Jason M. DeFreitas

The natural progression of age can result in motor neuron degeneration. Consequently, this leads to slowing of nerve conduction, denervation, and reduced motor function. Slower neural conduction can negatively alter an individual’s response time, which could increase the risk of falls. Further investigation is needed to determine the potential role exercise interventions may afford in mitigating age-related nerve deterioration. Purpose: The purpose was two-fold: first, to determine the effects of resistance training on nerve conduction velocity (NCV), and second, to determine if changes in NCV are dependent on age. We hypothesized that training would result in faster nerve conduction in both young and older adults, albeit to a lesser extent in older adults. Methods: Forty-eight subjects (18-84 yrs) completed this study (training: n = 14 younger, 14 older; control: n = 12 younger, 8 older). Median motor NCV and maximal strength were recorded before and after 4-weeks of handgrip training in both limbs. Training was conducted 3×/week with the use of a grip training kit. Results: Mixed-factorial ANOVAs revealed significant increases in NCV for both the young (p < 0.001, Cohen’s d = 0.749) and older training groups (p < 0.001, Cohen’s d = 0.679), but neither control groups (young: p = 0.353, Cohen’s d = 0.326; older: p = 0.108, Cohen’s d = -0.184). Conclusions: The results of this study suggest that resistance training may be a viable method to counteract age-related nerve deterioration. These outcomes have the potential to improve quality of life and generate greater independence for our older populations.

SignificanceA shared understanding of terminology is essential for clear scientific communication and minimizing misconceptions. This is particularly challenging in rapidly expanding, interdisciplinary domains that utilize functional near-infrared spectroscopy (fNIRS), where researchers come from diverse backgrounds and apply their expertise in fields such as engineering, neuroscience, and psychology.AimThe fNIRS Glossary Project was established to develop a community-sourced glossary covering key fNIRS terms, including those related to the continuous-wave (CW), frequency-domain (FD), and time-domain (TD) NIRS techniques.ApproachThe glossary was collaboratively developed by a diverse group of 76 fNIRS researchers, representing a wide range of career stages (from PhD students to experts) and disciplines. This collaborative process, structured across five phases, ensured the glossary’s depth and comprehensiveness.ResultsThe glossary features over 300 terms categorized into six key domains: analysis, experimental design, hardware, neuroscience, mathematics, and physics. It also includes abbreviations, symbols, synonyms, references, alternative definitions, and figures where relevant.ConclusionsThe fNIRS glossary provides a community-sourced resource that facilitates education and effective scientific communication within the fNIRS community and related fields. By lowering barriers to learning and engaging with fNIRS, the glossary is poised to benefit a broad spectrum of researchers, including those with limited access to educational resources.

It is well known that the natural progression of age can result in motor neuron degeneration. Consequently, this leads to slowing of nerve conduction, denervation, and reduced motor function. Slower neural conduction can negatively alter an individual’s response time, which could increase the risk of falls. Further investigation is needed to determine the potential role exercise interventions may afford in mitigating age-related nerve deterioration. The purpose was two-fold: first, to determine the effects of resistance training on nerve conduction velocity (NCV), and second, to determine if changes in NCV are dependent on age. We hypothesized that training would result in faster nerves in both young and older adults, albeit to a lesser extent in older adults. Forty-eight subjects (18-84 yrs) completed this study (n = 26 younger, 22 older). Motor NCV and maximal strength were recorded before and after 4-weeks of handgrip training in both limbs. Training was conducted 3×/week with the use of a grip training kit. Mixed-factorial ANOVAs revealed significant increases in NCV for both the young ( p < 0.001) and older training groups ( p < 0.001), but neither control groups (p > 0.05). The young training group showed increased handgrip strength after four weeks ( p = 0.004), while the other groups did not. The results of this study suggest that resistance training may be a viable method to counteract age-related nerve deterioration. These outcomes have the potential to improve quality of life and generate greater independence for our older populations. NEW & NOTEWORTHY: Aging is characterized by a slowing in nerve conduction speed, which could potentially affect how quickly an individual responds to unexpected incidents. Consequently, this slowing could increase the risk of falls and injuries. This study utilized resistance training as an intervention to mitigate age-related nerve deterioration. We show that motor nerve conduction velocity significantly increases in both young and older adults after 4-weeks of training.

