Counterweight mass influences single-leg cycling biomechanics.

INTRODUCTION: Single-leg cycling is a commonly used intervention in exercise physiology that has applications in exercise training and rehabilitation. The addition of a counterweight to the contralateral pedal helps single-leg cycling mimic cycling patterns of double-leg cycling. To date, no research has tested (a) the influence of a wide range of counterweight masses on a person's cycling biomechanics and (b) the optimal counterweight mass to emulate double-leg cycling. OBJECTIVES: The purpose of this study was to determine the effects of varying counterweights on the kinematics (joint angles) and kinetics (joint moments, work) of cycling using a 3D analysis. METHODS: Twelve participants cycled at 50W or 100W with different counterweight masses (0 to 30 lbs, 2.5 lbs increments), while we analyzed the pedal force data, joint angles, joint moments, and joint power of the lower limb using 3D motion capture and 3D instrumented pedals to create participant-specific musculoskeletal models. RESULTS: The results showed that no single-leg cycling condition truly emulated double-leg cycling with respect to all measured variables, namely pedal forces (p ≤ 0.05), joint angles (p ≤ 0.05), joint moments(p ≤ 0.05), and joint powers (p ≤ 0.05), but higher counterweights resulted in single-leg cycling that was statistically similar (p > 0.05), but descriptively, asymptotically approached the biomechanics of double-leg cycling. CONCLUSION: We suggest that a 20-lb counterweight is a conservative estimate of the counterweight required for using single-leg cycling in exercise physiology studies, but further modifications are needed to the cycle ergometer for the biomechanics of single-leg cycling to match those of double-leg cycling.

Comparison of muscle activity of the lower limbs while running on different treadmill models.

Treadmill running is a common method of exercise and to study human locomotion. Research has examined the kinematics and kinetics of overground and treadmill running, but there has been less focus on the levels of muscle activity during treadmill running. We investigated if muscle activity is different while running overground compared to running on a variety of treadmills. A total of 11 healthy individuals ran at 3 speeds (2.6, 3.6, 4.5 m/s) under 4 different running conditions (3 treadmills, overground). The three treadmills included a typical home exercise treadmill, a midsize commercial research treadmill, and a large, instrumented research treadmill. Surface EMG of the tibialis anterior (TA), gastrocnemius medialis (GM), rectus femoris (RF) and biceps femoris (BF) muscles were measured for each running condition. The integrated EMG was computed for each running condition for the stance and swing phase, as well as 100 ms before and after the heel-strike. Friedman analysis revealed significant effects during the stance phase for GM and RF at all speeds, such that muscle activation was lower on the treadmills relative to overground. During the stance phase at faster speeds, the muscle activity was higher for the TA and lower for the BF while running on the different treadmills compared to overground running. Before heel-strike, the TA was significantly less active during treadmill compared to overground running at 2.6 m/s and the RF showed significantly higher activity at 3.6 m/s and 4.5 m/s while running on the different treadmills. Summarizing, differences were mainly observed between the different treadmill conditions relative to overground running. Muscle activation differences between the different treadmill conditions were observed at faster running speeds for RF during the pre-heel-strike phase only. Different types of treadmills with different mechanical properties affects the muscle activity during stance phase as well as in preparation to heel-strike. Additionally, the muscle activity is greater during overground compared to treadmill running during the stance phase for the GM, BF, and RF.

Validation of Polar ElixirTM Pulse Oximeter against Arterial Blood Gases during Stepwise Steady State Inspired Hypoxia.

