Cross-education is a neural adaptation defined as the increase in strength or functional performance of the untrained contralateral limb after unilateral training of the opposite homologous limb. Since cross-education can improve strength in an untrained limb, there is therapeutic potential to apply cross-education to clinical rehabilitation settings; however, a large gap in the literature remains. The first objective of this thesis was to determine if cross-education could improve strength and functional performance (i.e. active range of motion (AROM), self-reported function) of an immobilized limb using a shoulder sling model in both healthy and injured participants. The second objective was to determine if cross-education could improve strength and functional performance (i.e. AROM, self-reported function) of wrist fracture rehabilitation after unilateral training of the non-fractured limb. Study 1 applied cross-education to non-injured participants who wore a shoulder sling and swathe and strength trained the non-immobilized limb. Strength (NORM dynamometer), muscle size (ultrasound), electromyography, and interpolated twitch were measured. Results showed cross-education increased strength and maintained muscle size in the immobilized limb after training the non-immobilized limb. Study 2 applied cross-education using a clinically relevant at-home resistance tubing shoulder strength training program to healthy participants. Results showed significant cross-education effects for untrained shoulder external and internal rotation strength (handheld dynamometer), and increased muscle size (ultrasound) in the trained supraspinatus and anterior deltoid. Study 3 applied cross-education using the clinically relevant strength training program (in Study 2) to post-shoulder surgery rehabilitation and measured strength (handheld dynamometer), muscle size (ultrasound), AROM (goniometer), and self-reported function (Western Ontario Rotator Cuff Questionnaire) (WORC). Results showed the training group had significantly greater supraspinatus muscle thickness at 6 months post-surgery compared to the control group; however, there were no cross-education effects for strength, AROM, or the WORC. Study 4 applied cross-education during rehabilitation from wrist fractures and measured strength (handgrip dynamometer), AROM (goniometer), and self-reported function (Patient Rated Wrist Evaluation) (PRWE). Results showed cross-education improved strength and AROM in the fractured limb 12 weeks post-fracture. In conclusion, there was evidence for cross-education to benefit a healthy immobilized limb and to use a clinically relevant shoulder strength training program to produce cross-education effects. When cross-education was applied to shoulder surgeries there were improvements in muscle size but no effect for strength, AROM or function. However, when applied to wrist fractures, strength and AROM were improved for the injured limb. These findings represent the first well-controlled evidence that cross-education may improve rehabilitation after unilateral injuries.

Coordinated movements are regulated by the brain, spinal cord and sensory feedback. The interaction between the spinal cord and sensory feedback also play a significant role in facilitating plasticity and functional recovery after neural trauma. Cross-education describes training one side of the limb to enhance the strength of the homologous muscle on the contralateral side. Previous study with chronic stroke participants found significant strength gains in the more affected leg following unilateral dorsiflexion training on the less affected side, which suggested cross-education can be used to boost strength gain when training the more affected side is hard to initiate. However, there is lack of evidence showing cross-education in the arm muscles after stroke and the neural pathways mediating strength cross-education in stroke participants require further study. The modulatory role of sensory feedback in movement control has been studied by using cutaneous stimulation as a proxy of the sensory input from skin. Mechanistic studies on neurological intact participants show that cutaneous reflex pathways are widespread in the cervical and lumbar spinal cord and have a global effect on the muscles in the non-stimulated limbs. In rehabilitation training, sensory enhancement from prolonged electrical stimulation has been used to facilitate training outcomes for those had stroke and other neurological disorders. Therefore, cutaneous pathways may be important in regulating cross-education training-induced strength gain. The purpose of this dissertation was to explore the effects of upper limb cross-education strength training in chronic stroke participants and the role of sensory inputs in regulating intra- and interlimb neural excitability in neurologically intact participants. In the first project (Chapter 2), we explored the efficacy of cross-education strength training in wrist extensor muscles of chronic stroke participants. Strength improvements were found bilaterally with altered excitabilities in the cutaneous