There is no nonvolitional method of assessing quadriceps strength which both supramaximally activates the muscle and is acceptable to subjects. In 10 normal subjects and 10 patients with suspected muscle weakness we used magnetic stimulation of the femoral nerve to elicit an isometric twitch and measured twitch tension (TwQ), surface electromyogram in addition to the maximum voluntary contraction force (MVC). Supramaximality was achieved in all subjects at a mean of 83% of maximum stimulator output. When supramaximal, TwQ was reproducible (mean coefficient of variation 3.6%, range 0.7–10.9) and correlated well with MVC (r2 = 0.83, P < 0.001). In 7 normal subjects we measured TwQ before and after a fatiguing protocol; after 20 min TwQ was a mean of 55% (range 29–77%) of baseline and remained substantially reduced at 90 min. Magnetic femoral nerve stimulation is a painless, supramaximal method of assessing quadriceps strength and fatigue which is likely to be of value in clinical and physiological studies. © 1996 John Wiley & Sons, Inc.

The diaphragm is normally the main inspiratory muscle and diaphragm strength in chronic obstructive pulmonary disease (COPD) is therefore of interest. We assessed diaphragm strength in 20 patients with severe stable COPD (mean FEV1 0.61, mean thoracic gas volume [Vtg] 5.31) and seven normal control subjects, measuring both maximal sniff transdiaphragmatic pressure (sniff Pdi(max)) and twitch transdiaphragmatic pressure (Tw Pdi) elicited by cervical magnetic stimulation (CMS) of the phrenic nerve roots at FRC. Acute-on-chronic hyperinflation was examined in four patients. Mean Tw Pdi in patients and control subjects was 18.5 cm H2O and 25.4 cm H2O, respectively (p < 0.01), and mean sniff Pdi was 81.9 cm H2O and 118 cm H2O, respectively (p < 0.001). Reduction in mean intrathoracic pressures was more marked; twitch esophageal pressure (Tw Pes) was -7.3 cm H2O and -16.3 cm H2O, respectively (p < 0.001) and sniff Pes was -67 cm H2O and -97.8 cm H2O (p < 0.001). During acute-on-chronic hyperinflation there was a linear negative correlation of Tw Pdi with increasing lung volume of 3.5 cm H2O/L. The ability of the diaphragm to generate transdiaphragmatic, and particularly a negative intrathoracic, pressure is reduced in COPD and these changes are exaggerated with acute-on-chronic hyperinflation.

Background: The effects of providing ventilatory assistance to patients with severe chronic obstructive pulmonary disease (COPD) during a high intensity outpatient cycle exercise programme were examined. Methods: Nineteen patients (17 men) with severe COPD (mean (SD) forced expiratory volume in 1 second (FEV 1 ) 27 (7)% predicted) underwent a 6 week supervised outpatient cycle exercise programme. Ten patients were randomised to exercise with ventilatory assistance using proportional assist ventilation (PAV) and nine (two women) to exercise unaided. Before and after training patients performed a maximal symptom limited incremental cycle test to determine peak work rate (Wpeak) followed by a constant work rate (CWR) test at 70% of Wpeak achieved in the baseline incremental test. Minute ventilation (VE), heart rate, and arterialised venous plasma lactate concentration [La

We investigated whether bilateral magnetic stimulation of the phrenic nerves from an anterolateral approach (BAMPS) could combine the reproducibility and ease of use of cervical magnetic stimulation (CMS) with the specificity of bilateral electrical stimulation (BES) and whether it could be used in supine subjects. We placed two double 43-mm coils over the phrenic nerves in the neck. BAMPS produced supramaximal phrenic stimulation by electromyogram (EMG) assessment in six of seven subjects. There was no significant difference in the twitch gastric pressure/twitch esophageal pressure ratio (twitch Pgas/Pes) between BAMPS (1.2) and BES (1.3). Both differed from CMS (0.9, p < 0.001). The effect of a change in posture on twitch transdiaphragmatic pressure (TwPdi) and Pgas/Pes ratio was the same for BAMPS and BES. In normal subjects and patients BAMPS correlated significantly with BES (r = 0.97), maximal sniffs (r = 0.85), and CMS (r = 0.92). The mean difference between BAMPS and BES was 0.3 cm H2O (SD = 2.3). Two-minute maximal isocapnic ventilation produced a 19% fall in TwPdi elicited by BAMPS. BAMPS is easy, well tolerated and can be used in the supine subject. TwPdi and partitioning of Pes and Pgas were very close for BAMPS and BES, suggesting similar specificity for the diaphragm.

It has previously been reported that aging is associated with a substantial decrease in diaphragm strength. To test this hypothesis we studied 15 (10 male, 5 female) subjects with a mean age of 29 (range 21 to 40) and 15 elderly (10 male, 5 female) subjects, mean age 73 (range 67 to 81). We measured transdiaphragmatic pressure (Pdi) during a maximal sniff (Sniff Pdi) and during bilateral cervical magnetic stimulation (CMS) of the phrenic nerve roots (Tw Pdi). Additionally in 17 subjects (9 elderly and 8 young) the Pdi elicited by paired CMS (pTw Pdi) was obtained at interstimulus intervals ranging from 10 to 999 ms (1 to 100 Hz). There was considerable overlap between groups. Mean Sniff Pdi in the elderly was 119 cm H2O compared with 136 cm H2O for the young subjects; this represented a median reduction of 18 cm H2O or 13% (p = 0.05, 95% Cl of difference 0 to 33 cm H2O). Mean Twitch Pdi in the elderly was 26.8 cm H2O compared with 35.2 cm H2O, a median reduction of 8 cm H2O or 23% (p = 0.004, 95% Cl 3 to 13 cm H2O). At 10 Hz the elderly tended to generate a higher fraction of the Pdi obtained at 100 Hz than the young, but this trend did not achieve statistical significance (p = 0.11). We conclude that aging is associated with a reduction in diaphragm strength. However the magnitude of the reduction is small and may be offset by a leftward shift of the force-frequency relationship.