PURPOSE: The purpose of this study was to evaluate the accuracy of peripheral oxygen saturation (SpO2) measurements from Polar ElixirTM pulse oximetry technology compared to arterial oxygen saturation (SaO2) measurements during acute stepwise steady state inspired hypoxia at rest. A post hoc objective was to determine if SpO2 measurements could be improved by recalibrating the Polar ElixirTM algorithm with SaO2 values from a random subset of participants. METHODS: The International Organization for Standardization (ISO) protocol (ISO 80601-2-61:2017) for evaluating the SpO2 accuracy of pulse oximeter equipment was followed whereby five plateaus of SaO2 between 70-100% were achieved using stepwise reductions in inspired O2 during supine rest. Blood samples drawn through a radial arterial catheter from 25 participants were first used to compare SaO2 to SpO2 measurements from Polar ElixirTM. Then the Polar ElixirTM algorithm was recalibrated using SaO2 data from 13 random participants and SpO2 estimates were recalculated for the other 12 participants. For SaO2 values between 70-100%, root mean square error (RMSE), intraclass correlations (ICC), Pearson correlations, and Bland-Altman plots were used to assess the accuracy, agreement, and strength of relationship between SaO2 values and SpO2 values from Polar ElixirTM. RESULTS: The initial RMSE for Polar ElixirTM was 4.13%. After recalibrating the algorithm, the RMSE was improved to 2.67%. The ICC revealed excellent levels of agreement between SaO2 and Polar ElixirTM SpO2 values both before (ICC(3,1) = 0.837, df = 574, p < 0.001) and after (ICC(3,1) = 0.942, df = 287, p < 0.001) recalibration. CONCLUSIONS: Relative to ISO standards, Polar ElixirTM yielded accurate SpO2 measurements during stepwise inspired hypoxia at rest when compared to SaO2 values, which were improved by recalibrating the algorithm using a subset of the SaO2 data.

2021 ISB World Athletics Award for Biomechanics: The Subtalar Joint Maintains "Spring-Like" Function While Running in Footwear That Perturbs Foot Pronation.

Humans have the remarkable ability to run over variable terrains. During locomotion, however, humans are unstable in the mediolateral direction and this instability must be controlled actively-a goal that could be achieved in more ways than one. Walking research indicates that the subtalar joint absorbs energy in early stance and returns it in late stance, an attribute that is credited to the tibialis posterior muscle-tendon unit. The purpose of this study was to determine how humans (n = 11) adapt to mediolateral perturbations induced by custom-made 3D-printed "footwear" that either enhanced or reduced pronation of the subtalar joint (modeled as motion in 3 planes) while running (3 m/s). In all conditions, the subtalar joint absorbed energy (ie, negative mechanical work) in early stance followed by an immediate return of energy (ie, positive mechanical work) in late stance, demonstrating a "spring-like" behavior. These effects increased and decreased in footwear conditions that enhanced or reduced pronation (P ≤ .05), respectively. Of the recorded muscles, the tibialis posterior (P ≤ .05) appeared to actively change its activation in concert with the changes in joint energetics. We suggest that the "spring-like" behavior of the subtalar joint may be an inherent function that enables the lower limb to respond to mediolateral instabilities during running.

The Effects of Increased Midsole Bending Stiffness of Sport Shoes on Muscle-Tendon Unit Shortening and Shortening Velocity: a Randomised Crossover Trial in Recreational Male Runners.

BACKGROUND: Individual compliances of the foot-shoe interface have been suggested to store and release elastic strain energy via ligamentous and tendinous structures or by increased midsole bending stiffness (MBS), compression stiffness, and resilience of running shoes. It is unknown, however, how these compliances interact with each other when the MBS of a running shoe is increased. The purpose of this study was to investigate how structures of the foot-shoe interface are influenced during running by changes to the MBS of sport shoes. METHODS: A randomised crossover trial was performed, where 13 male, recreational runners ran on an instrumented treadmill at 3.5 m·s-1 while motion capture was used to estimate foot arch, plantar muscle-tendon unit (pMTU), and shank muscle-tendon unit (sMTU) behaviour in two conditions: (1) control shoe and (2) the same shoe with carbon fibre plates inserted to increase the MBS. RESULTS: Running in a shoe with increased MBS resulted in less deformation of the arch (mean ± SD; stiff, 7.26 ± 1.78°; control, 8.84 ± 2.87°; p ≤ 0.05), reduced pMTU shortening (stiff, 4.39 ± 1.59 mm; control, 6.46 ± 1.42 mm; p ≤ 0.01), and lower shortening velocities of the pMTU (stiff, - 0.21 ± 0.03 m·s-1; control, - 0.30 ± 0.05 m·s-1; p ≤ 0.01) and sMTU (stiff, - 0.35 ± 0.08 m·s-1; control, - 0.45 ± 0.11 m·s-1; p ≤ 0.001) compared to a control condition. The positive and net work performed at the arch and pMTU, and the net work at the sMTU were significantly lower in the stiff compared to the control condition. CONCLUSION: The findings of this study showed that if a compliance of the foot-shoe interface is altered during running (e.g. by increasing the MBS of a shoe), the mechanics of other structures change as well. This could potentially affect long-distance running performance.

