Exercise-induced depression of the diaphragm motor evoked potential is not affected by non-invasive ventilation
Introduction
The reasons for the development of muscle fatigue after exercise are complex. Fatigue is defined as ‘any exercise-induced reduction in the ability to exert muscle force or power’ (Bigland-Ritchie and Woods, 1984). Depending on the protocol, some contribution is generally considered to arise from the central nervous system, and this has been termed supraspinal or ‘central’ fatigue (Gandevia, 2001). Since the diaphragm is the principle muscle of inspiration, mechanisms of diaphragm fatigue are of clinical and physiological interest.
Transcranial magnetic stimulation has been used to investigate the function of supraspinal structures in humans through its ability to excite the cortico-spinal tract. A single stimulus of sufficient magnitude to the motor cortex produces an electrical response in a muscle (motor evoked potential, MEP), although this response may be influenced by other structures in the cortico-spinal pathway as well as the motor cortex.
During and after significant muscular activity, there is a characteristic pattern of change in the MEP. During the contraction and for a short period of time after (seconds to minutes), the MEP is potentiated (Brasil-Neto et al., 1999, Balbi et al., 2002). Following this, a prolonged decline in the MEP amplitude, without change in peripheral transmission or conduction time, typically reaching a nadir between 5 and 15 min, has been described both after exercise of single muscle groups (Brasil-Neto et al., 1993, McKay et al., 1995, Gandevia, 1996, Samii et al., 1996a, Samii et al., 1996b), and also dynamic whole body exercise (Hollge et al., 1997, Fulton et al., 2002, Verin et al., 2004, Jonville et al., 2005). In those experiments that have focused on single muscle groups, the response is thought to be muscle specific.
Our group has previously demonstrated a persistent significant decline in the diaphragm MEP after exhaustive treadmill exercise (Verin et al., 2004). We hypothesised that exercise-induced MEP depression of the diaphragm was related to whole body exercise itself, rather than to the work done by the diaphragm muscle during exercise. If this hypothesis is correct, then it implies that mechanisms which are not necessarily muscle specific are involved in inducing cortical MEP depression.
Two separate studies were performed. In the first study we used non-invasive ventilation to reduce the work of the diaphragm during exercise (study 1—exercise and non-invasive ventilation). In the second study we compared exercise with hyperventilation (study 2—exercise versus hyperventilation).
Section snippets
Subjects
Subjects, who were without neurological, psychiatric, cardiac, respiratory or locomotor disease and on no medication, were requested to refrain from intense exercise in the 24 h before the study, from significant alcohol consumption on the night before the study, and from caffeine on the day of the study. All gave their written informed consent and the experiment was approved by the Royal Brompton Hospital ethics committee and performed according to the declaration of Helsinki.
Study 1
The mean peak of the subjects was 39.9 ± 8.3 ml/kg/min (range 29.6–55.7 ml/kg/min) which was 113 ± 28% predicted (Fig. 3). The mean anaerobic threshold was 132 ± 18 W.
The work of breathing during the three conditions varied significantly (p < 0.001). The pressure time product during exercise was substantially and significantly reduced by the ventilator (101 ± 68 cm H2O/s/min versus 278 ± 95 cm H2O/s/min, p < 0.001). The end-tidal CO2 was not different between the two exercise conditions (exercise with
Discussion
Our data confirm that the motor evoked potential of the diaphragm and quadriceps elicited by transcranial magnetic stimulation is reduced after moderate whole body exercise. Furthermore, the data suggest that the reduction seen in the diaphragmatic motor evoked potential is not proportional to the work of breathing per se. An equivalent reduction of the MEP was seen when the diaphragm was rested during exercise by using a non-invasive ventilator. Moreover hyperventilation, without exercise did
Acknowledgements
We are grateful to all of the subjects who took part in this particularly demanding experiment, especially those who came back for a second go. We are also grateful to the research fellows who allowed us to use all of the equipment in the muscle laboratory for days at a time, preventing them from doing any physiology experiments for themselves. Mark Dayer was supported by a British Heart Foundation project grant (PG/2001042). Dr. Hopkinson was principally supported by the Wellcome Trust (Grant
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