Introduction: The neural pathways that contribute to force production in humans are currently poorly understood, as the relative roles of the corticospinal tract and brainstem pathways, such as the reticulospinal tract (RST), vary substantially across species. Using functional magnetic resonance imaging (fMRI) we aimed to measure activation in the pontine reticular nuclei (PRN) during different submaximal handgrip contractions to determine the potential role of the PRN in force modulation. Methods: Thirteen neurologically intact participants (age: 28 ± 6 years) performed unilateral handgrip contractions at 25%, 50%, 75% of maximum voluntary contraction during brain scans. We quantified the magnitude of PRN activation from the contralateral and ipsilateral sides during each of the three contraction intensities. Results: A repeated measures ANOVA demonstrated a significant main effect of force (p = 0.012, η_p² = 0.307) for PRN activation, independent of side (i.e., activation increased with force for both contralateral and ipsilateral nuclei). Further analyses of these data involved calculating the linear slope between the magnitude of activation and handgrip force for each region of interest (ROI) at the individual-level. One-sample t-tests on the slopes revealed significant group-level scaling for the PRN bilaterally, but only the ipsilateral PRN remained significant after correcting for multiple comparisons. Conclusions: We show evidence of task dependent activation in the PRN that was positively related to handgrip force. These data build on a growing body of literature that highlights the RST as a functionally relevant motor pathway for force modulation in humans.

The determination of active motor threshold (AMT) is a critical step in transcranial magnetic stimulation (TMS) research protocols involving voluntary muscle contractions. As AMT is frequently determined using an absolute electromyographic (EMG) threshold (e.g., 200µV peak-to-peak amplitude), wide variation in EMG recordings across participants has given reason to consider a relative threshold (e.g., = 2× background EMG) for AMT determination. However, these approaches have not been systemically compared. PURPOSE: We sought to compare the AMT estimations derived from absolute and relative criteria commonly used to determine AMT in the quadriceps muscles, and assess the test-retest reliability of each approach (absolute = 200µV vs. relative = 2× background EMG). METHODS: Eighteen young adults (9 males and 9 females; mean ± SD age = 23 ± 2 years) visited the research laboratory on two occasions. All testing was conducted on the dominant limb. During each laboratory visit, maximal voluntary isometric contraction (MVIC) quadriceps torque was measured, with all subsequent TMS procedures conducted as participants maintained 10% of MVIC torque. AMT was derived from each criterion using motor evoked potentials recorded from the vastus lateralis (VL) and defined as the lowest stimulator output (SO%) needed to meet the specified criteria within ≥ 5/10 pulses. The order of criteria (i.e., absolute vs. relative) was randomized during the first laboratory visit, and counterbalanced during the second visit. A paired samples t -test, 95% confidence intervals and the effect size were used to compare mean differences in AMT estimations obtained from each criterion during the second laboratory visit. Paired samples t -tests, intraclass correlation coefficients (ICC _2,1 ), standard errors of measurement (SEMs), and the minimal difference (MD) scores were calculated to assess test-retest reliability of each AMT criterion. RESULTS: Differences between the AMT criteria were small and not statistically significant (absolute criterion mean = 48.9%, relative criterion mean = 47.4%; p = .309, Cohen’s d = 0.247). The absolute criterion demonstrated moderate to excellent reliability (ICC _2,1 = .866 [0.648 – 0.950], SEM = 7.9%, MD = 10.4%), but higher AMTs were observed in the second visit compared to the first ( p = 0.043). The relative criteria demonstrated good-to-excellent test-retest reliability (ICC _2,1 = .894 [0.746 – 0.959], SEM = 6.9%, MD = 8.9%) and AMTs were not different between visits ( p = 0.420). CONCLUSION: Quantifying AMT with an absolute voltage threshold of 200µV peak-to-peak amplitude and a relative voltage threshold 2× background EMG resulted in similar estimations within a single testing session. However, the relative voltage criterion demonstrated superior test-retest reliability. TMS researchers aiming to track corticospinal characteristics across visits should consider implementing relative criterion approaches during their AMT determination protocol.

