Respiratory muscle strength and cough capacity in patients with Duchenne muscular dystrophy

The function of inspiratory muscles is crucial for effective cough as well as expiratory muscles in patients with Duchenne muscular dystrophy (DMD). However, there is no report on the correlation between cough and inspiratory muscle strength. To investigate the relationships of voluntary cough capacity, assisted cough techniques, and inspiratory muscle strength as well as expiratory muscle strength in patients with DMD (n= 32). The vital capacity (VC), maximum insufflation capacity (MIC), maximal inspiratory pressure (MIP), and maximal expiratory pressure (MEP) were measured. Unassisted peak cough flow (UPCF) and three different techniques of assisted PCF were evaluated. The mean value of MICs (1918 +/- 586 mL) was higher than that of VCs (1474 +/- 632 mL) (p < 0.001). All three assisted cough methods showed significantly higher value than unassisted method (212 +/- 52 L/min) (F = 66.13, p < 0.001). Combined assisted cough technique (both manual and volume assisted PCF; 286 +/- 41 L/min) significantly exceeded manual assisted PCF (MPCF; 246 +/- 49 L/ min) and volume assisted PCF (VPCF; 252 +/- 45 L/min) (F = 66.13, p < 0.001). MIP (34 +/- 13 cmH2O) correlated significantly with both UPCF and all three assisted PCFs as well as MEP (27 +/- 10 cmH2O) (p < 0.001). Both MEP and MIP, which are the markers of respiratory muscle weakness, should be taken into account in the study of cough effectiveness.     

膝关节最大和次最大等速运动至疲劳过程中主动肌与拮抗肌肌电特征分析

背景：人体关节力量输出主要取决于关节周围主动肌与拮抗肌的激活效果,而拮抗肌通过共激活形式参与肢体活动,从而协助维持关节稳定性。但是肌肉不同收缩模式下拮抗肌共激活的不同中枢策略的报道较少。目的：观察健康受试者膝关节伸肌在最大和次最大等速运动至疲劳过程中主动肌与拮抗肌表面肌电信号的特征与差别。方法：对16名在校学生使用Biodex等速训练仪诱发下肢右侧膝关节伸肌等速屈伸运动直至疲劳,同时使用表面肌电仪采集单侧股内侧肌、股外侧肌以及股二头肌的表面肌电信号。分析时域指标均方根振幅百分比以及频域指标平均频率的变化规律。结果与结论：在膝关节最大等速屈伸运动中峰值力矩下降了40.3%(P < 0.05),主动肌活动先增加后下降(P < 0.05),而拮抗肌电活动保持不变。在次最大等速运动中峰值力矩平均下降了40.0%(P < 0.05),主动肌肌电活动逐渐增加(P < 0.05),而拮抗肌电活动先增加随后又降低。在指定的交互收缩屈伸运动中作为拮抗肌的股内和股外侧肌的均方根振幅和平均频率没有显著变化(P > 0.05)。结果提示,在不同强度的等速疲劳运动中中枢神经系统对主动肌和拮抗肌采取了不同神经支配策略。中国组织工程研究杂志出版内容重点：组织构建;骨细胞;软骨细胞;细胞培养;成纤维细胞;血管内皮细胞;骨质疏松;组织工程     

Respiratory muscle performance with stretch-shortening cycle manoeuvres: maximal inspiratory pressure-flow curves

AIM: To test the hypothesis that the maximal inspiratory muscle (IM) performance, as assessed by the maximal IM pressure-flow relationship, is enhanced with the stretch-shortening cycle (SSC). METHODS: Maximal inspiratory flow-pressure curves were measured in 12 healthy volunteers (35 +/- 6 years) during maximal single efforts through a range of graded resistors (4-, 6-, and 8-mm diameter orifices), against an occluded airway, and with a minimal load (wide-open resistor). Maximal inspiratory efforts were initiated at a volume near residual lung volume (RV). The subjects exhaled to RV using slow (S) or fast (F) manoeuvres. With the S manoeuvre, they exhaled slowly to RV and held the breath at RV for about 4 s prior to maximal inspiration. With the F manoeuvre, they exhaled rapidly to RV and immediately inhaled maximally without a post-expiratory hold; a strategy designed to enhance inspiratory pressure via the SSC. RESULTS: The maximal inspiratory pressure-flow relationship was linear with the S and F manoeuvres (r2 = 0.88 for S and r2 = 0.88 for F manoeuvre, P < 0.0005 in all subjects). With the F manoeuvre, the pressure-flow relationship shifted to the right in a parallel fashion and the calculated maximal power increased by approximately 10% (P < 0.05) over that calculated with the S manoeuvre. CONCLUSION: The maximal inspiratory pressure-flow capacity can be enhanced with SSC manoeuvres in a manner analogous to increases in the force-velocity relationship with SSC reported for skeletal muscles.     

