The Role of Erythrocytic Acetylcholinesterase in Hormonal Regulation of Adaptation to Physical Exercise

Acetylcholinesterase (AchE) activity in erythrocytes and blood levels of cortisol and insulin were investigated in athletes training under different bioenergetic conditions (sprinters, middle-distance runners, and marathoners). The groups of sprinters and marathoners had a decreased enzyme activity compared to nonathletes (p < 0.05). In response to a standard exercise, load AchE activity increased in the groups of middle-distance runners and marathoners. A relationship was observed between the level of AchE activity and the cortisol-to-insulin ratio in the blood. This ratio is specific to the type of bioenergetic conditions and increases in the following order: controls, middle-distance runners, sprinters, and marathoners. In vitro experiments revealed an effect of insulin on AchE activity. This effect was significantly lower in sprinters than in the control group. A reduction in AchE activity and an increase in the cortisol-to-insulin ratio are considered as factors increasing metabolic turnover in athletes, mainly, lipid turnover. This mechanism ensures effective mobilization of substrates at the start of physical exercise and their recovery after. The observed relationship between the insulin level and the AchE activity may prove to be a mechanism of regulation of the insulin level. This relationship may change during adaptation to physical exercise, as in the case of sprinters, when the sensitivity of AchE to the inhibitory effect of insulin is decreased. A high blood level of cortisol and insulin is a distinctive feature of sprinters, which provides for a higher turnover of carbohydrates. In marathoners, low AchE activity leads to an increased effect of acetylcholine, which is manifested by an increased cortisol level and a decreased insulin level, thus providing for a higher lipid turnover.     

橘皮素与乳清蛋白的合用对短跑运动员冬训期间血清睾酮、皮质醇水平的影响*

目的:探讨补充橘皮素对短跑运动员冬训期间血清睾酮、皮质醇水平的影响。方法:将24名短跑运动员配对、随机分为试验组和对照组。冬训期间,试验组补充橘皮素补剂(含橘皮素200 mg),对照组补充安慰剂,为期4周。分别在每周第一天(T1、T2、T3、T4)及干预结束次日(T5)采集血液样本,检测血清睾酮、皮质醇、超氧化物歧化酶(SOD)、促肾上腺皮质激素(ACTH)等指标,并在干预开始和结束时测试身体成分。结果:干预4周后,①对照组的血清皮质醇水平上升,血清睾酮和SOD活性水平显著下降(P＜0.05);②试验组的血清皮质醇(P＜0.05)和ACTH(P＜0.01)均显著下降,血清睾酮维持稳定,SOD活性水平小幅上升;③试验组和对照组的肌肉量均有增加,但前者的增加量显著大于后者。结论:补充4周橘皮素能减少冬训期间高强度运动引起的机体氧化应激反应,维持机体血清睾酮水平稳定,抑制皮质醇过度分泌,促进肌肉合成。     

Sweating responses and the muscle metaboreflex under mildly hyperthermic conditions in sprinters and distance runners

To investigate the effects of different training methods on nonthermal sweating during activation of the muscle metaboreflex, we compared sweating responses during postexercise muscle occlusion in endurance runners, sprinters, and untrained men under mild hyperthermia (ambient temperature, 35°C; relative humidity, 50%). Ten endurance runners, nine sprinters, and ten untrained men (maximal oxygen uptakes: 57.5 ± 1.5, 49.3 ± 1.5, and 36.6 ± 1.6 ml·kg(-1)·min(-1), respectively; P < 0.05) performed an isometric handgrip exercise at 40% maximal voluntary contraction for 2 min, and then a pressure of 280 mmHg was applied to the forearm to occlude blood circulation for 2 min. The Δ change in mean arterial blood pressure between the resting level and the occlusion was significantly higher in sprinters than in untrained men (32.2 ± 4.4 vs. 17.3 ± 2.6 mmHg, respectively; P < 0.05); however, no difference was observed between distance runners and untrained men. The Δ mean sweating rate (averaged value of the forehead, chest, forearm, and thigh) during the occlusion was significantly higher in distance runners than in sprinters and untrained men (0.38 ± 0.07, 0.19 ± 0.03, and 0.11 ± 0.04 mg·cm(-2)·min(-1), respectively; P < 0.05) and did not differ between sprinters and untrained men. Our results suggest that the specificity of training modalities influences the sweating response during activation of the muscle metaboreflex. In addition, these results imply that a greater activation of the muscle metaboreflex does not cause a greater sweating response in sprinters.     