pathways on the untrained side. These results show the potential role of cutaneous pathways in mediating strength transfer after unilateral strength training which led us to further explore the factors may affect the cutaneous modulation. In neurologically intact participants, we investigated the effects forearm position (Chapter 4), stimulation trigger mode and parameters (Chapter 5) on the cutaneous reflexes in the stimulated limb. Following the findings from Chapter 3, 4, and 5, the interlimb effects of self-induced sensory enhancement on the cutaneous reflexes were examined in Chapter 6. Taken together, data from this thesis confirms the clinical application of cross-education in strength training after stroke. It addresses that exaggerated bilateral strength gains and neural plasticity can be induced following unilateral strength training on the less affected side. In addition, sensory enhancement may be applied to amplify cross-education effects in strength training. / Graduate / 2020-09-12

Cross-education of strength is a neural adaptation defined as the increase in strength of the untrained contralateral limb after unilateral training of the opposite homologous limb. The neural mechanisms of the effect have remained elusive, although it appears to be a motor learning adaptation. Despite cross-education of strength being an inter-limb effect, no previous study has determined the influence of handedness and the direction of transfer (dominant to non-dominant or the reverse). Arguably, this is partly responsible for massive variation in the literature regarding the magnitude of the effect. The primary purpose of this document is to attempt to determine the central and peripheral neuro-physiological mechanisms controlling cross-education of muscular strength. Prior to determining the mechanisms of the effect, the influence of handedness and the preferred direction of transfer for cross-education of strength must be addressed. The secondary purpose is to determine the preferential direction of transfer of cross-education of strength in order to isolate the circumstances in which the effect is more pronounced. Two experiments were necessary to meet these objectives. <p>Experiment 1: The purpose was to determine the effect of the direction of transfer on cross-education in right-handed individuals. Subjects were randomized into a left-hand training (LEFT), right-hand training (RIGHT), or non-training control (CON) group. Strength training was 6 weeks of maximal isometric ulnar deviation, 4 times per week. The change in strength in the untrained limb was greatest in the RIGHT group (39.2%; p<.01), whereas no significant changes in strength were observed for the untrained limb of the LEFT group (9.3%) or for either of the CON group limbs (10.4% and 12.2%). There were no changes in muscle thickness of untrained limbs compared to CON. Changes in untrained limb EMG were not different compared to CON. Cross-education with hand strength training occurs only in the right-to-left direction of transfer in right-handed individuals. Cross-education of arm muscular strength is most pronounced to the non-dominant arm. <p>Experiment 2: The purpose of this study was to determine the central and peripheral mechanisms of cross-education of strength after actual and imagery training. Subjects were randomized into an actual training, imagery training, or non-training control group. A sub-sample of 8 subjects (4 actual, 4 imagery training) had brain activity during exercise assessed with functional magnetic resonance imaging (fMRI). Strength training was 6 weeks of maximal isometric handgrip ulnar deviation (Biodex) of the right arm, 4 times per week. Actual training was highly effective for increasing strength in trained (45.3%; p<.01) and untrained (47.1%; p<.01) limbs. Imagery training and control groups had no increases in strength for either arm. Muscle thickness increased only in the trained arm of the actual group (8.4%; p<.001). After actual training, there was an increase in activation of contralateral sensorimotor cortex and left temporal lobe during actual contractions with the untrained left arm (p<.001). Actual training was associated with a significantly greater change in agonist muscle activation pooled over both limbs, compared to the imagery and control groups (p<.05). Cross-education of strength is only significant after actual training, indicating that peripheral feedback is necessary for the effect. Cross-education of strength is accompanied by changes in cortical activation indicative of motor learning and the retrieval of memory of movement acquired by the trained limb. <p>General Conclusion: The neuro-physiological mechanism of cross-education of strength is that changes in cortical activation indicative of motor learning occur in both brain hemispheres after unilateral training. Cross-education of strength is influenced by strength asymmetries related to handedness, and the preferential direction of transfer is from dominant to non-dominant limb. Cross-education is a motor learning adaptation also reliant on peripheral feedback during training.