The use of functional near-infrared spectroscopy (fNIRS) for brain imaging during human movement continues to increase. This technology measures brain activity non-invasively using near-infrared light, is highly portable, and robust to motion artifact. However, the spatial resolution of fNIRS is lower than that of other imaging modalities. It is unclear whether fNIRS has sufficient spatial resolution to differentiate nearby areas of the cortex, such as the leg areas of the motor cortex. Therefore, the purpose of this study was to determine fNIRS’ ability to discern laterality of lower body contractions. Activity in the primary motor cortex was recorded in forty participants (mean = 23.4 years, SD = 4.5, female = 23, male = 17) while performing unilateral lower body contractions. Contractions were performed at 30% of maximal force against a handheld dynamometer. These contractions included knee extension, knee flexion, dorsiflexion, and plantar flexion of the left and right legs. fNIRS signals were recorded and stored for offline processing and analysis. Channels of fNIRS data were grouped into regions of interest, with five tolerance conditions ranging from strict to lenient. Four of five tolerance conditions resulted in significant differences in cortical activation between hemispheres. During right leg contractions, the left hemisphere was more active than the right hemisphere. Similarly, during left leg contractions, the right hemisphere was more active than the left hemisphere. These results suggest that fNIRS has sufficient spatial resolution to distinguish laterality of lower body contractions. This makes fNIRS an attractive technology in research and clinical applications in which laterality of brain activity is required during lower body activity.

Introduction: The neural pathways that contribute to force production in humans are currently poorly understood, as the relative roles of the corticospinal tract and brainstem pathways, such as the reticulospinal tract (RST), vary substantially across species. Using functional magnetic resonance imaging (fMRI) we aimed to measure activation in the pontine reticular nuclei during different submaximal handgrip contractions to determine the potential role of the RST in force modulation. Methods: Thirteen neurologically intact participants (age: 28 ± 6 years) performed unilateral handgrip contractions at 25%, 50%, 75% of maximum voluntary contraction during brain scans. We quantified the magnitude of RST activation from the contralateral and ipsilateral sides in addition to the contralateral primary motor cortex during each of the three contraction intensities. Results: A repeated measures ANOVA demonstrated a significant main effect of force ( p = 0.012, η _p ² = 0.307) for RST activation, independent of side (i.e., activation increased with force for both contralateral and ipsilateral nuclei). Further analyses of these data involved calculating the linear slope between the magnitude of activation and handgrip force for each ROI at the individual-level. One-sample t- tests on the slopes revealed significant group-level scaling for the RST bilaterally, but only the ipsilateral RST remained significant after correcting for multiple comparisons. Conclusions: Here, we show evidence of task dependent activation in the RST pontine nuclei that was positively related to handgrip force. These data build on a growing body of literature that highlights the RST as a functionally relevant motor pathway for force modulation in humans. New & Noteworthy: In this short report, we used a task-based fMRI paradigm to show that activity in the reticulospinal tract, but not the contralateral motor cortex, scales linearly with increasing force during a handgrip task. These findings directly support recently proposed hypotheses that the reticulospinal tract may play an important role in modulating force production in humans.

OBJECTIVE: Having participants perform 50 maximal isokinetic contractions, as introduced by Thorstensson and Karlsson (1976), is to this day one of the most commonly used fatigue protocols. Purpose: To determine to what extent a Thorstensson fatiguing protocol affects isokinetic and isometric performance characteristics. METHODS: Twenty-five college-aged men performed an isokinetic fatigue protocol consisting of 50 maximal elbow flexions at a moderate speed (180[ ] [ ]s[ ]). Pre- and post-tests were used to calculate fatigue indices (FI%) for the following variables: isometric and isokinetic peak torque (PT), isometric rate of torque development (RTD), and maximum acceleration (ACC[ ]). RESULTS: Each of the four dependent variables significantly decreased with fatigue (each demonstrated [ ] 0.001), but not of the same magnitude. Isometric PT FI ([ ]18.7%) was significantly less than isokinetic PT FI ([ ]45.1%, [ ] 0.001) and ACC[ ] FI ([ ]26.3%) was significantly less than isometric RTD FI ([ ]54.9%, [ ] 0.001). There was a significant positive relationship between isometric and isokinetic PT FI% ([ ] 0.60, [ ] 0.002) as well as between isometric PT FI% and RTD FI% ([ ] 0.40, [ ] 0.046). There was no significant relationship observed between RTD FI% and ACC[ ] FI% nor between isokinetic PT FI% and ACC[ ] FI%. CONCLUSIONS: This investigation observed different patterns of response in maximal strength between isometric and isokinetic assessments following a isokinetic fatigue protocol. Additionally, the ability to rapidly generate strength and velocity had significantly different responses to fatigue between isometric and dynamic assessments. Due to these different responses, we recommend that assessments of fatigue at least include the same testing modality as the modality of the fatigue protocol.