Peak blood ammonia and lactate after submaximal, maximal and supramaximal exercise in sprinters and long-distance runners

The purpose of this study was to elucidate the difference in peak blood ammonia concentration between sprinters and long-distance runners in submaximal, maximal and supramaximal exercise. Five sprinters and six long-distance runners performed cycle ergometer exercise at 50% maximal, 75% maximal, maximal and supramaximal heart rates. Blood ammonia and lactate were measured at 2.5, 5, 7.5, 10 and 12.5 min after each exercise. Peak blood ammonia concentration at an exercise intensity producing 50% maximal heart rate was found to be significantly higher compared to the basal level in sprinters (P less than 0.01) and in long-distance runners (P less than 0.01). The peak blood ammonia concentration of sprinters was greater in supra-maximal exercise than in maximal exercise (P less than 0.05), while there was no significant difference in long-distance runners. The peak blood ammonia content after supramaximal exercise was higher in sprinters compared with long-distance runners (P less than 0.01). There was a significant relationship between peak blood ammonia and lactate after exercise in sprinters and in long-distance runners. These results suggest that peak blood ammonia concentration after supramaximal exercise may be increased by the recruitment of fast-twitch muscle fibres and/or by anaerobic training, and that the processes of blood ammonia and lactate production during exercise may be strongly linked in sprinters and long-distance runners.     

Fibre conduction velocity and fibre composition in human vastus lateralis

The relationship between muscle fibre composition and fibre conduction velocity was investigated in 19 male track athletes, 12 sprinters and 7 distance runners, aged 20-24 years, using needle biopsy samples from vastus lateralis. Cross sectional areas of the fast twitch (FT) and slow twitch (ST) fibres were determined by histochemical analysis. The percentage of FT fibre areas ranged from 22.6 to 93.6%. Sprinters had a higher percentage of FT fibres than distance runners. Muscle fibre conduction velocity was measured with a surface electrode array placed along the muscle fibres, and calculated from the time delay between 2 myoelectric signals recorded during a maximal voluntary contraction. The conduction velocity ranged from 4.13 to 5.20 m.s-1. A linear correlation between conduction velocity and the relative area of FT fibres was statistically significant (r = 0.84, p less than 0.01). This correlation indicates that muscle fibre composition can be estimated from muscle fibre conduction velocity measured noninvasively with surface electrodes.     

Distribution of motor unit potential velocities in the biceps brachii muscle of sprinters and endurance athletes during short static contractions at low force levels

In surface electromyography (sEMG), the distribution of motor unit potential (MUP) velocities has been shown to reflect the proportion of faster and slower propagating MUPs. This study investigated whether the distribution of MUP velocities could distinguish between sprinters and endurance athletes in not-specifically trained muscle (biceps brachii). sEMG results were acquired from 15 sprinters and 18 endurance athletes during short static contractions (3.8 s) at three force levels: unloaded, 10% and 20% of maximum voluntary contraction. The features extracted from the sEMG were: the mean muscle conduction velocity (CV) – estimated using the inter-peak latency and the cross-correlation methods, the within-subject skewness of MUP velocities (expressing the relative proportions of faster and slower propagating MUPs), and the within-subject standard deviation of MUP velocities. Sprinters had a higher CV than endurance athletes using both methods. Sprinters also demonstrated a greater proportion of fast propagating MUPs, as indicated by the skewness. Thus, the distribution of MUP velocities was able to demonstrate physiological differences between sprinters and endurance athletes during short contractions at low forces. The findings can be extrapolated to the motor unit level. Since the investigated muscle was not involved in specific training, the differences seem to reflect inherited properties.     

