system in the cardiac adaptations to physiological loads, such as chronic exercise conditioning, has not been established. We therefore studied the effect of angiotensin receptor 1 (AT1) blockade on the chronic cardiac responses of rats subjected to an 8-wk swimming program. Compared with matched sedentary controls, untreated swimmers increased their left ventricular weights by 13%, and swimmers treated with the AT1 antagonist L-158809 increased their left ventricular weights by 11% (both P < 0.05 vs. sedentary controls). The incorporation of labeled amino acids into the heart at the time of death was unchanged in all groups, and therefore the increase in heart weight in both swim-conditioned groups appeared to reflect a decrease in the rate of protein degradation in the heart. Hearts from both swim-conditioned groups manifested an increase in the V1-predominant myosin isoform pattern but not an increase in atrial natriuretic factor mRNA expression or protein kinase C translocation. The fact that these patterns of adaptation are preserved in exercised conditioned animals treated with an AT1 antagonist suggests that the chronic hypertrophic response of the heart to physiological loads is not influenced by the renin-angiotensin system.

Swimming training improves brown-adipose-tissue activity in young and old mice

The impairment of brown adipose tissue (BAT) thermogenic activity with aging has been well documented. The current study investigated the effect of swimming training on BAT activity in 2-month-old (young) and 26-month-old (old) male mice. The trained mice underwent a 6-week swimming program (1 h/day, 5 days/week) in water at 35–36°C. Compared with young sedentary mice, the BAT-to-body mass ratio was markedly smaller in old sedentary mice, accompanied by the decreased amount of protein, whereas there was no significant difference in uncoupling protein (UCP) content, UCP mRNA expression, or guanosine 5′-diphosphate (GDP) binding (an index of UCP activity) between young and old mice. Meanwhile, the swimming training definitely increased BAT mass and its protein content in both the young and old mice, suggesting hypertrophy and hyperplasia. In addition, after the swimming training, the amounts of protein, UCP antigen, and GDP binding in the mitochondria recovered from BAT of both mice increased significantly as compared with the respective sedentary groups, while the expression of UCP mRNA did not vary substantially. These findings suggest that, irrespective of age, swimming training enhances the thermogenic activity and capacity in BAT of mice.

Injuries in female collegiate swimmers due to swimming and cross training.

To identify and compare the nature and frequency of training and cross-training injuries incurred by members of a women's collegiate swim team. A longitudinal survey of training-room and medical records for 7 years, classifying injuries by diagnosis and time lost from participation. Division I women's collegiate swimming program, United States. All swimmers in a Division I women's collegiate swimming program over 7 years, for a total of 68 swimmers. ASSESSMENT OF RISK FACTOR: Not applicable. Not applicable. "Injury" was defined as any contact with a trainer or physician that resulted in evaluation or treatment. Each injury was categorized with respect to (a) activity during which injury was incurred; (b) diagnosis, including body part injured; (c) time lost from participation in practice or competition; and (d) severity, i.e., minor (< or = 7 days), moderate (7-21 days), and major (> 21 days). An "Exposure" was defined as participation in one practice session or competition. The overall injury rate per 1,000 exposures per athlete was 2.12; 44% of injuries were due to swimming, 44% to cross training, and 11% to activities unrelated to athletics. Cross-training injuries occurred primarily in the lower extremities, while swimming injuries occurred more commonly in the upper extremities. The ratio of upper to lower extremity injuries due to swimming was 3:1, whereas the ratio for cross training was 1:4. Injuries to swimmers occur at a lower rate per exposure than to other collegiate athletic populations. Swimming injuries occurred primarily in the upper extremities, especially the shoulder. Lower extremity injuries occurred primarily due to cross training. We conclude that swimming is relatively safe compared to other collegiate sports, but special care should be used in designing injury-avoiding cross-training programs.

SWIMMING INCREASES COLD TOLERANCE IN YOUNG AND OLD MICE: WITH EMPHASIS ON BROWN ADIPOSE TISSUE 1204

Brown adipose tissue (BAT) expresses an unusual protein as uncoupling protein (UCP). Activation of UCP leads to the uncoupled passage of protons through the mitochondrial channel and the generation of heat. Currently, it is generally accepted that the impairment of BAT thermogenic activity with aging is a major cause for the decline of cold tolerance. Several studies have suggested, on the other hand, that, in addition to physical training, swimming training serves cold acclimation. The present study was, thus, undertaken to investigate the effect of swimming training on interscapular BAT activity in young and old mice. Both young and old mice (n = 62), aged 8 weeks and 26 months respectively, were divided into either the untrained groups or the trained groups. The trained mice underwent a 6-week swimming program (1 h/day, 5 days/week) in water at 35-36°C (to prevent hypothemia). Compared with young mice, the BAT mass was markedly smaller in old mice, accompanied by the decreased amount of UCP antigen, whereas there was no significant difference in the expression of UCP mRNA between young and old mice. Meanwhile, the swimming training significantly increased BAT mass and its protein content in both the young and old mice, suggesting hyperplasia. In addition, after the swimming training, the amounts of UCP antigen in both mice increased two to threefold over those of the respective untrained groups, while the expression of UCP mRNA did not vary substantially. Results on protein and guanosine 5′-diphosphate (GDP) binding (an index of UCP activity) in the mitochondria (means ± SEM) were: Table These findings indicate that swimming training enhances the nonshivering thermogenesis of BAT (irrespective of age), with resultant increases in tolerance to cold.