Cross-education of strength is a neural adaptation defined as the increase in strength of the untrained contralateral limb after unilateral training of the opposite homologous limb. The neural mechanisms of the effect have remained elusive, although it appears to be a motor learning adaptation. Despite cross-education of strength being an inter-limb effect, no previous study has determined the influence of handedness and the direction of transfer (dominant to non-dominant or the reverse). Arguably, this is partly responsible for massive variation in the literature regarding the magnitude of the effect. The primary purpose of this document is to attempt to determine the central and peripheral neuro-physiological mechanisms controlling cross-education of muscular strength. Prior to determining the mechanisms of the effect, the influence of handedness and the preferred direction of transfer for cross-education of strength must be addressed. The secondary purpose is to determine the preferential direction of transfer of cross-education of strength in order to isolate the circumstances in which the effect is more pronounced. Two experiments were necessary to meet these objectives. <p>Experiment 1: The purpose was to determine the effect of the direction of transfer on cross-education in right-handed individuals. Subjects were randomized into a left-hand training (LEFT), right-hand training (RIGHT), or non-training control (CON) group. Strength training was 6 weeks of maximal isometric ulnar deviation, 4 times per week. The change in strength in the untrained limb was greatest in the RIGHT group (39.2%; p<.01), whereas no significant changes in strength were observed for the untrained limb of the LEFT group (9.3%) or for either of the CON group limbs (10.4% and 12.2%). There were no changes in muscle thickness of untrained limbs compared to CON. Changes in untrained limb EMG were not different compared to CON. Cross-education with hand strength training occurs only in the right-to-left direction of transfer in right-handed individuals. Cross-education of arm muscular strength is most pronounced to the non-dominant arm. <p>Experiment 2: The purpose of this study was to determine the central and peripheral mechanisms of cross-education of strength after actual and imagery training. Subjects were randomized into an actual training, imagery training, or non-training control group. A sub-sample of 8 subjects (4 actual, 4 imagery training) had brain activity during exercise assessed with functional magnetic resonance imaging (fMRI). Strength training was 6 weeks of maximal isometric handgrip ulnar deviation (Biodex) of the right arm, 4 times per week. Actual training was highly effective for increasing strength in trained (45.3%; p<.01) and untrained (47.1%; p<.01) limbs. Imagery training and control groups had no increases in strength for either arm. Muscle thickness increased only in the trained arm of the actual group (8.4%; p<.001). After actual training, there was an increase in activation of contralateral sensorimotor cortex and left temporal lobe during actual contractions with the untrained left arm (p<.001). Actual training was associated with a significantly greater change in agonist muscle activation pooled over both limbs, compared to the imagery and control groups (p<.05). Cross-education of strength is only significant after actual training, indicating that peripheral feedback is necessary for the effect. Cross-education of strength is accompanied by changes in cortical activation indicative of motor learning and the retrieval of memory of movement acquired by the trained limb. <p>General Conclusion: The neuro-physiological mechanism of cross-education of strength is that changes in cortical activation indicative of motor learning occur in both brain hemispheres after unilateral training. Cross-education of strength is influenced by strength asymmetries related to handedness, and the preferential direction of transfer is from dominant to non-dominant limb. Cross-education is a motor learning adaptation also reliant on peripheral feedback during training.