Twitch contractile adaptations are not dependent on the intensity of isometric exercise in the human triceps surae

Ultrastructural and twitch contractile characteristics of the human triceps surae were determined in six healthy but very sedentary subjects before and after 16 weeks of isometric training at 30% maximal voluntary contraction (MVC). Following training, twitch contraction time was approximately 16% shorter, although no differences were observed in one-half relaxation time or peak twitch torque. Percent fibre type was not changed by training. The mean area of type I and type II fibres in the soleus increased by approximately 30% but only type II fibres showed an increase in area in the lateral gastrocnemius (30%). Despite such changes in fibre area the volume density of the sarcoplasmic reticulum-transverse tubular network averaged 3.2 +/- 0.6% and 5.9 +/- 0.9% in type I and type II fibres respectively, before and after training in the two heads of the gastrocnemius. The results indicate that contractile adaptations to isometric training at 30% MVC were limited to twitch contraction time and were not directly related to changes in percent fibre distribution or the volume of sarcoplasmic reticulum and transverse tubules in either type I or type II fibres. The data further demonstrate that substantial fibre hypertrophy is achieved by training with low-intensity contractions.     

Force-velocity relationship and stretch-shortening cycle function in sprint and endurance athletes

This study examined the torque-velocity and power-velocity relationships of quadriceps muscle function, stretch shortening cycle function, and leg-spring stiffness in sprint and endurance athletes. Isokinetic maximal knee extension torque was obtained from 7 sprinters and 7 endurance athletes using a Con-trex isokinetic dynamometer. Torque and power measures were corrected for lean-thigh cross-sectional area and lean-thigh volume, respectively. Stretch-shortening cycle function and muscle stiffness measurements were obtained while subjects performed single-legged squat, countermovement, and drop-rebound jumps on an inclined sledge and force-plate apparatus. The results indicated that sprinters generated, on average, 0.15 +/- 0.05 N.m.cm(-2) more torque across all velocities compared with endurance athletes. Significant differences were also found in the power-velocity relationships between the 2 groups. The sprinters performed significantly better than the endurance athletes on all jumps, but there were no differences in prestretch augmentation between the groups. The average vertical leg stiffness during drop jumps was significantly higher for sprinters (5.86 N.m(-1)) compared with endurance runners (3.38 N.m(-1)). The findings reinforce the need for power training to be carried out at fast contraction speeds but also show that SSC function remains important in endurance running.     

Carbon dioxide storage capacity of endurance and sprint-trained athletes in exercise

The purpose of this study was to compare CO2 storage capacity of endurance and sprint-trained athletes during steady state exercise. Ten subjects, five sprinters and five distance runners, performed a submaximal treadmill exercise at two different work rates, 45% and 65% of VO2max. CO2 storage capacity was determined by measuring the excess CO2 washout associated with hyperventilation, normalized for body weight and expressed per unit change in mixed venous PCO2 (ml kg-1 Torr-1). Mixed venous PCO2 (PvCO2) was measured by rebreathing equilibration. It was found that CO2 storage capacities of the runners were significantly (P less than 0.05) greater than the sprinters at the two work rates. The sprinters CO2 storage capacities were 2.69 and 2.14 ml kg-1 Torr-1 at low and high work rates, respectively. The corresponding mean values for the runners were 4.56 and 3.92 ml kg-1 Torr-1, respectively. These results may be explained by the metabolic differences between the sprinters and runners. The sprinters' musculature depends more heavily on the glycolytic metabolic pathway, which is associated with an increased lactate production and hence a reduction in the combining power of the blood for CO2 during exercise. At the low work rate, the body's storage capacity for CO2 was significantly (P less than 0.05) greater than the higher work rate for both groups. Obviously, at the higher work level more blood would be presented to the lungs per unit time allowing an increase in CO2 clearance from the body stores.     

短跑训练对运动员跟腱影响的超声测量研究

目的：研究短跑训练对运动员跟腱的影响。方法：选择从事运动训练4-6年的健康男、女短跑运动员为实验组（n=12）,同时选同龄健康非运动员男、女为对照组（n=12）,采用高频超声检测平静状态跟腱长度、横截面积,以及小腿三头肌等长收缩最大力量跖屈跟腱长度的变化。结果：跟腱长度：对照组男子168.5±9.2mm,女子162.4±9.8mm,实验组男子170.9±10.7mm,女子164.0±7.0mm。实验组和对照组组内、组间差别均无统计学意义。跟腱横截面积：对照组男子62.2±6.2mm2,女子47.1±4.5mm2,实验组男子65.6±2.9mm2,女子49.6±1.9mm2。同组内男子比女子跟腱横截面积大,差别有统计学意义（P＆lt;0.05）,但实验组和对照组组间差别无统计学意义。小腿三头肌等长收缩最大力跖屈时跟腱拉长值：对照组男子7.6±3.6mm,女子4.9±2.8mm,实验组男子11.1±2.9mm,女子7.9±3.1mm。男子比女子跟腱拉长值大、实验组比对照组拉长值大,差别均有统计学意义（P＆lt;0.05）。结论：短跑训练可增加跟腱可拉伸长度,跟腱的长度和横截面积未发生明显变化。高频超声可作为重要的测量手段用于...     

