Manda L. Keller

Supraspinal Fatigue Is Similar in Men and Women for a Low-Force Fatiguing Contraction

PURPOSE This study determined the contribution of supraspinal fatigue to the sex difference in neuromuscular fatigue for a low-intensity fatiguing contraction. Because women have greater motor responses to arousal than men, we also examined whether cortical and motor nerve stimulation, techniques used to quantify central fatigue, would alter the sex difference in muscle fatigue. METHODS In study 1, cortical stimulation was elicited during maximal voluntary contractions (MVC) before and after a submaximal isometric contraction at 20% MVC with the elbow flexor muscles in 29 young adults (20 ± 2.6 yr, 14 men). In study 2, 10 men and 10 women (19.1 ± 2.9 yr) performed a fatiguing contraction in the presence and absence of cortical and motor nerve stimulation. RESULTS Study 1: Men had a briefer time to task failure than women (P = 0.009). Voluntary activation was reduced after the fatiguing contraction (P < 0.001) similarly for men and women. Motor-evoked potential area and the EMG silent period increased similarly with fatigue for both sexes. Peak relaxation rates, however, were greater for men than women and were associated with time to task failure (P < 0.05). Force fluctuations, RPE, HR, and mean arterial pressure increased at a greater rate for men than for women during the fatiguing contraction (P < 0.05). Study 2: Time to task failure, force fluctuations, and all other physiological variables assessed were similar for the control session and stimulation session (P > 0.05) for both men and women. CONCLUSIONS Supraspinal fatigue was similar for men and women after the low-force fatiguing contraction, and the sex difference in muscle fatigue was associated with peripheral mechanisms. Furthermore, supraspinal fatigue can be quantified in both men and women without influencing motor performance.

Anxiety and stress can predict pain perception following a cognitive stress

Hoeger Bement, M.K., A. Weyer, M. Keller, A. Harkins, and S.K. Hunter. Anxiety and stress can predict pain perception following a cognitive stressor. PHYSIOL BEHAV 000-000. The purpose of this study was to investigate the influence of a cognitive stressor on pain perception and determine individual characteristics that may predict the pain response. Twenty-five subjects participated in three sessions: one familiarization and two experimental. The experimental sessions involved measurement of pain perception before and after 1) mental math tasks (stressor session) and 2) quiet rest (control session). Pain threshold and ratings were assessed with a mechanical noxious stimulus. Changes in stress and anxiety were examined with self-reported and physiological measures including questionnaires, visual analogue scales, and salivary cortisol levels. During the control session, stress and anxiety decreased and pain reports remain unchanged. During the stressor session, stress and anxiety increased and pain reports were variable among subjects. Based on the pain response to mental math, subjects were divided into three groups (increase, decrease or no change in pain). The increase-pain group (n=8) had lower baseline stress and anxiety, lower baseline pain reports, and large anxiety response following the mental math. In contrast, the decrease-pain group (n=9) had higher baseline stress and anxiety levels, higher baseline pain reports, and a large increase in cortisol levels. Thus, the differential response in the changes in pain perception was related to anxiety and stress levels prior to and during the cognitive stressor, indicating that psychosocial characteristics can help determine the stress-induced pain response.

The role of the menstrual cycle phase in pain perception before and after an isometric fatiguing contraction

The purpose of this study was to compare exercise-induced analgesia in young women after a fatiguing isometric contraction during different phases of the menstrual cycle. Twenty female subjects performed a submaximal (25% maximal voluntary contraction) isometric contraction until task failure during both the mid-follicular and mid-luteal phases of their menstrual cycle. Pain perception (i.e., pain threshold and pain ratings) was measured before and after the isometric fatiguing contraction. Other measures included mean arterial pressure, heart rate, and anxiety levels. Time to task failure of the fatiguing contraction was similar for the two phases of the menstrual cycle. Following the performance of the isometric contraction: (1) pain thresholds increased and pain ratings decreased; (2) anxiety levels increased; and (3) mean arterial pressure and heart rate increased. These changes were not dependent on the phase of the menstrual cycle. Thus, the menstrual cycle phase does not influence the magnitude of exercise-induced analgesia.

Supraspinal Fatigue Impedes Recovery From a Low-Intensity Sustained Contraction in Old Adults

This study determined the contribution of supraspinal fatigue and contractile properties to the age difference in neuromuscular fatigue during and recovery from a low-intensity sustained contraction. Cortical stimulation was used to evoke measures of voluntary activation and muscle relaxation during and after a contraction sustained at 20% of maximal voluntary contraction (MVC) until task failure with elbow flexor muscles in 14 young adults (20.9 ± 3.6 yr, 7 men) and 14 old adults (71.6 ± 5.4 yr, 7 men). Old adults exhibited a longer time to task failure than the young adults (23.8 ± 9.0 vs. 11.5 ± 3.9 min, respectively, P < 0.001). The time to failure was associated with initial peak rates of relaxation of muscle fibers and pressor response (P < 0.05). Increments in torque (superimposed twitch; SIT) generated by transcranial magnetic stimulation (TMS) during brief MVCs, increased during the fatiguing contraction (P < 0.001) and then decreased during recovery (P = 0.02). The increase in the SIT was greater for the old adults than the young adults during the fatiguing contraction and recovery (P < 0.05). Recovery of MVC torque was less for old than young adults at 10 min post-fatiguing contraction (75.1 ± 8.7 vs. 83.6 ± 7.8% of control MVC, respectively, P = 0.01) and was associated with the recovery of the SIT (r = -0.59, r(2) = 0.35, P < 0.001). Motor evoked potential (MEP) amplitude and the silent period elicited during the fatiguing contraction increased less for old adults than young adults (P < 0.05). The greater fatigue resistance with age during a low-intensity sustained contraction was attributable to mechanisms located within the muscle. Recovery of maximal strength after the low-intensity fatiguing contraction however, was impeded more for old adults than young because of greater supraspinal fatigue. Recovery of strength could be an important variable to consider in exercise prescription of old populations.

Understanding the mechanisms of neuromuscular fatigue with paired-pulse stimulation

Transcranial magnetic stimulation (TMS), using both single and paired-pulse techniques, has provided insight in understanding the balance of excitation and inhibition within the corticomotor system during maximal and submaximal exercise. A single pulse of TMS produces a short-latency motor-evoked potential (MEP), indicating the net excitation of the motor pathway which includes the balance within supraspinal (cortical) and spinal sources. The MEP is followed by an interruption of ongoing electromyographic (EMG) activity known as the silent period. Classically, the silent period has been interpreted to represent the inhibition within the motor cortex (i.e. intracortical inhibition) which probably involves γ-aminobutyric acid (GABAB) receptors. Intracortical inhibition is clinically important to understand because chronic pain disorders (migraines) and psychiatric disorders (post-traumatic stress disorder) have demonstrated less cortical inhibition during a voluntary contraction while other disorders such as Parkinsons and Alzheimers diseases have demonstrated abnormal corticospinal excitatory circuits at rest. Additionally, the fatigue-induced reduction in cortical inhibition may be related to the motor impairments during exercise. The alterations in the MEP and silent period during exercise indicate that the central nervous system (CNS) may necessitate increases in both excitatory and inhibitory activity during fatiguing contractions; however, the mechanisms underlying these effects are yet to be completely understood.