Chest wall volume changes during inspiratory loaded breathing

We assessed the effect of inspiratory loaded breathing (ILB) on respiratory muscle strength and investigated the extent to which respiratory muscle fatigue is associated with chest wall volume changes during ILB. Twelve healthy subjects performed ILB at 76 ± 11% of maximal inspiratory mouth pressure (MIP) for 1h. MIP and breathing pattern during 3 min of normocapnic hyperpnea (NH) were measured before and after ILB. Breathing pattern and chest wall volume changes were assessed by optoelectronic plethysmography. After ILB, six subjects decreased MIP significantly (-16 ± 10%; p < 0.05), while the other six subjects did not (0 ± 7%, p = 0.916). Only subjects with decreased MIP after ILB lowered end-expiratory rib cage volume (volume at which inspiration is initiated) below resting values during ILB. During NH after ILB, tidal volume was smaller in subjects with decreased MIP (-19 ± 16%, p < 0.05), while it remained unchanged in the other group (-3 ± 11%, p = 0.463). These results suggest that respiratory muscle fatigue depends on the lung volume from which inspiratory efforts are made during ILB.     

Mouth pressure twitches induced by cervical magnetic stimulation to assess inspiratory muscle fatigue

This study aimed at determining whether twitch mouth pressure (TwPmo) induced by cervical magnetic stimulation (CMS) was sensitive to inspiratory muscle fatigue produced by whole body exercise (WBE) in normal subjects. Twenty subjects performed one or two of the following protocols: (i). cycling at 85% V(O(2),max) until exhaustion; (ii). inspiratory resistive load (IRL) breathing at 62% of maximal inspiratory pressure until task failure. In eight subjects, oesophageal (TwPoes), gastric (TwPga) and transdiaphragmatic (TwPdi) pressures were recorded. The TwPmo was significantly reduced (P<0.05) 20 min after both WBE and IRL, from 17.5+/-4.4 to 15.9+/-3.9 cmH(2)O and from 19.4+/-4.9 to 17.7+/-4.5 cmH(2)O, respectively. Subsequently to IRL, the TwPdi decrease was associated with a reduction in TwPoes/TwPga ratio; not after WBE. Independently of the mode of ventilatory loading, inspiratory muscle fatigue was detected. Thus, inspiratory muscle fatigue after WBE can be assessed in normal subjects with a noninvasive technique.     

Influence of the subcutaneous fat layer,as measured by ultrasound,skinfold calipers and BMI,on the EMG amplitude

Surface electromyography (sEMG) is an important tool to estimate muscular activity at work. There is, however, a great inter-individual variation, even in carefully standardized work tasks. The sEMG signal is attenuated in the subcutaneous tissues, differently for each subject, which requires normalization. This is commonly made in relation to a reference contraction, which by itself, however, introduces a variance. A normalization method that is independent of individual motivation, motor control and pain inhibition would be desirable. The aim of the study was to explore the influence of the subcutaneous tissue thickness on sEMG amplitude. Ultrasound measurements of the muscle to skin surface distance were made bilaterally over the trapezius muscle in 12 females. Skinfold caliper measurements from these sites, as well as from four other sites, were made, body mass index (BMI) was recorded, and sEMG was recorded at maximal and submaximal contractions. The muscle–electrode distance, as measured by ultrasound, explained 33% and 31% (on the dominant and non-dominant sides respectively) of the variance of the sEMG activity at a standardized submaximal contraction (average between the sides, 46%); for maximal contractions the explained variance was 21%. Trapezius skinfold measurements showed poor correlations with sEMG. Instead, the mean of skinfold measurements from other sites explained as much as 68% (submaximal contraction). The corresponding figure for BMI was 67%. In conclusion, skinfold thickness explains a major part of the inter-individual variance in sEMG amplitude, and normalization to this measure is a possibility worth further evaluation.     