Cumulative Metrics of Tendon Load and Damage Vary Discordantly with Running Speed.

PURPOSE: Cumulative load has become a popular metric in running biomechanics research to account for potential spatiotemporal changes associated with different locomotion strategies. This study investigated how incorporating mechanical fatigue principles into Achilles tendon cumulative load measurements affected their relationship with running speed. METHODS: Achilles tendon forces and strains were estimated from a dynamometry/ultrasound session followed by a motion capture session, where participants ran at three speeds. Three cumulative measures of increasing complexity were calculated using Achilles tendon force/strain: 1) cumulative load, defined as the product of the stance phase time integral of Achilles tendon force/strain and the stride count for 1 km of running; 2) cumulative damage, which accounted for the nonlinear relationship between load magnitude and fatigue life by exponentially weighting the time integral of Achilles tendon force/strain before multiplication with stride count; and (3) the probability of fatigue failure, which expanded upon the cumulative damage measure of Achilles tendon strain by fitting a probabilistic Weibull model to existing fatigue life data to account for the inherent variability that exists in the fatigue life of biological samples. RESULTS: Cumulative load measures significantly decreased with running speed, whereas the cumulative damage and probabilistic measures either increased or did not change significantly with running speed. CONCLUSIONS: The choice of cumulative metric has an important influence on the interpretation of overuse injury risk with changes in running speed. Although cumulative load metrics certainly provide meaningful information about the load experienced over a given distance, they do not account for the tissue damage incurred by such load. Cumulative load metrics should therefore be interpreted with caution when making inferences to overuse injury risk.

Reliability and validity of a novel device for quantifying ankle dorsiflexion force in persons with multiple sclerosis.

BACKGROUND: With emerging treatment modalities and therapeutics for Multiple Sclerosis (MS), there is a critical need for improved measures of disability. Routine clinical practice and trials will benefit from devices that are capable of objectively quantifying muscle strength/weakness. We have developed a device for measuring Tibialis Anterior (TA) force that is both objective and easy to use - the Rapid Objective Quantification - TA (ROQ-TA). The purpose of this study was to determine the reliability and validity of the ROQ-TA versus Manual Muscle Testing and Isokinetic Dynamometry (IKD) for evaluating TA force in persons with MS (PwMS). METHODS: Ankle dorsiflexion of 20 PwMS was assessed by three modalities: ROQ-TA, MMT, and IKD over 2 testing sessions. ICC(2,1) values and Bland-Altman plots were used to assess reliability and validity of the ROQ-TA. RESULTS: The ICC(2,1) for reliability for the ROQ-TA was found to be 0.884 (0.690-0.957) while the IKD produced a similar ICC(2,1) of 0.919 (0.784-0.970). The mean difference between the two sessions for the ROQ-TA was -6.4 N with limits of agreement of 42.5 to -55.4 N as inferred by the Bland-Altman plots. With respect to validity, the ROQ-TA versus IKD yielded similar values for both sessions- the mean bias was 9.3 N (SE range: -3.4 to 22 N) for session 1 and 9.9 N for session 2 (SE range: -3.2 to 23.0 N). The ICC(2,1) values between the two devices were in moderate agreement - session 1: 0.579 (-0.125-0.843) and session 2: 0.490 (-0.363-0.809). CONCLUSION: The ROQ-TA is a valid and highly reliable device to test dorsiflexion force in PwMS.

Does increased midsole bending stiffness of sport shoes redistribute lower limb joint work during running?

OBJECTIVES: To investigate if lower limb joint work is redistributed when running in a shoe with increased midsole bending stiffness compared to a control shoe. DESIGN: Within-subject with two conditions: (1) commercially available running shoe and (2) the same shoe with carbon fibre inserts to increase midsole bending stiffness. METHODS: Thirteen male, recreational runners ran on an instrumented treadmill at 3.5m/s in each of the two shoe conditions while motion capture and force platform data were collected. Positive and negative metatarsophalangeal (MTP), ankle, knee, and hip joint work were calculated and statistically compared between conditions. RESULTS: Running in the stiff condition (with carbon fibre inserts) resulted in significantly more positive work and less negative work at the MTP joint, and less positive work at the knee joint. CONCLUSIONS: Increased midsole bending stiffness resulted in a redistribution of positive lower limb joint work from the knee to the MTP joint. A larger MTP joint plantarflexor moment due to increased vGRF at the instant of peak positive power and an earlier onset of MTP joint plantarflexion velocity were identified as the reasons for lower limb joint work redistribution.