Similar to quadrupedal animals, there is evidence in humans of interlimb signalling during upper and lower limb muscular activation. A product of these interconnections is modulation of motor output via remote neural input. Such remote communication can take several forms; for example, movement modifies activity between upper and lower limbs (e.g. arms to legs) and between a limb pair (e.g. one leg to the other). A specific form of modulation between homologous muscles bilaterally (i.e. the corresponding motor unit pool across the spinal cord) is also seen with strength training. However, details of these motor connections are not well known. Improved understanding of remote influences on motor output and coordination patterns may be valuable in an applied motor re-training setting. Abnormal excitability within reflex pathways of lower limb musculature is common among various neurological disorders. Thus, it is of interest whether remote inputs could be exploited to help normalize dysfunctional motor output. The primary goal of this thesis was to better our understanding of neural interlimb connections; specifically, to examine modulatory responses within the ankle flexor and extensor muscles induced by remote muscular activation associated with both rhythmic arm movement and contralateral resistance training. Further, the final objective of this work was to apply these earlier observations in the context of a post-stroke rehabilitation paradigm, aimed at normalizing muscle activation patterns within the more-affected limb. Initially, this thesis examined spinal reflex excitability within functional antagonists of the lower leg, the ankle flexors and extensor muscles, and the impact of transient, rhythmic movement on these neural networks. Hoffmann (H-) reflexes were first used as a measurement probe. Rhythmic arm cycling significantly suppressed reflex amplitude in extensors, but revealed a bidirectional (i.e. either suppression or facilitation) reflex modulation in flexor muscles. Thus, differential regulation of ankle flexor and extensor H-reflex amplitudes was evidenced during rhythmic arm movement. This may stem from differences in locomotor pattern generator output to these groups as well as increased involvement of cortical drive to the flexors relative to the extensors during rhythmic movement. These results support the presence of interlimb neural coupling, such that remote motor action (arm movement) influences lumbar spinal cord excitability. Additionally, these descending signals impact ankle flexors and extensors differentially, which illustrates a method of producing facilitative modulation of ankle flexor motor responses. Second, reciprocal inhibition (RI) was used to examine regulation of excitability between these same lower limb functional antagonists during rhythmic arm movement. Arm cycling significantly increased RI in ankle extensors, but had no effect in the flexors. This extends observation of remote motor activity-induced modulation on spinal excitability to the core circuitry that comprises the interaction between functional agonist/antagonist pairs. Moreover, the asymmetry of this effect highlights differences in descending supraspinal inputs to ankle flexors vs. extensors, and may be related to functional dorsiflexion requirements during locomotion. Subsequently, this thesis explored long term plasticity of interlimb neural modulation resulting from remote motor activation in the form of resistance training. Specifically, the within limb pair ‘cross-education’ phenomenon was investigated via unilateral isometric strength training of the ankle flexors. The first of these training interventions was implemented in a cohort of neurologically intact subjects who performed five weeks of one-sided maximal isometric dorsiflexion training. H-reflex recruitment curves were used to probe for training-induced spinal plasticity within the agonist (flexor) and antagonist (extensor) muscles bilaterally. Post-intervention, dorsiflexor torque significantly increased in the trained and untrained limbs. Further, significant changes in H-reflex excitability were detected in the trained flexor (agonist) muscle and in both extensor (antagonist) muscles. These findings reveal that muscular crossed effects can be obtained in the ankle dorsiflexor muscles, and provide novel information on agonist and antagonist spinal adaptations that accompany unilateral training. They also suggest potential for application of remote motor activation (resistance training) to induce interlimb neural plasticity within a clinical context, such as improving one-sided weakness and/or motor dysfunction following neurotrauma. The final training intervention was implemented in a chronic (>6mo post-infarct) stroke clinical group who completed six weeks of maximal isometric dorsiflexion training in the less-affected leg. Voluntary isometric strength (dorsiflexion torque, muscle activation), reciprocal inhibition (RI), walking ability and clinical function were used to quantify training effects. Post-intervention, dorsiflexion torque and maximal flexor muscle activation significantly increased in both the more-affected (untrained) and less-affected (trained) legs. Further, the relation between size of RI and level of muscle activation in the more-affected flexor muscle was significantly altered by training, and the