Oral creatine supplementation decreases plasma markers of adenine nucleotide degradation during a 1-h cycle test

We investigated the effect of oral creatine supplementation (20 g d(-1) for 7 days) on metabolism during a 1-h cycling performance trial. Twenty endurance-trained cyclists participated in this double-blind placebo controlled study. Five days after familiarization with the exercise test, the subjects underwent a baseline muscle biopsy. Thereafter, a cannula was inserted into a forearm vein before performing the baseline maximal 1-h cycle (test 1 (T1)). Blood samples were drawn at regular intervals during exercise and recovery. After creatine (Cr) loading, the muscle biopsy, 1-h cycling test (test 2 (T2)) and blood sampling were repeated. Resting muscle total creatine (TCr), measured by high performance liquid chromatography, was increased (P < 0.001) in the creatine group from 123.0 +/- 3.8 - 159.8 +/- 7.9 mmol kg(-1) dry wt, but was unchanged in the placebo group (126.7 +/- 4.7 - 127.5 +/- 3.6 mmol kg(-1) dry wt). The extent of Cr loading was unrelated to baseline Cr levels (r=0.33, not significant). Supplementation did not significantly improve exercise performance (Cr group: 39.1 +/- 0.9 vs. 39.8 +/- 0.8 km and placebo group: 39.3 +/- 0.8 vs. 39.2 +/- 1.1 km) or change plasma lactate concentrations. Plasma concentrations of ammonia (NH(3)) (P < 0.05) and hypoxanthine (Hx) (P < 0.01) were lower in the Cr group from T1 to T2. Our results indicate that Cr supplementation alters the metabolic response during sustained high-intensity submaximal exercise. Plasma data suggest that nett intramuscular adenine nucleotide degradation may be decreased in the presence of enhanced intramuscular TCr concentration even during submaximal exercise.     

Neuromuscular fatigue after a ski skating marathon.

The aim of this study was to characterize neuromuscular fatigue in knee extensor muscles after a marathon skiing race (mean +/- SD duration = 159.7 +/- 17.9 min). During the 2 days preceding the event and immediately after, maximal percutaneous electrical stimulations (single twitch, 0.5-s tetanus at 20 and 80 Hz) were applied to the femoral nerve of 11 trained skiers. Superimposed twitches were also delivered during maximal voluntary contraction (MVC) to determine maximal voluntary activation (% VA). EMG was recorded from the vastus lateralis muscle. MVC decreased with fatigue from 171.7 +/- 33.7 to 157.3 +/- 35.2 Nm (-8.4%; p < 0.005) while % VA did not change significantly. The RMS measured during MVC and peak-to-peak amplitude of the compound muscle action potential (PPA) from the vastus lateralis decreased with fatigue by about 30% (p < 0.01), but RMS.PPA-1 was similar before and after the ski marathon. Peak tetanus tension at 20 Hz and 80 Hz (P(0)20 and P(0)80, respectively) did not change significantly, but P(0)20.P(0)80-1 increased (p < 0.05) after the ski marathon. Data from electrically evoked single twitches showed greater peak mechanical response, faster rate of force development, and shorter contraction time in the fatigued state. From these results it can be concluded that a ski skating marathon (a) alters slightly but significantly maximal voluntary strength of the knee extensors without affecting central activation, and (b) induces both potentiation and fatigue.     

Acute and chronic responses of the upper airway to inspiratory loading in healthy awake humans: an MRI study

We assessed upper airway responses to acute and chronic inspiratory loading. In Experiment I, 11 healthy subjects underwent T(2)-weighted magnetic resonance imaging (MRI) of upper airway dilator muscles (genioglossus and geniohyoid) before and up to 10 min after a single bout of pressure threshold inspiratory muscle training (IMT) at 60% maximal inspiratory mouth pressure (MIP). T(2) values for genioglossus and geniohyoid were increased versus control (p<0.001), suggesting that these airway dilator muscles are activated in response to acute IMT. In Experiment II, nine subjects underwent 2D-Flash sequence MRI of the upper airway during quiet breathing and while performing single inspirations against resistive loads (10%, 30% and 50% MIP); this procedure was repeated after 6 weeks of IMT. Lateral narrowing of the upper airway occurred at all loads, whilst anteroposterior narrowing occurred at the level of the laryngopharynx at loads > or =30% MIP. Changes in upper airway morphology and narrowing after IMT were undetectable using MRI.     