The effects of high intensity training upon respiratory gas exchanges during fixed term maximal incremental exercise in man

The response of respiratory gas exchanges to a 6 week high intensity training program was examined in 5 healthy males during fixed term maximal incremental treadmill exercise. Training was performed 3 d.wk-1 and consisted of a progressive series of repeated 15 sec and 30 sec maximal runs, and weight training exercises for the leg extensor muscles. Respiratory gases during the tests were continuously monitored using an on-line system. Muscle biopsy samples were obtained from the m. vastus lateralis before and after training for histochemical determination of fibre distribution based on myosin ATP-ase activity, and fibre cross-sectional area based on NADH-Tetrazolium Reductase activity. Training significantly increased the proportion of type IIa fibres (+5.9 +/- 2.0%, p less than 0.001) and decreased type I fibres (-6.3 +/- 2.0%, p less than 0.001), the distribution of type IIb fibres remained unchanged (+0.4 +/- 0.9%). Muscle cross-sectional area also showed a significant increase after training in type I (+318 +/- 215 microns 2, p less than 0.05), IIa (+652 +/- 207 microns 2, p less than 0.001) and IIb (+773 +/- 196 microns 2, p less than 0.001) fibres. During fixed term maximal incremental exercise the mean carbon dioxide output (VCO2) and mean respiratory exchange ratio (R = VCO2/VO2) were significantly increased (p less than 0.01) after training. The R-time relationship was at all times shifted to the left after training, being significantly (p less than 0.01) so over the final five min of exercise. No changes in mean exercise oxygen uptake (VO2), maximum oxygen uptake (VO2max) and maximum heart rate (FHRmax) were observed between tests.(ABSTRACT TRUNCATED AT 250 WORDS)     

On the implications of a sex difference in the reaction times of sprinters at the Beijing Olympics

Elite sprinters offer insights into the fastest whole body auditory reaction times. When, however, is a reaction so fast that it represents a false start? Currently, a false start is awarded if an athlete increases the force on their starting block above a given threshold before 100 ms has elapsed after the starting gun. To test the hypothesis that the fastest valid reaction times of sprinters really is 100 ms and that no sex difference exists in that time, we analyzed the fastest reaction times achieved by each of the 425 male and female sprinters who competed at the 2008 Beijing Olympics. After power transformation of the skewed data, a fixed effects ANOVA was used to analyze the effects of sex, race, round and lane position. The lower bounds of the 95, 99 and 99.9% confidence intervals were then calculated and back transformed. The mean fastest reaction time recorded by men was significantly faster than women (p<0.001). At the 99.9% confidence level, neither men nor women can react in 100 ms, but they can react in as little as 109 ms and 121 ms, respectively. However, that sex difference in reaction time is likely an artifact caused by using the same force threshold in women as men, and it permits a woman to false start by up to 21 ms without penalty. We estimate that female sprinters would have similar reaction times to male sprinters if the force threshold used at Beijing was lowered by 22% in order to account for their lesser muscle strength.     

Training-induced alterations in muscle glycogen utilization in fibre-specific types during prolonged exercise

Using the glycogen depletion technique, we have examined utilization of specific fibre types during prolonged submaximal exercise to investigate the recruitment pattern employed by the central nervous system to sustain force generation in the face of a progressive glycogen depletion. Six male subjects (Vo2 max, 52.8 +/- 2.5 mL.kg-1.min-1, mean +/- SE) cycled at 59% of pretraining Vo2 max (the same absolute power output) for 99.5 +/- 6 min on two occasions, before training and after 10-12 days of intensive training, involving 2 h of cycling per day. Prior to the training, glycogen concentration during exercise in the type I and type IIA fibres of the vastus lateralis muscle as measured by microphotometric techniques was progressively reduced during exercise. The pattern of depletion in both of these fibre types was parallel and showed an early marked depletion amounting to 51 (p less than 0.05) and 35% (p less than 0.05) in the type I and type IIA fibres, respectively, during the first 15 min of exercise. At the end of exercise, glycogen levels in type I and type IIA fibres were reduced to 9 and 44% of initial levels, respectively. In contrast, glycogen concentration in type IIB fibres was not significantly (p less than 0.05) altered throughout the exercise. Following training, a pronounced glycogen sparing occurred that was conspicuous in only the type I and type IIA fibres, which was most pronounced during the first 15 min of the exercise. Similar to pretraining, glycogen concentrations in type IIB fibres were unaffected by either exercise or training.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differences in H-reflex between athletes trained for explosive contractions and non-trained subjects