Effect of long-term intense swimming training on the upper body peak oxygen uptake of prepubertal girls.

The purpose of the present investigation was to determine the effect of a long-term intense swimming programme on the aerobic potential of prepubertal girls. Five girls [GS, aged 9.3 (SD 0.5) years] participated in a 40-month intense training period. The girls trained on average 10-12 h*week(-1), approximately 1 h-1.5 h twice a day, 5 days each week. Nine girls [CG aged 9.3 (SD 0.4) years] who were engaged in various activities (on average 1-4 h*week(-1)), but not in sports involving upper body muscle mass, served as the control group. All the children completed, on a special swim bench, an incremental maximal exercise prior to (pretest) and after (post-test) the swimming programme. Biometric parameters and the peak oxygen uptake (VO(2peak)) were determined using the same procedure at the pre- and post-test sessions. There was no significant difference between the two groups for any of the variables at the beginning of the study. The biometric characteristics remained similar at the post-test session, indicating that intense swimming training early in life has no influence on the physical growth of prepubertal children. The VO(2peak) expressed in absolute values, however, increased over a year in GS and CG by 38 percent and 13 percent, respectively. The improvement in CG VO(2peak) was related to normal growth and development while that of GS was much higher (P<0.01) than would have been expected due to growth factors alone. The reason for such an improvement could be attributed to an increase in the stroke volume and/or in the difference of the arteriovenous concentration of oxygen since the maximal O(2) pulse was different between CG and GS only at the post-test. Moreover, it increased after 10 months only in GS (delta maximal O(2) pulse: GS 1.09, P<0.01; CG 0.27 NS, ml.beat(-1)). Thus, the results of this study show that physiological adaptations can occur in prepubertal children as a consequence of intense physical training.

Effects of exhaustive exercise and vitamin b 6 deficiency on free radical oxidative process in male trained rats

There is growing evidence that oxygen free radical production and subsequent lipid peroxidation are normal sequelae to the rise in oxygen consumption concomitant with exercise. In addition, increased lipid peroxidation has also been shown in vitamin B6-deficient rat plasma, liver, and kidney. To investigate the potential for a role of vitamin B 6 in exercise-induced oxidative stress, 36 male Sprague-Dawley rats received 0 ( n = 12), 2 ( n = 12) and 8 mg pyridoxine (PN)-HCl/kg diet ( n = 12) and were trained by a 9-week swimming program. After 9-weeks of training, six rats (exhausted: E rats) of each vitamin group were exercised to exhaustion by swimming while the other six rats rested (nonexhausted: NE rats). Ascorbate, ascorbate free radical and antilipoperoxidant capability (AC) were evaluated in plasma. These parameters were higher in E rats than in NE rats. Free radical-mediated lipid peroxidation was measured in tissue and plasma by evaluation of thiobarbituric acid reactive substances (TBARS) content. This index of peroxidation was significantly increased in liver of E rats but not in plasma, heart, and gastrocnemius muscle. Concentration of TBARS in liver was the highest in vitamin B 6-deficient rats (consuming 0 mg PN-HCl/kg diet) and the lowest in vitamin B 6-sufficient rats (consuming 8 mg PN-HCl/kg diet). Vitamin E (α-tocopherol) levels in liver and heart were negatively related to vitamin B 6 levels in the diet. Independently of vitamin B 6, liver and muscle a-tocopherol levels were significantly higher in E animals than in NE animals. There is good evidence according to our results that exercise induced an oxidative stress, as indicated by a significant increase of ascorbyl radical levels in the plasma. The effects of vitamin B 6 deficiency on the free radical metabolism are low in trained rats. On the contrary, exhaustive exercise induced modifications in the metabolism pathways of vitamin C and E objectivated by variations of levels of vitamin C in the plasma and vitamin E in liver.

Variations in bone status of contralateral and regional sites in young athletic women

To determine if volleyball (VB), basketball (BB), soccer (SO) and swimming (SW) programs were associated with site-specific differences in contralateral, regional, and total body bone mineral density (BMD), 62 eumenorrheic female athletes [BB (N = 7), VB (N = 11), SO (N = 9), and SW (N = 7)] and controls participated in the study. The controls were categorized as either moderately active control (MOD) (N = 17) or sedentary control (SED) (N = 11) based on fitness and activity assessments. Contralateral, total body, lumbar (L2-L4), and femur BMD were measured (Lunar DPX). The between sport contralateral comparisons indicated that VB and BB had significantly greater leg and arm measurements than all other groups, while the within contralateral comparisons revealed significantly greater right arm measurements for all groups, except SW. No significant differences were found for the within group contralateral leg comparisons, except VB. VB and BB had significantly higher (P ≤ 0.05) total body and lumbar BMD values than SW, MOD, and SED. At the femur neck, trochanter, and Ward's triangle, BB showed significantly higher BMD than SW, MOD, and SED. Only BB had significantly higher Ward's triangle BMD than SW, MOD, and SED. Our findings show site-specific differences in BMD associated with selected sports' programs.

