Nicolas Caesar Petersen

Reduced muscle activation during exercise related to brain oxygenation and metabolism in humans

Maximal exercise may be limited by central fatigue defined as an inability of the central nervous system to fully recruit the involved muscles. This study evaluated whether a reduction in the cerebral oxygen‐to‐carbohydrate index (OCI) and in the cerebral mitochondrial oxygen tension relate to the ability to generate a maximal voluntary contraction and to the transcranial magnetic stimulated force generation. To determine the role of a reduced OCI and in central fatigue, 16 males performed low intensity, maximal intensity and hypoxic cycling exercise. Exercise fatigue was evaluated by ratings of perceived exertion (RPE), arm maximal voluntary force (MVC), and voluntary activation of elbow flexor muscles assessed with transcranial magnetic stimulation. Low intensity exercise did not produce any indication of central fatigue or marked cerebral metabolic deviations. Exercise in hypoxia ( 0.10) reduced cerebral oxygen delivery ∼25% and decreased 11 ± 4 mmHg (P < 0.001) together with OCI (6.2 ± 0.7 to 4.8 ± 0.5, P < 0.001). RPE increased while MVC and voluntary activation were reduced (P < 0.05). During maximal exercise declined 8 ± 4 mmHg (P < 0.05) and OCI to 3.8 ± 0.5 (P < 0.001). RPE was 18.5, and MVC and voluntary activation were reduced (P < 0.05). We observed no signs of muscular fatigue in the elbow flexors and all control MVCs were similar to resting values. Exhaustive exercise provoked cerebral deoxygenation, metabolic changes and indices of fatigue similar to those observed during exercise in hypoxia indicating that reduced cerebral oxygenation may play a role in the development of central fatigue and may be an exercise capacity limiting factor.

Modulation of reciprocal inhibition between ankle extensors and flexors during walking in man

1 The modulation of disynaptic reciprocal inhibition between antagonistic ankle muscles during walking was investigated in 17 healthy human subjects. Inhibition from ankle dorsiflexors to ankle plantar flexors was evoked by stimulation of the common peroneal nerve (CPN) and evaluated as the stimulus‐induced depression of rectified soleus EMG activity (latency approx. 40 ms) or the short‐latency depression of the soleus H‐reflex (conditioning‐test intervals around 2–3 ms). In some experiments the inhibition from ankle plantar flexors to ankle dorsiflexors was investigated. In these experiments the tibial nerve was stimulated and the amount of inhibition was evaluated from the short‐latency depression of the voluntary rectified tibialis anterior (TA) EMG. 2 The short‐latency inhibition of the soleus H‐reflex following the CPN stimulation (1.1 × motor threshold; MT) was strongly modulated during walking, being large in the swing phase and absent in the stance phase. 3 A smaller amount of EMG depression following the CPN stimulation (1.1–1.2 × MT) was observed in the stance phase of walking as compared to tonic or dynamic plantar flexion at a similar background EMG activity level in standing or sitting subjects. 4 In four subjects a depression of the TA EMG activity was produced by stimulation of the tibial nerve (1.1–1.2 × MT). In all subjects a smaller amount of inhibition was observed in the swing phase of walking as compared to tonic dorsiflexion at a comparable EMG activity level. 5 It is concluded that the transmission in the disynaptic Ia reciprocal pathway between ankle plantar flexors and dorsiflexors is modulated during walking. Inhibition from dorsiflexors to plantar flexors seems to be large in swing and small in stance, whereas inhibition from plantar flexors to dorsiflexors seems to be small in swing.

The effect of transcranial magnetic stimulation on the soleus H reflex during human walking

1 The effect of transcranial magnetic stimulation (TMS) on the soleus H reflex was investigated in the stance phase of walking in seventeen human subjects. For comparison, measurements were also made during quiet standing, matched tonic plantar flexion and matched dynamic plantar flexion. 2 During walking and dynamic plantar flexion subliminal (0.95 times threshold for a motor response in the soleus muscle) TMS evoked a large short‐latency facilitation (onset at conditioning‐test interval: −5 to −1 ms) of the H reflex followed by a later (onset at conditioning‐test interval: 3–16 ms) long‐lasting inhibition. In contrast, during standing and tonic plantar flexion the short‐latency facilitation was either absent or small and the late inhibition was replaced by a long‐lasting facilitation. 3 When grading the intensity of TMS it was found that the short‐latency facilitation had a lower threshold during walking than during standing and tonic plantar flexion. Regardless of the stimulus intensity the late facilitation was never seen during walking and dynamic plantar flexion and the late inhibition was not seen, except for one subject, during standing and tonic plantar flexion. 4 A similar difference in the threshold of the short‐latency facilitation between walking and standing was not observed when the magnetic stimulation was replaced by transcranial electrical stimulation. 5 The lower threshold of the short‐latency facilitation evoked by magnetic but not electrical transcranial stimulation during walking compared with standing suggests that cortical cells with direct motoneuronal connections increase their excitability in relation to human walking. The significance of the differences in the late facilitatory and inhibitory effects during the different tasks is unclear.