The efficacy of type Ia synapse on alpha-motoneurons of soleus and lateral gastrocnemius muscles has been investigated, using the H-reflex technique, in athletes engaged in sports requiring very rapid and intense contractions (sprinters and volley-ball players) as well as in non-trained subjects. It has been observed, in both muscles, that the ratio between the mean value of the maximal reflex response (Hmax) and the mean value of the maximal direct response (Mmax) elicited upon electrical stimulation of the tibial nerve is significantly smaller in athletes trained for explosive-type movements than in non-trained subjects. This difference in the Hmax: Mmax ratio was dependent on a smaller amplitude of Hmax and not on a greater amplitude of Mmax. No significant differences were observed between sprinters and volley-ball players. In both trained and non-trained subjects, soleus and lateral gastrocnemius muscles displayed significant differences in Hmax: Mmax ratio and Mmax amplitude but not in Hmax amplitude. Since the H-response is considered to be due mainly to activation of the smallest motoneurons in the motoneuronal pools, the difference in Hmax amplitude and Hmax: Mmax ratio between athletes and non-trained subjects could have been dependent on a lower incidence of these motoneurons in the athletes. This is in accord with the mechanical needs of muscles during explosive-type power training. Although this difference may have been wholly determined genetically, the possibility is discussed as to whether the lower incidence in sprinters and volley-ball players of small motoneurons could have been related to a training-induced transformation of small and slow motoneurons into large and fast ones.     

Capillary density and capillary-to-fibre ratio in vastus lateralis muscle of untrained and trained men

Muscle fibre profile area (Af), volume density (Vv), capillary-to-fibre ratio (CF) and number of capillaries per fibre square millimetre (CD) were determined from needle biopsies of vastus lateralis of twenty-four male volunteers (mean +/- SD: age 25.4+/-5.8 years, height 178.6+/-5.5 cm, body mass 72.1+/-7.7 kg) of different training background. Seven subjects were untrained students (group A), nine were national and sub-national level endurance athletes (group B) with the background of 7.8+/-2.9 years of specialised training, and eight subjects were sprint-power athletes (group C) with 12.8+/-8.7 years of specialised training. Muscle biopsies of vastus lateralis were analysed histochemically for mATPase. Capillaries were visualized and counted using CD31 antibodies against endothelial cells. There were significant differences in the Vv of type I and type II muscle fibres in both trained groups, B (51.8%; 25.6%) and C (50.5%; 26.4%). However, in untrained group A that was treated as a reference group, the difference between Vv of type I and type II fibres was less prominent, nevertheless statistically significant (42.1%; 35.1%). There was also a significant difference in CF: 1.9 in group A and 2.1 in groups B and C. The number of capillaries per mm2 (CD) was 245 (group A), 308 (group B) and 325 (group C). Significant differences (P<0.05) in CF and CD, were found only between group A (1.9; 245) and both groups of trained men, B and C (2.1; 308 and 325). However, endurance athletes (group B), such as long-distance runners, cyclists and cross country skiers, did not differ from the athletes representing short term, high power output sports (group C) such as ice hockey, karate, ski-jumping, volleyball, soccer and modern dance.     