QUANTITATIVE BEHAVIORAL STUDY OF BOTTLENOSE DOLPHINS IN SWIM‐WITH‐DOLPHIN PROGRAMS IN THE UNITED STATES

AbstractThe behavior of dolphins in four Swim‐With‐Dolphin programs was compared by type of Swim encounter, defined by the presence (“Controlled“) or absence (“Not‐Controlled“) of explicit trainer regulation of interactions between dolphins and human swimmers. Dolphin‐swimmer interactions involving aggressive, submissive, or sexual behavior were designated as “high‐risk” in the Swim context; sexual behavior was included as high‐risk based on analyses that demonstrated co‐occurrence of sexual and agonistic behaviors. High‐risk activity comprised a substantial proportion of dolphin‐swimmer social activity during Not‐Controlled Swims. In contrast, high‐risk activity rarely occurred during Controlled Swims, even though agonistic and sexual behaviors were normal components of the same dolphins’ free‐time social repertoire. These results indicated that direct trainer control of dolphin‐swimmer interactions virtually eliminated high‐risk activity from the Swim context, and thereby diminished the potential for dolphin distress, swimmer injury, and rejection of dolphins from Swim programs due to swimmer injury. This study illustrates effective use of quantitative behavioral sampling techniques for evaluation of captive management concerns and promotes broader use of these techniques for a better understanding of cetacean behavior.

VARIATIONS IN THE BONE STATUS OF CONTRALATERAL AND REGIONAL SITES IN YOUNG ATHLETIC WOMEN.

To determine if volleyball (VB), basketball (BB), soccer (SO) and swimming (SW) programs were associated with site-specific differences in contralateral, regional, and total body bone mineral density (BMD), 62 eumenorrheic female athletes [BB (N = 7), VB (N = 11), SO (N = 9), and SW (N = 7)] and controls participated in the study. The controls were categorized as either moderately active control (MOD) (N = 17) or sedentary control (SED) (N = 11) based on fitness and activity assessments. Contralateral, total body, lumbar (L2-L4), and femur BMD were measured (Lunar DPX). The between sport contralateral comparisons indicated that VB and BB had significantly greater leg and arm measurements than all other groups, while the within contralateral comparisons revealed significantly greater right arm measurements for all groups, except SW. No significant differences were found for the within group contralateral leg comparisons, except VB. VB and BB had significantly higher (P < or = 0.05) total body and lumbar BMD values than SW, MOD, and SED. At the femur neck, trochanter, and Ward's triangle, BB showed significantly higher BMD than SW, MOD, and SED. Only BB had significantly higher Ward's triangle BMD than SW, MOD, and SED. Our findings show site-specific differences in BMD associated with selected sports' programs.

Properties of networks controlling locomotion and significance of voltage dependency of NMDA channels: stimulation study of rhythm generation sustained by positive feedback

1. We have built a realistic 24-neuron model based on data from the spinal pattern generator for swimming in Xenopus embryos with the use of the SWIM programs. The neurons have dendrite, soma, and axon compartments with voltage-gated Na+ and K+ channels. Dendritic synapses were modeled as modulated ionic conductances with currents that have different reversal levels. One of these conductances was voltage dependent to model N-methyl-D-aspartate ("NMDA") synapses in the presence of Mg2+. 2. In this model, rhythm generation is initiated by a brief excitation, depends on rebound from reciprocal inhibition, and is sustained by long-duration "NMDA-dependent" feedback excitation. 3. Without NMDA voltage dependency, rhythmic activity is stable over a wide range of synaptic conductances. Its frequency decreases with more inhibition and increases with more excitation. The introduction of normally distributed variation in soma size or excitatory synaptic conductance extends the lower stable frequency range. Without such variation the frequency of the 24-neuron model is the same as a 4-neuron model provided that the synaptic conductances for each neuron are the same. 4. The effect of introducing NMDA voltage dependency on rebound after negative current injections or synaptic inhibition was investigated in single depolarized model neurons. With NMDA voltage dependency, hyperpolarizations and rebound spike responses were increased. 5. Network activity with NMDA voltage dependency was similar to that without it, but inhibitory postsynaptic potentials (IPSPs) and spikes were larger, and frequencies were lower and more sensitive to changes in excitatory and inhibitory conductance. 6. We conclude that in the model, mutual reexcitation among excitatory spinal interneurons can sustain rhythm generation by positive feedback and that NMDA voltage dependency can enhance postinhibitory rebound, stabilize swimming activity and extend its lower frequency range, and steepen the dependency of frequency on synaptic drive.