The Submaximal Lateral Shuffle Test: A reliability and sensitivity analysis.

Lateral ankle stability and how it changes in different footwear has been investigated for years. Research, however, has shown a lack of reliability or sensitivity of available methodologies. This study aimed to evaluate the test-retest reliability and sensitivity of a novel lateral stability protocol, the Submaximal Lateral Shuffle Test (SLST). We recruited 11 and 40 participants to assess reliability and sensitivity of the SLST, respectively. Participants performed the SLST in footwear that differed in collar height and upper stiffness. ICC values showed good to excellent reliability in peak ankle angles and moments, ground reaction forces, impulses, stance time, and performance time. Significantly lower peak inversion and adduction angles and lower medio-lateral push off peak forces were found in the high cut shoes compared to the low cut shoes. The medio-lateral landing peak force showed lower forces in the high cut shoes. The smallest worthwhile change indicated meaningful differences in 70.0-82.5% of participants for inversion, adduction, medio-lateral landing peak, and push off peak forces. These results, however, were not systematic such that there was not a consistent direction of the difference for all participants. In conclusion, the SLST is a promising protocol to further investigate lateral stability in footwear.

Portable fixed dynamometry to quantify ankle dorsiflexion force.

INTRODUCTION: Quantifying muscle strength is critical in clinical and research settings. A rapid and objective method is ideal. The primary objective of this study was to examine the reliability of a novel device, the rapid objective quantification- tibialis anterior (ROQ-TA), which quantifies the dorsiflexion force of the tibialis anterior, and to assess its validity against isokinetic dynamometry (IKD). METHODS: Ankle dorsiflexion of 20 healthy subjects was assessed by 3 modalities, ROQ-TA, manual muscle testing, and isokinetic dynamometry, over 2 testing sessions. RESULTS: The intraclass correlation coefficient [ICC(2,1) ] for reliability was 0.872 (0.677-0.949) for the ROQ-TA and 0.892 (0.728-0.957) for IKD. For validity, the ICC(2,1) values for the ROQ-TA and IKD were in good agreement, with 0.672 (0.17-0.87) in the first testing session and 0.769 (0.42-0.91) in the second session. DISCUSSION: The ROQ-TA is a valid and reliable device to test ankle dorsiflexion force in a healthy population. Muscle Nerve, 2018.

The effect of torsional shoe sole stiffness on knee moment and gross efficiency in cycling.

Altering torsional stiffness of cycling shoe soles may be a novel approach to reducing knee joint moments and overuse injuries during cycling. We set out to determine if the magnitude of three-dimensional knee moments were different between cycling shoe soles with different torsional stiffnesses. Eight trained male cyclists cycled at 90% lactate threshold power output in one of two cycling shoe conditions in a randomized crossover design. The shoe sole was considered torsionally flexible (FLEX) compared to a relatively stiffer (STIFF) sole. Gross efficiency (GE) and knee joint moments were quantified. No significant effect of shoe condition was seen in GE (21.4 ± 1.1% and 20.9 ± 1.6% for FLEX and STIFF, respectively, P = 0.12), nor in three-dimensional knee moments. 4 of the 8 subjects had reduced knee moments in at least 2 of the 3 moment directions. These "responders" were significantly shorter (1.73 ± 0.02 m vs 1.81 ± 0.04 m, P = 0.017) and had a higher relative maximal aerobic power (MAP) (4.6 ± 0.3 W∙kg-1 vs 3.9 ± 0.3 W∙kg-1, P = 0.024) compared to non-responders. These results suggest that certain shoe characteristics may influence certain individuals differently because these participants belong to different "functional groups"; certain individuals may respond positively to FLEX, while others may not. Further studies should test this proposed hypothesis.

Force measurements during running on different instrumented treadmills.