124例健康成人吸气肌强度和耐力测定

本文测定了124例健康成人最大吸气压和最大持续吸气压,探讨吸气肌强度和耐力的分布及影响因素。发现最大吸气压和最大持续吸气压受年龄、体重和性别响影,吸气肌张力时间指数0.30可作为检测吸气肌疲劳的临界阈值。     

Inspiratory muscles do not limit maximal incremental exercise performance in healthy subjects

We investigated whether the inspiratory muscles affect maximal incremental exercise performance using a placebo-controlled, crossover design. Six cyclists each performed six incremental exercise tests. For three trials, subjects exercised with proportional assist ventilation (PAV). For the remaining three trials, subjects underwent sham respiratory muscle unloading (placebo). Inspiratory muscle pressure (P(mus)) was reduced with PAV (-35.9+/-2.3% versus placebo; P<0.05). Furthermore, V(O2) and perceptions of dyspnea and limb discomfort at submaximal exercise intensities were significantly reduced with PAV. Peak power output, however, was not different between placebo and PAV (324+/-4W versus 326+/-4W; P>0.05). Diaphragm fatigue (bilateral phrenic nerve stimulation) did not occur in placebo. In conclusion, substantially unloading the inspiratory muscles did not affect maximal incremental exercise performance. Therefore, our data do not support a role for either inspiratory muscle work or fatigue per se in the limitation of maximal incremental exercise.     

Glucose infusion attenuates muscle fatigue in rat plantaris muscle during prolonged indirect stimulation in situ

Carbohydrate ingestion increases both endurance time to exhaustion during prolonged exercise, and the ability to perform resistance exercise. The mechanism(s) underlying the increased performance following glucose ingestion remain(s) unclear. The purpose of the present study was to verify the hypothesis that glucose infusion could attenuate peripheral muscle fatigue in the anaesthetized rat during prolonged indirect electrical stimulation in situ. For this purpose the plantaris muscle was electrically stimulated (50 Hz for 200 ms every 2.7 s; 5 V; pulse width, 0.05 ms) in situ through the sciatic nerve to perform concentric contractions for 60 min while infusing intravenously either saline alone (7.25 ml kg(-1) h(-1)), or saline and glucose (1 g kg(-1) h(-1): plasma glucose 11 +/- 1.1, vs. 4.9 +/- 0.2 mm with infusion of saline) (8 rats per group). Glucose infusion attenuated the reduction in submaximal peak dynamic force (55% decrease vs. 70% decrease in rats infused with saline alone, P < 0.05). In a third group of rats (n = 8), infusion of glucose 30 min after the start of stimulation partially restored submaximal peak dynamic force (P < 0.05). Maximum dynamic and isometric forces at the end of the period of stimulation were also higher (P < 0.05) in rats infused with glucose (4.0 +/- 0.2 and 4.3 +/- 0.2 N, respectively) than saline alone (3.0 +/- 0.2 and 3.5 +/- 0.2 N, respectively). The beneficial effect of glucose infusion on peripheral muscle force during prolonged stimulation was not associated with a reduction in muscle glycogen utilisation, nor with a reduction of fatigue at the neuromuscular junction, as assessed through maximal direct muscle stimulation (200 Hz for 200 ms; 150 V; pulse width, 0.05 ms). However, changes in M-wave peak-to-peak amplitude, duration and total area suggest that glucose infusion, and/or the associated increase in plasma insulin concentration, may prevent the deterioration of electrical properties of the muscle fibre membrane.     