Timed Up and Go clinical test was significantly improved. Thus, significant gains in voluntary strength, muscle activation and spinal excitability on the untrained, more-affected side after stroke can be invoked through training the opposite limb. This translates into small but observable functional improvements. Taken together, the data in this thesis provide a basis for novel motor re-training approaches. Improved understanding has been gained of the similarities and differences between remote motor influences received by ankle flexor and extensor muscles in the lower leg. These observations culminate in the implementation of a novel post-stroke training paradigm, which shows that remote muscle activation, i.e. the cross-education effect, can induce strength and functional gains in the more-affected limb. / Graduate

Cross education is the phenomenon by which the untrained limb will experience a gain in strength following a unilateral resistance training program. However, little is known as to the underlying adaptation occurring in the untrained limb. Purpose: To examine the effect of dynamic unilateral resistance training on the strength and neuromuscular adaptations of both the trained and untrained legs. Methods: Eight previously untrained males (22.38±2.92 y, 1.73±0.08 m, 75.26±14.53 kg) completed a four-week unilateral resistance training program, while another eight untrained males (24.00±4.57 y, 1.84±0.05 m, 94.21±16.14 kg) served as controls. Isometric leg extension strength, leg press 1 repetition maximum (1RM), leg extension 1RM, root mean square of the maximal electromyographic amplitude (EMG), submaximal EMG, dynamic neuromuscular economy (NME) and the slope of NME-power output relationship were determined before and after training to assess the changes in strength and neuromuscular adaptations of the vastus lateralis (VL) and rectus femoris (RF) in both the trained and untrained legs. The unilateral resistance training program was conducted on the dominant leg (DOM) in the unilateral resistance training group (URT) and was compared to the dominant leg of the control group (CON). Cross education was measured in the nondominant leg (NON) for both groups. The unilateral resistance training program was completed three days per week for a total of twelve training sessions. Exercises included in the training program were unilateral leg press, unilateral leg extension, bilateral chest press and bilateral low row. All data was analyzed using one-way analysis of covariance of the post-testing values using the pre-testing values as the covariate. Further analysis of the EMG and NME data was performed using magnitude-based inferences. Results: The URT group improved their isometric (DOM:11.03%, NON:4.98%), leg press (DOM:77.63%, NON:64.88%) and leg extension (DOM:46.76%, NON:16.43%) strength after the four weeks of resistance training. There was no difference between the groups in isometric strength in the dominant (p=0.188) or nondominant (p=0.948) leg. For leg extension 1RM, there was a significant difference between groups in the dominant leg (p=0.018), but not the nondominant leg (p=0.482). However, there were significant group differences in both the dominant (p=0.003) and nondominant (p=0.034) leg for leg press 1RM. In terms of maximal EMG, the training groups improved in the vastus lateralis (DOM:29.81%, NON:31.44%) and rectus femoris (DOM:20.71%, NON:6.26%) individually, as well as in total EMG (DOM:24.78%, NON:17.57%). There was a Likely Positive or Very Likely Positive effect of unilateral resistance training on the changes in maximal EMG of the vastus lateralis and rectus femoris in both the dominant and nondominant legs. There was a Likely Positive effect of unilateral resistance training on the submaximal EMG of the dominant vastus lateralis at 75 and 125 watts. Conversely, in the rectus femoris, there was Unclear effects of unilateral resistance training on the submaximal EMG of the dominant leg. There was no consistent effect of unilateral resistance training on submaximal EMG values of the vastus lateralis in the nondominant leg. However, the rectus femoris in the nondominant leg experienced a Likely Positive effect of unilateral resistance training on submaximal EMG. NME improved in the URT group in the VL at 75 (DOM:9.73%, NON:13.42%), 100 (DOM:8.76%, NON:8.21%), and 125(DOM:24.26%, NON:12.8%) watts and in the RF at 75 (DOM:22.25%, NON:15.73%), 100(DOM:24.85%, NON:17.05%) and 125 (DOM:30.99%) watts. In terms of neuromuscular economy, there was a Likely Positive or Very Likely Positive effect of unilateral resistance training on most measures of NME on both the vastus lateralis and rectus femoris in both the dominant and nondominant legs. In terms of NME slope, there was only a Likely Positive effect of unilateral resistance training on the dominant vastus lateralis. Conclusion: Based on these results, it appears that the cross education of strength from unilateral resistance training is modality-specific. Furthermore, the NME of both the vastus lateralis and rectus femoris in both legs appear to improve following unilateral resistance training. However, in the nondominant leg, the improvement in NME appears to be due solely to the increase in maximal EMG, whereas the improved NME in the dominant leg is due to both an increase in maximal EMG and a decrease in submaximal EMG.