Changes in the effect of magnetic brain stimulation accompanying voluntary dynamic contraction in man.

1. The soleus (Sol) H reflex was conditioned by magnetic stimulation of the contralateral motor cortex at rest and during voluntary contraction in healthy human subjects. The intensity of the magnetic stimulus was adjusted so as to have no effect on the H reflex at rest. During tonic voluntary contraction the same magnetic stimulus produced a facilitation with a short latency and a long duration, thus reflecting an increased excitation of Sol motoneurones by the magnetic stimulus during voluntary contraction. 2. The amount of reflex facilitation produced by brain stimulation within the initial 0.5‐1 ms after its onset was investigated at different times during dynamic ramp‐and‐hold plantar flexion. The facilitation was largest at the onset of voluntary activity in the Sol muscle. It then decreased abruptly within 100 ms after the onset of the voluntary contraction. Neither the voluntary Sol activity nor the control H reflex decreased at this time. 3. Electrical stimulation of the brain with the anode placed lateral to the vertex produced a facilitation of the H reflex, which preceded the facilitation evoked by magnetic stimulation by 1‐2 ms. The facilitation produced by the magnetic stimulus occurred or increased at the onset of contraction in relation to rest in all experiments. However, this was the case in only two out of eight experiments, when the brain was stimulated electrically. 4. The size of the reflex facilitation measured at the onset of contraction was larger the faster the contraction. Positive correlations were found between the size of the facilitation and the peak of the first and second derivative of the torque and the peak Sol EMG activity. 5. It is suggested that the observed changes in the size of the short‐latency reflex facilitation produced by magnetic brain stimulation mainly reflects changes in the excitability of corticospinal cells, since similar changes were not observed in the size of the unconditioned Sol H reflex or in the short‐latency reflex facilitation produced by electrical brain stimulation. The data support the hypothesis that fast conducting corticospinal fibres with monosynaptic projections to spinal motoneurones are involved in the initiation of voluntary movement in man.

Evaluation of reciprocal inhibition of the soleus H-reflex during tonic plantar flexion in man

Changes in reciprocal inhibition from ankle dorsiflexors to ankle plantar flexors were evaluated at increasing levels of tonic plantar flexion in 11 healthy subjects. Stimulation of the common peroneal nerve (CPN) evoked a short-latency depression of the rectified and averaged soleus electromyogram (average latency of depression: 40 ms) and a short-latency inhibition of the soleus H-reflex (conditioning-test interval: 2-3 ms). When the intensity of the CPN stimulation was below approximately 1.2 x motor threshold (x MT) the inhibition of both the soleus EMG (expressed as the amount of EMG during the inhibition as percentage of the background EMG) and the soleus H-reflex (expressed as the size of the conditioned reflex as percentage of the control H-reflex size) were seen to decrease with increasing levels of plantar flexion. At intensities of stimulation higher than approximately 1.2 x MT the inhibition of the EMG and the H-reflex was very strong and was not modulated with contraction. It is suggested that the decrease of reciprocal inhibition with increasing levels of plantar flexion is due to a decreased excitability of the Ia inhibitory interneurones which are responsible for the inhibition. It is emphasized that submaximal stimulation is necessary to demonstrate this modulation of inhibition and that the functional contribution of reciprocal inhibition to motor performance cannot be revealed from the amount of inhibition evoked by artificial electrical stimulation of a peripheral nerve.

Probing the corticospinal link between the motor cortex and motoneurones: some neglected aspects of human motor cortical function.

This review considers the operation of the corticospinal system in primates. There is a relatively widespread cortical area containing corticospinal outputs to a single muscle and thus a motoneurone pool receives corticospinal input from a wide region of the cortex. In addition, corticospinal cells themselves have divergent intraspinal branches which innervate more than one motoneuronal pool but the synergistic couplings involving the many hand muscles are likely to be more diverse than can be accommodated simply by fixed patterns of corticospinal divergence. Many studies using transcranial magnetic stimulation of the human motor cortex have highlighted the capacity of the cortex to modify its apparent excitability in response to altered afferent inputs, training and various pathologies. Studies using cortical stimulation at ‘very low’ intensities which elicit only short‐latency suppression of the discharge of motor units have revealed that the rapidly conducting corticospinal axons (stimulated at higher intensities) drive motoneurones in normal voluntary contractions. There are also major non‐linearities generated at a spinal level in the relation between corticospinal output and the output from the motoneurone pool. For example, recent studies have revealed that the efficacy of the human corticospinal connection with motoneurones undergoes activity‐dependent changes which influence the size of voluntary contractions. Hence, corticospinal drives must be sculpted continuously to compensate for the changing functional efficacy of the descending systems which activate the motoneurones. This highlights the need for proprioceptive monitoring of movements to ensure their accurate execution.