Skeletal muscle and cardiac changes with training in patients with angina pectoris

Cardiovascular and skeletal muscle adaptations were studied before and after 6 mo of physical training in patients with coronary artery disease and exertional angina pectoris. Symptom-limited exercise capacity increased by 41% (470 +/- 30 to 665 +/- 35 kg.m.min-1; n = 29, P less than 0.001) with training as did skeletal muscle succinate dehydrogenase activity (1.75 +/- 0.24 to 3.31 +/- 0.24 IU; n = 23, P less than 0.001) and the areas of muscle fibers (type I from 43.6 +/- 3.3 to 54.4 +/- 3.3 micrometers 2 X 10(2); n = 21, P less than 0.05 and type II from 43.9 +/- 2.4 to 57.2 +/- 5.1 micrometers 2 X 10(2); P less than 0.01). At the same submaximal exercise intensity (mean 355 +/- 100 km.m.min-1), plasma catecholamines (1.31 +/- 0.14 to 1.07 +/- 0.09 ng.ml-1; n = 13, P less than 0.05), heart rate (115 +/- 3 to 97 +/- 3 beats/min; n = 29, P less than 0.001), and systolic blood pressure (171 +/- 4 to 143 +2- 4 mmHg; n = 29, P less than 0.001) were significantly reduced after training. Maximal coronary sinus blood flow (192 +/- 10 to 208 +/- 9 ml.min-1; n = 29, P less than 0.05) and left ventricular oxygen consumption (23.2 +/- 1.5 to 25.8 +/- 1.6 ml.min-1; n = 24, P less than 0.05) were increased by 8 and 11%, respectively, after training. The improvement in exercise capacity with training in patients with exercise is secondary to a reduction in myocardial oxygen requirements during subangina levels of exercise and partly to a small increase in maximal myocardial oxygen consumption. The skeletal muscle adaptations with training were not related to other indices of training such as the reduced exercise heart rate or increased symptom-limited exercise capacity.     

The influence of inspiratory muscle work history and specific inspiratory muscle training upon human limb muscle fatigue

The purpose of this study was to assess the influence of the work history of the inspiratory muscles upon the fatigue characteristics of the plantar flexors (PF). We hypothesized that under conditions where the inspiratory muscle metaboreflex has been elicited, PF fatigue would be hastened due to peripheral vasoconstriction. Eight volunteers undertook seven test conditions, two of which followed 4 week of inspiratory muscle training (IMT). The inspiratory metaboreflex was induced by inspiring against a calibrated flow resistor. We measured torque and EMG during isometric PF exercise at 85% of maximal voluntary contraction (MVC) torque. Supramaximal twitches were superimposed upon MVC efforts at 1 min intervals (MVC_TI); twitch interpolation assessed the level of central activation. PF was terminated ( T _lim) when MVC_TI was <50% of baseline MVC. PF T _lim was significantly shorter than control (9.93 ± 1.95 min) in the presence of a leg cuff inflated to 140 mmHg (4.89 ± 1.78 min; P = 0.006), as well as when PF was preceded immediately by fatiguing inspiratory muscle work (6.28 ± 2.24 min; P = 0.009). Resting the inspiratory muscles for 30 min restored the PF T _lim to control. After 4 weeks, IMT, inspiratory muscle work at the same absolute intensity did not influence PF T _lim, but T _lim was significantly shorter at the same relative intensity. The data are the first to provide evidence that the inspiratory muscle metaboreflex accelerates the rate of calf fatigue during PF, and that IMT attenuates this effect.     

The influence of muscle fiber type composition on the patterns of responses for electromyographic and mechanomyographic amplitude and mean power frequency during a fatiguing submaximal isometric muscle action