This study examined the effects of six weeks of velocity specific isokinetic training on peak torque (PT), and the estimated cross-sectional area of the upper arm (AG) in the trained. Thirty volunteers (M=15, F=15) were randomly assigned to an experimental, slow velocity group (S), 60 degrees-per-second (n=9; 25.4±..6.5yr), a fast velocity group (F), 450 degrees-per-second (n=ll, 23.7 ±..S.4yr), or control group (C) (n=10, 26 ± 3.2yr). One limb was randomly selected for isokinetic training (3 d/wk-elbow flexion) using a Biodex System 2 isokinetic dynamometer. The contralateral limb served as a control and as the basis for measurements measure of cross education (CE). Both experimental conditions (S) and (F) were assigned equal training workloads, calcu1ated from an isokinetic pre-test. Pre- and post-tests (PT) were recorded for both limbs at the training velocities of 60 and 450 degrees-per-second, as well as the velocity of 210 degrees-per-second. Pre and post-test (AG's) were measured on the training limb. The (S) condition was significantly different in strength gains from the control at 60 degrees-per-second, but not different from the fast velocity group in the trained limb. The (F) condition was significantly different in strength gains from the control at 450 degrees-per-second, but not different from the slow velocity group in the the trained limb. The conditions were not significantly different from each other in the trained limb at the test velocity of 210 degrees-per-second. The three conditions significantly different from each at the test velocity of 60 degrees-per-second in the trained limb. The conditions did not differ in strength at velocities of 210 and 450 degrees-per-second in the trained limb. The conditions did not differ in the cross-sectional area of the upper arm in the trained limb. The (S) and (F) training conditions improved (PT) by 12.36% and 18.84% at their respective training velocities of 60 and 450 degrees-per-second. These improvements were significantly (p<.05) larger than (C). The (S) and (F) training conditions also increased (PT) by 11.56% and 11.24% at the non-training velocity of 210 degrees-per-second (p<.05). Significant 10.77% (p<.05) improvement in (AG) was recorded in the (S) condition. No changes in (PT) were recorded in the contralateral limb within the three conditions. These data support the concept of limited (S) and (F) hi-directional (PT) overflow and (S) velocity hypertrophy enhancement. The presence of cross education (CE) was not supported by this investigation. / Master of Science

Abstract    Aim    The main purpose of this study was to investigate Cross Education (CE), and how gender, detraining and leg dominance affects CE in previously untrained subjects when conducting a unilateral resistance training program. We also investigated if unilateral resistance training can give a hypertrophic response.    Method   Twenty healthy previously untrained individuals, 10 females and 10 males, were recruited as volunteer participants. The participants were randomly assigned to train either left or right leg. The training intervention was 10 weeks (34 sessions) of unilateral resistance training in the leg press and leg extension, sixteen of the participants fulfilled the criteria for inclusion. After two initial familiarization the participants trained conventional resistance training three times a week (week 1-3, 5-7 and 9-10) and Blood Flow Restriction Training (BFRT) five times a week (week 4 and 8). One repetition maximum for both legs in the leg press and leg extension was tested pre-, post and post20 to the training intervention as well as ultrasound measurements of muscle thickness.   