Anita D. Christie

The purpose of this study was to perform a novel exploration of sex‐based differences in various single‐ and paired‐pulse transcranial magnetic stimulation (TMS)‐based measures of corticospinal excitability and inhibition. Thirty participants (15 females) attended one laboratory visit where responses evoked by single‐ and paired‐pulse TMS were recorded using electromyography from the first dorsal interosseous muscle. Excitability was assessed via the motor‐evoked potential (MEP) and intracortical facilitation (ICF). Inhibition was assessed via the cortical silent period (CSP), short‐interval intracortical inhibition (SICI), and long‐interval intracortical inhibition (LICI). Each measure was compared between sexes. Overall, males and females did not significantly differ in excitability (MEP: P = 0.070; ICF: P = 0.194). Males displayed significantly greater inhibition compared to females for the SICI ( P = 0.016) and LICI ( P = 0.003) measures but not CSP ( P = 0.612). These findings suggest that sex may be an important consideration for some (SICI and LICI), but not all TMS‐based measures of corticospinal excitability and inhibition.

This study provides a unique, comprehensive comparison of the control of force and motor unit firing rates between a muscle in the upper and lower limb, and between sexes. Our findings identify differences in neural control between muscles and sexes; however, these differences were not observed in force control.

Motor unit firing rate (MUFR) and pennation angle were measured concurrently in males and females from submaximal to maximal intensities. Thirty participants, (16 female, 14 male) performed isometric dorsiflexion contractions at 20%, 40%, 60%, 80%, and 100% of maximal voluntary contraction (MVC). During each contraction, measures of MUFR were obtained via surface electromyography decomposition, and muscle fiber pennation angle and fascicle length were obtained via ultrasound. There was no significant interaction effect of sex and contraction intensity present for mean MUFR ( p = 0.24), pennation angle ( p = 0.98), or fascicle length ( p = 0.81). Males had greater mean MUFR ( p < 0.001), pennation angle ( p = 0.02), and fascicle length ( p = 0.03) compared to females. In general, mean MUFR ( p < 0.001) and pennation angle ( p < 0.02) increased with increasing contraction intensity; however, fascicle length ( p = 0.30) was similar across contraction intensities. There were no significant relationships between mean MUFR and pennation angle for males ( r = 0.18, p = 0.13) or females ( r = 0.20, p = 0.09), nor between mean MUFR and fascicle length for males ( r = 0.20, p = 0.10) or females ( r = 0.21, p = 0.07). Although sex-related differences in MUFR, pennation angle, and fascicle length were present, there were no relationships between MUFR and the muscle properties. These results suggest that sex-related differences in mean MUFR may not be associated with the sex-related differences in the muscle architectural properties currently investigated.

We examined sex‐specific changes to neuromuscular function in response to mental fatigue. Twenty‐five young, healthy adults (13 F, 12 M) performed a mentally fatiguing task and control condition for 30 min on two separate days. Neuromuscular function was assessed in the first dorsal interosseous before and after each condition. Reaction time decreased after the mentally fatiguing task (P < 0.001, η² = 0.47). Males and females reported higher levels of subjective fatigue after the mentally fatiguing task (P < 0.02, η² = 0.07). Motor unit firing rate increased over time at 10% maximal voluntary contraction (MVC; P < 0.04, η² = 0.16), and decreased over time at 50% MVC (P < 0.01, η² = 0.14); however, this was not unique to either sex. During a variable force contraction, error decreased in females over time and increased in males (P < 0.05, η² = 0.13), although changes were not unique to mental fatigue. Physiological function of the neuromuscular system was not specifically affected by mental fatigue in males or females.

Sex-related differences in changes in functional fitness over time were longitudinally assessed in older adults participating in a group-based multimodal exercise program. From a database, functional fitness scores were obtained for 89 older adults (71.6 ± 6.5 years old) who had completed two assessments, 5–8 years apart. Lower body strength, upper body strength, aerobic endurance, flexibility, and change of direction performances were compared over time and with normative values. Females (p = .02), but not males, had an improvement in upper body strength over time. Females were also more flexible than males at both assessments (p ≤ .02). Of those who had five consecutive assessments, females were more flexible than males (p ≤ .05) and had a faster change of direction ability (p < .001). When compared with normative values, our results indicate that typical time-related functional fitness loss can be attenuated with group exercise. Our results further support the need to tailor exercise prescription according to the individual.

It has been consistently demonstrated that females report greater numbers of concussions in sex-comparable sports and take longer to recover from concussive symptoms than males. However, it is unknown if the neurophysiological consequences of concussion may contribute to these sex differences in concussion symptoms and recovery. The purpose of this study was to examine potential sex-related differences in neurophysiology in healthy and concussed individuals. Twenty-one (nine F) concussed individuals (20.9 ± 4.1 years; CONC) and twenty-one age-, sex-, height-, weight-, and activity-matched controls (21.2 ± 4.2 years; CONT) participated in the study. The CONC group reported to the lab within 72 h, 1-week, and 2-weeks post-injury and the CONT group followed a similar measurement schedule. Using transcranial magnetic stimulation, motor evoked potential (MEP) amplitude and cortical silent period (CSP) duration were measured from the first dorsal interosseous muscle to assess corticospinal excitability and inhibition, respectively. There were no significant differences across time (p ≥ 0.13) or between the CONC and CONT group in MEP amplitude (p = 0.72) or CSP duration (p = 0.54). Overall, males (119.08 ± 29.91 ms) had significantly longer CSP durations compared with females (101.24 ± 33.43 ms), indicating greater corticospinal inhibition in males, regardless of injury status (p = 0.04). An important and novel finding of this study was the lack of differences in these neurophysiological measures between males and females following concussion. To our knowledge, this is the first study to document greater corticospinal inhibition in males compared with females.