One method to determine the forces produced during running is to conduct extensive kinematic and kinetic analysis. These analyses can be performed by having an individual perform repeated over-ground running trials or simply run continuously on an instrumented treadmill. The forces produced during over-ground running may not be the same as the forces during treadmill running and these differences could be attributed to a number of factors, including the design of the instrumented treadmill. The purpose of this paper was to determine whether there are differences in force measurements on different instrumented treadmill setups in comparison to over-ground running and to correct for any of these differences using a theoretical model. 11 participants ran on three different treadmills and performed over-ground running at 2.7, 3.6, and 4.5 m/s. Ground reaction forces were measured via force plates and an instrumented pressure insole. We found that the magnitude of the vertical ground reaction force differed between the three treadmills and over-ground running. The difference in ground reaction forces estimated by the pressure insole and the treadmill-force-plate system or instrumented treadmill can be explained by a three degree of freedom mechanical model of a person running on a treadmill and this model could potentially be used to correct for errors in force measurement from instrumented treadmills. The model included a force plate, a treadmill, and a wobbling mass with varying natural frequencies and damping characteristics, and constant masses. These findings provide researchers a method to correct forces from an instrumented treadmill set-up to determine a close approximation of the actual forces experienced by a participant during treadmill running.

Motor Unit Action Potential Clustering-Theoretical Consideration for Muscle Activation during a Motor Task.

During dynamic or sustained isometric contractions, bursts of muscle activity appear in the electromyography (EMG) signal. Theoretically, these bursts of activity likely occur because motor units are constrained to fire temporally close to one another and thus the impulses are "clustered" with short delays to elicit bursts of muscle activity. The purpose of this study was to investigate whether a sequence comprised of "clustered" motor unit action potentials (MUAP) can explain spectral and amplitude changes of the EMG during a simulated motor task. This question would be difficult to answer experimentally and thus, required a model to study this type of muscle activation pattern. To this end, we modeled two EMG signals, whereby a single MUAP was either convolved with a randomly distributed impulse train (EMG-rand) or a "clustered" sequence of impulses (EMG-clust). The clustering occurred in windows lasting 5-100 ms. A final mixed signal of EMG-clust and EMG-rand, with ratios (1:1-1:10), was also modeled. A ratio of 1:1 would indicate that 50% of MUAP were randomly distributed, while 50% of "clustered" MUAP occurred in a given time window (5-100 ms). The results of the model showed that clustering MUAP caused a downshift in the mean power frequency (i.e., ~30 Hz) with the largest shift occurring with a cluster window of 10 ms. The mean frequency shift was largest when the ratio of EMG-clust to EMG-rand was high. Further, the clustering of MUAP also caused a substantial increase in the amplitude of the EMG signal. This model potentially explains an activation pattern that changes the EMG spectra during a motor task and thus, a potential activation pattern of muscles observed experimentally. Changes in EMG measurements during fatiguing conditions are typically attributed to slowing of conduction velocity but could, per this model, also result from changes of the clustering of MUAP. From a clinical standpoint, this type of muscle activation pattern might help describe the pathological movement issues in people with Parkinson's disease or essential tremor. Based on our model, researchers moving forward should consider how MUAP clustering influences EMG spectral and amplitude measurements and how these changes influence movements.

Short-latency afferent inhibition determined by the sensory afferent volley.

Short-latency afferent inhibition (SAI) is characterized by the suppression of the transcranial magnetic stimulation motor evoked potential (MEP) by the cortical arrival of a somatosensory afferent volley. It remains unknown whether the magnitude of SAI reflects changes in the sensory afferent volley, similar to that observed for somatosensory evoked potentials (SEPs). The present study investigated stimulus-response relationships between sensory nerve action potentials (SNAPs), SAI, and SEPs and their interrelatedness. Experiment 1 (n = 23, age 23 ± 1.5 yr) investigated the stimulus-response profile for SEPs and SAI in the flexor carpi radialis muscle after stimulation of the mixed median nerve at the wrist using ∼25%, 50%, 75%, and 100% of the maximum SNAP and at 1.2× and 2.4× motor threshold (the latter equated to 100% of the maximum SNAP). Experiment 2 (n = 20, age 23.1 ± 2 yr) probed SEPs and SAI stimulus-response relationships after stimulation of the cutaneous digital nerve at ∼25%, 50%, 75%, and 100% of the maximum SNAP recorded at the elbow. Results indicate that, for both nerve types, SAI magnitude is dependent on the volume of the sensory afferent volley and ceases to increase once all afferent fibers within the nerve are recruited. Furthermore, for both nerve types, the magnitudes of SAI and SEPs are related such that an increase in excitation within somatosensory cortex is associated with an increase in the magnitude of afferent-induced MEP inhibition.