Results    The ten-week training period resulted in a significant increase of maximal strength for the untrained leg 18,9 %, (16,6) (p &lt; 0,01) in the leg press and 6,7 %, (3,7) (p &lt; 0,05) in the leg extension. When comparisons between gender were made only men had a significant increase 26,5 %, (16,7) (p &lt; 0,01) in the leg press and 9,9 %, (4,7) (p &lt; 0,05) in the leg extension. Also, we saw a significant difference between women and men on a group level. Comparisons of dominant vs non-dominant leg showed that training the dominant leg resulted in a significant increase of maximal strength in the untrained leg in both the leg press 22 %, (17,9) (p &lt; 0,01) and leg extension 10,1 %, (4,3) (p &lt; 0,05). The maximal strength in the untrained leg was not significantly altered by the detraining period and a significant increase of muscle thickness could be seen in the untrained leg at MP50 4,7 %, (1,3) (p &lt; 0,01).   Conclusion    The conclusions are that a ten week unilateral resistance training intervention results in a CE effect for men but not for women and that this type of training also can result in an increased muscle thickness in the untrained leg. Our findings also supports that training the dominant limb has superior effect on achieving a CE effect. Lastly we conclude that a twenty week detraining period did not affect the CE achieved strength. / Abstrakt    Syfte     Studiens huvudsakliga syfte var att undersöka Cross Education (CE) och hur kön, viloperiod och ben-dominans påverkar CE hos otränade individer när man undergår ett unilateralt styrketräningsprogram. Vi undersökte även om ett unilateral styrketräning kunde ge ett hypertrofisvar.    Metod    Tjugo friska otränade och för närvarande inaktiva individer, tio kvinnor och tio män rekryterades som frivilliga deltagare. Deltagarna randomiserades för att träna antingen vänster eller höger ben. Träningsperioden var tio veckor (trettiofyra pass) av unilateral styrketräning i benpress och benspark, sexton deltagare uppfyllde kriterierna för inkludering. Två initiala familjäriseringspass hölls varefter träningen delades in i två typer av träning, dels konventionell styrketräning tre gånger i veckan (vecka 1-3, 5-7 och 9-10) och dels Blood Flow Restriction Training (BFRT) fem gånger i veckan (vecka 4 och 8). Före, efter och efter tjugo veckor testades one repetition maximum för båda benen i benpress och benspark samt att ultraljudsmätningar för muskeltjocklek utfördes.     Resultat    Den tio veckor långa träningsperioden resulterade i en signifikant ökning av den maximala styrkan för det otränade benet 18,9 % (16,6) (p &lt; 0,010) i benpressen och 6,7 % (3,7) (p &lt; 0,050) i bensparken. När jämförelser gjordes mellan könen så hade enbart män en signifikant ökning, 26,5 % (16,7) (p &lt; 0,010) i benpressen och 9,9 % (4,7) (p &lt; 0,050) i bensparken. Vi fann även att det var en signifikant skillnad mellan kvinnor och män på gruppnivå.   Jämförelser mellan dominant och icke-dominant ben visade att träning av det dominanta benet resulterade i en signifikant styrkeökning i både benpress 22 % (17,9) (p &lt; 0,010) och benspark 10,1 % (4,3) (p &lt; 0,050). Den maximala styrkan i det otränade benet påverkades inte signifikant av en viloperiod och en signifikant ökning i muskeltjocklek kunde ses i det otränade benet i MP50 4,7 % (1,3) (p &lt; 0,010)    Konklusion    Slutsatserna är att en tio veckors unilateral styrketräningsintervention resulterar i en CE effekt hos män men inte hos kvinnor, och att denna typ av träning kan resultera i en ökad muskeltjocklek i det otränade benet. Våra fynd styrker att träning av den dominanta lemmen har större effekt på CE. Slutligen drar vi slutsatsen att en tjugo veckors viloperiod inte påverkar CE-styrkan.