Neck pain and postural balance among workers with high postural demands - a cross-sectional study

BackgroundNeck pain is related to impaired postural balance among patients and is highly prevalent among workers with high postural demands, for example, cleaners. We therefore hypothesised, that cleaners with neck pain suffer from postural dysfunction. This cross-sectional study tested if cleaners with neck pain have an impaired postural balance compared with cleaners without neck pain.MethodsPostural balance of 194 cleaners with (n = 85) and without (N = 109) neck pain was studied using three different tests. Success or failure to maintain the standing position for 30 s in unilateral stance was recorded. Participants were asked to stand on a force platform for 30 s in the Romberg position with eyes open and closed. The centre of pressure of the sway was calculated, and separated into a slow (rambling) and fast (trembling) component. Subsequently, the 95% confidence ellipse area (CEA) was calculated. Furthermore a perturbation test was performed.ResultsMore cleaners with neck pain (81%) failed the unilateral stance compared with cleaners without neck pain (61%) (p < 0.01). However, the risk of failure in unilateral stance was statistically elevated in cleaners with concurrent neck/low back pain compared to cleaners without neck/low back pain (p < 0.01), whereas pain at only neck or only low back did not increase the risk. Impaired postural balance, measured as CEA (p < 0.01), rambling (p < 0.05) and trembling (p < 0.05) was observed among cleaners with neck pain in comparison with cleaners without neck pain in the Romberg position with eyes closed, but not with eyes open.ConclusionsPostural balance is impaired among cleaners with neck pain and the current study suggests a particular role of the slow component of postural sway. Furthermore, the unilateral stance test is a simple test to illustrate functional impairment among cleaners with concurrent neck and low back pain.Trial registrationISRCTN96241850

Distribution of non-monosynaptic excitation to early and late recruited units in human forearm muscles

Abstract. The distribution of monosynaptic and non-monosynaptic excitation was investigated within flexor carpi radialis (FCR) and extensor carpi radialis (ECR) motoneurone (MN) pools. FCR H reflexes of different size were conditioned by various conditioning stimuli eliciting different effects: (1) musculocutaneous-induced non-monosynaptic excitation of FCR MNs at the onset of biceps contraction, (2) heteronymous monosynaptic Ia facilitation, (3) reciprocal Ia inhibition, and (4) presynaptic inhibition of Ia terminals. Musculocutaneous-induced non-monosynaptic excitation increased continuously with the size of the unconditioned reflex. In contrast, heteronymous monosynaptic Ia excitation first increased and then decreased, with increases in the unconditioned reflex size, reciprocal inhibition and presynaptic inhibition showing an approximately similar tendency. This suggests that the non-monosynaptic excitation is distributed more evenly to early and late recruited MNs than monosynaptic Ia excitation, reciprocal inhibition and presynaptic inhibition. A different pattern of homonymous radial-induced monosynaptic and non-monosynaptic excitation was also found for individual ECR MNs investigated with the poststimulus time histogram (PSTH) method. Whereas the monosynaptic Ia excitation tended to be most marked in lower threshold MUs, the non-monosynaptic excitation was evenly distributed to lower and higher threshold MUs. We propose that the even distribution of the non-monosynaptic excitation in the motoneuronal pool may be of significance when it is necessary to activate a wide range of MNs more or less simultaneously.

Changes in presumed motor cortical activity during fatiguing muscle contraction in humans.

Aim:  Changes in sensory information from active muscles accompany fatiguing exercise and the force‐generating capacity deteriorates. The central motor commands therefore must adjust depending on the task performed. Muscle potentials evoked by transcranial magnetic stimulation (TMS) change during the course of fatiguing muscle activity, which demonstrates activity changes in cortical or spinal networks during fatiguing exercise. Here, we investigate cortical mechanisms that are actively involved in driving the contracting muscles.

Origin of the low-level EMG during the silent period following transcranial magnetic stimulation

OBJECTIVE The cortical silent period refers to a period of near silence in the electromyogram (EMG) after transcranial magnetic stimulation (TMS) of the motor cortex during contraction. However, low-level EMG of unknown origin is often present. We hypothesised that it arises through spinal reflexes. Sudden lengthening of the muscle as force drops during the silent period could excite muscle spindles and facilitate motoneurones. METHODS Subjects (n = 8) performed maximal isometric, shortening and lengthening contractions of the elbow flexors during which TMS (90-100% output) was delivered over the motor cortex. The rate of flexion during shortening contractions reduced muscle lengthening caused by muscle relaxation. Surface EMG was recorded from biceps brachii and brachioradialis, and the low-level EMG during silent periods produced by TMS was measured. RESULTS Low-level EMG activity was reduced on average by 68% in biceps and 63% in brachioradialis in the shortening contraction compared to all other contraction conditions (p < 0.001). Levels of pre-stimulus EMG were similar between conditions. CONCLUSIONS Muscle lengthening contributes to low-level EMG activity in the silent period, through spinal reflex facilitation by muscle spindle afferents. SIGNIFICANCE The silent period depth is not only dependent on cortical output but also reflex effects evoked by muscle lengthening.