The influence of parachute-resisted sprinting on running mechanics in collegiate track athletes

The influence of parachute-resisted sprinting on running mechanics in collegiate track athletes. The aim of this investigation was to compare the acute effects of parachute-resisted (PR) sprinting on selected kinematic variables. Twelve collegiate sprinters (mean age 19.58 ± 1.44 years, mass 69.32 ± 14.38 kg, height 1.71 ± 9.86 m) ran a 40-yd dash under 2 conditions: PR sprint and sprint without a parachute (NC) that were recorded on a video computer system (60 Hz). Sagittal plane kinematics of the right side of the body was digitized to calculate joint angles at initial ground contact (IGC) and end ground contact (EGC), ground contact (GC) time, stride rate (SR), stride length (SL), and the times of the 40-yd dashes. The NC 40-yd dash time was significantly faster than the PR trial (p < 0.05). The shoulder angle at EGC significantly increased from 34.10 to 42.10° during the PR trial (p < 0.05). There were no significant differences in GC time, SR, SL, or the other joint angles between the 2 trials (p > 0.05). This study suggests that PR sprinting does not acutely affect GC time, SR, SL and upper extremity or lower extremity joint angles during weight acceptance (IGC) in collegiate sprinters. However, PR sprinting increased shoulder flexion by 23.5% at push-off and decreased speed by 4.4%. While sprinting with the parachute, the athlete's movement patterns resembled their mechanics during the unloaded condition. This indicates the external load caused by PR did not substantially overload the runner, and only caused a minor change in the shoulder during push-off. This sports-specific training apparatus may provide coaches with another method for training athletes in a sports-specific manner without causing acute changes to running mechanics.     

Morphological differences of elite Croatian track-and-field athletes

In this study we present the morphological characteristics of 54 Croatian national level track-and-field athletes. 21 anthropometric body measures were taken on a sample of 15 sprinters (S), 16 endurance sprinters (S4), 10 middle-distance runners (MD) and 13 long-distance runners (LD). Body fat percentage, body mass index and somatotype were also calculated. Canonical discriminative analysis showed significant difference between the athletes of various running events, in the measures of body volume and body fat, while no significant difference was found in the variables of longitudinal and transversal dimensions of the skeleton. ANOVA and Student t-test for independent samples showed statistically significantly higher thigh and lower leg circumference in sprinters, as well as greater upper arm skinfold in middle-distance runners. The mesomorphic component is a dominant characteristic of somatotype of the runners in all events, whereas the ectomorphic component is the least marked.     

Creatine kinase MB and citrate synthase in type I and type II muscle fibres in trained and untrained men

Total creatine kinase (CK), creatine kinase MB (CK-MB) and citrate synthase (CS) were determined in isolated and pooled type I and type II skeletal muscle fibres. Determinations were made on biopsies from 3 sedentary men, 3 junior cyclists and 2 elite cyclists. CS and CK-MB activities were higher in the trained groups in both fibre types. The total CK activity was not related to training status, although it was lower in type I fibres than in type II fibres (p less than 0.05). The reverse relation was observed for CS and CK-MB activities (p less than 0.01). The ratio of type I/type II for CS was not related to training status, while the corresponding ratio for CK-MB increased with a greater degree of endurance training. For a given increase in CS activity, the increase in CK-MB activity was greater in type I fibres than in type II fibres (p less than 0.01). Thus, with endurance training there seems to be a specific adaptation for CK-MB, particularly in type I fibres.     

Presynaptic inhibition of spinal α motoneurons in humans adapted to different types of motor activity

Changes in the presynaptic inhibition of spinal α motoneurons were studied in athletes during motor activities of different types. In the state where muscles were at relative rest, the presynaptic inhibition of spinal α motoneurons of the m. soleus was stronger in samboists (athletes specializing in the martial art of sambo) and sprinters than in long-distance runners. In samboists performing repeated static efforts, the presynaptic inhibition of spinal α motoneurons became stronger from one trial to the next. Both technique training and strength training enhanced the presynaptic inhibition of spinal α motoneurons, this enhancement being greater after strength training.     

Adaptation of motor nerve fibers to physical activity

The effects induced by training on the H-reflex of soleus and lateral gastrocnemius muscles have been studied on 19 adult male volunteers; out of these, 10 were non-trained subjects and the remaining 9 were top level athletes engaged in sports requiring very rapid and intense contractions (sprinters and volley-ball players). It has been observed that the latency of the M response is significantly higher in the athletes than in the non-trained subjects. Instead, no significant differences were observed between these two groups, concerning the latency of H response. The increase of M response latency is likely due to a decrease of nerve conduction velocity in the terminal part of motor fibers. The possibility that this conduction speed decrease could be dependent on sprouting and/or terminal branching growth of the motor nerve ending is discussed.