The purpose of this thesis was to investigate the potential mechanisms and sites of neural adaptations that mediate cross-limb transfer of strength and motor learning that can occur subsequent to unilateral training. Better understanding of the mechanisms should allow therapeutic benefits of this effect to be assessed. There are two main classes of mechanisms that could contribute to cross-limb transfer. The first is described by the ??bilateral access?? hypothesis, which suggests that neural adaptations induced by training reside in bilaterally projecting motor areas that are accessible to the untrained (ipsilateral) hemisphere during task execution to facilitate performance. According to the alternative ??cross-activation?? hypothesis, activation of the untrained hemisphere during unilateral training leads to adaptations in the untrained hemisphere that cause improved performance with the opposite untrained limb. A series of studies were conducted in this research. We directly tested the cross-activation hypothesis via a reliable twitch interpolation technique involving transcranial magnetic stimulation (TMS). Four-weeks of strength training for the right wrist increased neural drive (from the untrained motor cortex) to the untrained left wrist. The data demonstrate that strength training of one limb can influence the efficacy of corticospinal pathways that project to the opposite untrained limb, consistent with the cross-activation hypothesis. To investigate the contribution of each hemisphere in cross-limb transfer, we applied repetitive TMS (rTMS) to the trained or the untrained motor cortex to disrupt brain processing after unilateral ballistic training. Learning to produce ballistic movements requires optimization of motor drive to the relevant muscles in a way that resembles high-force contractions performed during strength training. Ballistic skill transferred rapidly to the untrained hand and the improved performance was accompanied by bilateral increases in corticospinal excitability. Performance improvement in each hand was specifically suppressed by rTMS of the opposite hemisphere. Thus the motor cortex ipsilateral to the trained hand is critically altered during unilateral training; and neural adaptations within this untrained hemisphere are crucial in cross-limb transfer of ballistic skill. Overall, the data are in agreement with the cross-activation hypothesis for high-force and ballistic tasks, although they do not exclude the potential involvement of bilateral access mechanisms.

The purpose of this thesis was to investigate the potential mechanisms and sites of neural adaptations that mediate cross-limb transfer of strength and motor learning that can occur subsequent to unilateral training. Better understanding of the mechanisms should allow therapeutic benefits of this effect to be assessed. There are two main classes of mechanisms that could contribute to cross-limb transfer. The first is described by the ??bilateral access?? hypothesis, which suggests that neural adaptations induced by training reside in bilaterally projecting motor areas that are accessible to the untrained (ipsilateral) hemisphere during task execution to facilitate performance. According to the alternative ??cross-activation?? hypothesis, activation of the untrained hemisphere during unilateral training leads to adaptations in the untrained hemisphere that cause improved performance with the opposite untrained limb. A series of studies were conducted in this research. We directly tested the cross-activation hypothesis via a reliable twitch interpolation technique involving transcranial magnetic stimulation (TMS). Four-weeks of strength training for the right wrist increased neural drive (from the untrained motor cortex) to the untrained left wrist. The data demonstrate that strength training of one limb can influence the efficacy of corticospinal pathways that project to the opposite untrained limb, consistent with the cross-activation hypothesis. To investigate the contribution of each hemisphere in cross-limb transfer, we applied repetitive TMS (rTMS) to the trained or the untrained motor cortex to disrupt brain processing after unilateral ballistic training. Learning to produce ballistic movements requires optimization of motor drive to the relevant muscles in a way that resembles high-force contractions performed during strength training. Ballistic skill transferred rapidly to the untrained hand and the improved performance was accompanied by bilateral increases in corticospinal excitability. Performance improvement in each hand was specifically suppressed by rTMS of the opposite hemisphere. Thus the motor cortex ipsilateral to the trained hand is critically altered during unilateral training; and neural adaptations within this untrained hemisphere are crucial in cross-limb transfer of ballistic skill. Overall, the data are in agreement with the cross-activation hypothesis for high-force and ballistic tasks, although they do not exclude the potential involvement of bilateral access mechanisms.