The purpose of this investigation was to examine the influence of muscle fiber type composition on the patterns of responses for electromyographic (EMG) and mechanomyographic (MMG) amplitude and mean power frequency (MPF) during a fatiguing submaximal isometric muscle action. Five resistance-trained (mean +/- SD age = 23.2 +/- 3.7 yrs) and five aerobically-trained (mean +/- SD age = 32.6 +/- 5.2 yrs) men volunteered to perform a fatiguing, 30-sec submaximal isometric muscle action of the leg extensors at 50% of the maximum voluntary contraction (MVC). Muscle biopsies from the vastus lateralis revealed that the myosin heavy chain (MHC) composition for the resistance-trained subjects was 59.0 +/- 4.2% Type IIa, 0.1 +/- 0.1% Type IIx, and 40.9 +/- 4.3% Type I. The aerobically-trained subjects had 27.4 +/- 7.8% Type IIa, 0.0 +/- 0.0% Type IIx, and 72.6 +/- 7.8% Type I MHC. The patterns of responses and mean values for absolute and normalized EMG amplitude and MPF during the fatiguing muscle action were similar for the resistance-trained and aerobically-trained subjects. The resistance-trained subjects demonstrated relatively stable levels for absolute and normalized MMG amplitude and MPF across time, but the aerobically-trained subjects showed increases in MMG amplitude and decreases in MMG MPE The absolute MMG amplitude and MPF values for the resistance-trained subjects were also greater than those for the aerobi-cally-trained subjects. These findings suggested that unlike surface EMG, MMG may be a useful noninvasive technique for examining fatigue-related differences in muscle fiber type composition.     

A comparison of inspiratory muscle fatigue following maximal exercise in moderately trained males and females

Exercise-induced inspiratory muscle fatigue (IMF) has been reported in males but there are few reports of IMF in females. It is not known if a gender difference exists for inspiratory muscle strength following heavy exercise, as is reported in locomotor muscles. Therefore, the relationship between fatigue and subsequent recovery of maximal inspiratory pressure (MIP) following exercise to maximal oxygen consumption was examined in a group of moderately trained males and females. Eighteen males (23±3 years; mean ± SD) and 16 females (23±2 years) completed ten MIP and ten maximal handgrip (HG) strength maneuvers to establish baseline. Post-exercise MIP and HG were assessed successively immediately following a progressive intensity test on a cycle ergometer and at 1, 2, 3, 4, 5, 10, and 15 min. relative to fat-free mass was not statistically different between males (62±7 ml kg^–1 min^–1) and females (60±8 ml kg^–1 min^–1). Males had higher absolute MIP values than females at all time intervals (P<0.05). Immediately following exercise, MIP was significantly reduced in both genders (M=83±16%; F=78±15% of baseline) but HG values were not different than resting values. MIP values remained depressed for both males and females throughout the 15 min (P<0.05). Differences for MIP between males and females were not statistically significant at any measurement time (P>0.05). The findings in this study conclude that IMF, observed immediately following maximal exercise, demonstrated the same pattern of recovery for both genders.     

Inspiratory muscle training attenuates the human respiratory muscle metaboreflex

We hypothesized that inspiratory muscle training (IMT) would attenuate the sympathetically mediated heart rate (HR) and mean arterial pressure (MAP) increases normally observed during fatiguing inspiratory muscle work. An experimental group (Exp, n = 8) performed IMT 6 days per week for 5 weeks at 50% of maximal inspiratory pressure (MIP), while a control group (Sham, n = 8) performed IMT at 10% MIP. Pre- and post-training, subjects underwent a eucapnic resistive breathing task (RBT) (breathing frequency = 15 breaths min⁻¹, duty cycle = 0.70) while HR and MAP were continuously monitored. Following IMT, MIP increased significantly ( P < 0.05) in the Exp group (−125 ± 10 to −146 ± 12 cmH₂O; mean ± s.e.m. ) but not in the Sham group (−141 ± 11 to −148 ± 11 cmH₂O). Prior to IMT, the RBT resulted in significant increases in HR (Sham: 59 ± 2 to 83 ± 4 beats min⁻¹; Exp: 62 ± 3 to 83 ± 4 beats min⁻¹) and MAP (Sham: 88 ± 2 to 106 ± 3 mmHg; Exp: 84 ± 1 to 99 ± 3 mmHg) in both groups relative to rest. Following IMT, the Sham group observed similar HR and MAP responses to the RBT while the Exp group failed to increase HR and MAP to the same extent as before (HR: 59 ± 3 to 74 ± 2 beats min⁻¹; MAP: 84 ± 1 to 89 ± 2 mmHg). This attenuated cardiovascular response suggests a blunted sympatho-excitation to resistive inspiratory work. We attribute our findings to a reduced activity of chemosensitive afferents within the inspiratory muscles and may provide a mechanism for some of the whole-body exercise endurance improvements associated with IMT.     