Combining Multiple Data Acquisition Systems to Study Corticospinal Output and Multi-segment Biomechanics.

Transcranial magnetic stimulation techniques allow for an in-depth investigation into the neural mechanisms that underpin human behavior. To date, the use of TMS to study human movement, has been limited by the challenges related to precisely timing the delivery of TMS to features of the unfolding movement and, also, by accurately characterizing kinematics and kinetics. To overcome these technical challenges, TMS delivery and acquisition systems should be integrated with an online motion tracking system. The present manuscript details technical innovations that integrate multiple acquisition systems to facilitate and advance the use of TMS to study human movement. Using commercially available software and hardware systems, a step-by-step approach to both the hardware assembly and the software scripts necessary to perform TMS studies triggered by specific features of a movement is provided. The approach is focused on the study of upper limb, planar, multi-joint reaching movements. However, the same integrative system is amenable to a multitude of sophisticated studies of human motor control.

Intralimb coordination in children with and without developmental coordination disorder in one-handed catching.

There is a gap in the literature in regard to analysis of intralimb coordination exhibited by children with (n = 10) and without developmental coordination disorder (DCD; n = 9) in 1-handed catching. The functional data showed that children without DCD (M age = 10.6 years, SD = 1.08 years) were nearly perfect. Children with DCD (M age = 11.0 years, SD = 1.16 years) caught significantly fewer balls, and this was despite the fact that not all of them had difficulties organizing their actions at intralimb level of coordination. The analysis of the coinciding actions revealed differences at the distal (elbow-wrist relations), but not at the proximal joints where both groups exhibited decoupling between the shoulder and elbow joints. Large variability within the groups also suggested that the notion of universal coordinative tendencies at intralimb level of coordination has to be treated with caution. This is particularly true for children with DCD as different subgroups emerged in the present sample.

Modulation of short-latency afferent inhibition depends on digit and task-relevance.

Short-latency afferent inhibition (SAI) occurs when a single transcranial magnetic stimulation (TMS) pulse delivered over the primary motor cortex is preceded by peripheral electrical nerve stimulation at a short inter-stimulus interval (∼ 20-28 ms). SAI has been extensively examined at rest, but few studies have examined how this circuit functions in the context of performing a motor task and if this circuit may contribute to surround inhibition. The present study investigated SAI in a muscle involved versus uninvolved in a motor task and specifically during three pre-movement phases; two movement preparation phases between a "warning" and "go" cue and one movement initiation phase between a "go" cue and EMG onset. SAI was tested in the first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles in twelve individuals. In a second experiment, the origin of SAI modulation was investigated by measuring H-reflex amplitudes from FDI and ADM during the motor task. The data indicate that changes in SAI occurred predominantly in the movement initiation phase during which SAI modulation depended on the specific digit involved. Specifically, the greatest reduction in SAI occurred when FDI was involved in the task. In contrast, these effects were not present in ADM. Changes in SAI were primarily mediated via supraspinal mechanisms during movement preparation, while both supraspinal and spinal mechanisms contributed to SAI reduction during movement initiation.

Continuous theta-burst stimulation over primary somatosensory cortex modulates short-latency afferent inhibition.

OBJECTIVE: The present study investigated the effects of continuous theta-burst stimulation (cTBS) over primary somatosensory (SI) and motor (M1) cortices on motor-evoked potentials (MEPs) and short-latency afferent inhibition (SAI). METHODS: MEPs and SAI were recorded from the first dorsal interosseous (FDI) muscle of the right hand following 30Hz cTBS over left-hemisphere SI and M1 delivered to the same participants in separate sessions. Measurements were taken before and up to 60min following cTBS. RESULTS: CTBS over M1 suppressed MEPs and did not alter SAI. In contrast cTBS over SI facilitated MEPs and decreased median and digital nerve evoked SAI. CONCLUSIONS: These findings indicate that SAI amplitude is influenced by cTBS over SI but not M1, suggesting an important role for SI in the modulation of this circuit. These data provide further evidence that cTBS over SI versus M1 has opposite effects on corticospinal excitability. SIGNIFICANCE: To date, plasticity-inducing TMS protocols delivered over M1 have failed to modulate SAI, and the present research continues to support these findings. However, in young adults, cTBS over SI acts to reduce SAI and simultaneously increase corticospinal excitability. Future studies may investigate the potential to modulate SAI via targeting neural activity within SI.