Decreased motor unit discharge rate in the potentiated human tibialis anterior muscle

Aim The purpose of this study was to examine the influence of post‐activation potentiation (PAP), the transient increase in low‐frequency isometric force observed after muscle activity, on motor unit discharge rates measured during submaximal contractions. Methods A quadrifilar needle electrode was inserted into the tibialis anterior muscle to determine discharge rate of individual motor units while monopolar electrodes were used to monitor the root‐mean‐square (RMS) and mean power frequency (MPF) of the surface EMG signal. Control (unpotentiated) and experimental (potentiated) measures were obtained during a 5 s voluntary contraction at 50% of maximal. In between these measures, subjects performed a 10 s maximal voluntary contraction (MVC) to induce PAP. Results All subjects data are reported as means ± SEM (n = 10). Compared to baseline values measured prior to the MVC, isometric twitch force measured immediately after the MVC was increased by 260 ± 16% (day 3). Motor unit discharge rate in the potentiated tibialis anterior muscle decreased by approx. 10%, from 20.3 ± 0.8 (before) to 18.3 ± 0.99 pps (P = 0.01) (after). Moreover, the MPF was decreased by approx. 9% (from 58.1 ± 2.84 to 53.6 ± 2.85 Hz; P = 0.01) in the potentiated tibialis anterior. On the other hand, consistent with the absence of fatigue during the MVC, the RMS signal was not altered in the potentiated tibialis anterior (0.29 ± 0.03 vs. 0.33 ± 0.04 mV; P = 0.07). Conclusion Motor unit discharge rates determined during a brief, submaximal contraction were decreased in the potentiated human tibialis anterior muscle.     

Mitochondrial efficiency in rat skeletal muscle: influence of respiration rate, substrate and muscle type

AIM: To investigate the hypothesis that mitochondrial efficiency (i.e. P/O ratio) is higher in type I than in type II fibres during submaximal rates of respiration. METHODS: Mitochondria were isolated from rat soleus and extensor digitorum longus (EDL) muscles, representing type I and type II fibres, respectively. Mitochondrial efficiency (P/O ratio) was determined with pyruvate (Pyr) or palmitoyl-l-carnitine (PC) during submaximal (constant rate of adenosine diphosphate infusion) and maximal (V(max), state 3) rates of respiration and fitted to monoexponential functions. RESULTS: There was no difference in V(max) between PC and Pyr in soleus but in EDL V(max) with PC was only 58% of that with Pyr. The activity of 3-hydroxyacyl-CoA dehydrogenase was threefold higher in soleus than in EDL. P/O ratio at V(max) was 8-9% lower with PC [2.33 +/- 0.02 (soleus) and 2.30 +/- 0.02 (EDL)] than with Pyr [2.52 +/- 0.03 (soleus) and 2.54 +/- 0.03 (EDL)] but not different between the two muscles (P > 0.05). P/O ratio was low at low rates of respiration and increased exponentially when the rate of respiration increased. The asymptotes of the curves were similar to P/O ratio at V(max). P/O ratio at submaximal respirations was not different between soleus and EDL neither with Pyr nor with PC. CONCLUSION: Mitochondrial efficiency, as determined in vitro, was not significantly different in the two fibre types neither at V(max) nor at submaximal rates of respiration. The low V(max) for PC oxidation in EDL may relate to low activity of beta-oxidation.     

Maximal activation of the human diaphragm but not inspiratory intercostal muscles during static inspiratory efforts

It is widely held that transdiaphragmatic pressure is a reliable index of the extent of central activation of the diaphragm but the maximal voluntary transdiaphragmatic pressure is lower during inspiratory than expulsive efforts. To determine whether the diaphragm is fully activated during the two manoeuvres supramaximal stimuli were delivered to both phrenic nerves during maximal efforts. No discernible twitch was evoked during 30-55% of attempted maximal efforts with either voluntary manoeuvre. Thus the difference in maximal transdiaphragmatic pressure between the manoeuvres must reflect changes in chest-wall geometry or mechanics rather than in the phrenic motor outflow. Inspiratory intercostal muscle activity was consistently submaximal during maximal inspiratory efforts.