Cross education is the phenomenon by which strength or skill development achieved during the training of a single limb is transferred to the contralateral untrained limb (36). The cross education of strength due to unilateral resistance training (URT) has been shown to occur in the muscles of the hand (41), upper arm (9,11,30), and knee (8,16,19,20,34,40,43). A recent meta-analysis has shown the effect of URT on the maximal voluntary strength of the untrained limb to range from 3 to 20% with an average increase of 7.6% from pretraining to posttraining (32). Understanding and maximizing the effect of cross education may be beneficial for training individuals who are suffering from single limb immobilization (24). Recent research has shown URT to maintain (11,33) or possibly increase (26) the strength and size of an immobilized untrained limb. Studies have examined cross education of the knee extensor muscles after completing isometric URT (19,20) or isokinetic URT (16). Komi et al. (20) showed that 12 weeks of unilateral isometric knee extension training can significantly increase the strength of the untrained leg. Other studies employing isometric and isokinetic training have also revealed similar increases in strength of the untrained leg, but only in the modality of the training that was used (19,43). However, limited studies have assessed the effectiveness of dynamic constant external resistance URT on cross education in the knee extensors (4,18,40). Tracy et al. (40) showed that 9 weeks of dynamic constant external resistance lower-body URT increased contralateral knee extensor strength in older men and women. However, Coburn et al. (4) did not report any change in strength for the untrained limb when individuals completed 8 weeks of URT with the nondominant leg (NON). Houston et al. (18) performed 10 weeks of dynamic constant external resistance URT and observed increases in isometric and isokinetic strength of the untrained limb. However, this study did not observe any changes in muscle fiber area in the untrained leg (18). Previous research has shown that cross education of strength may be greater after URT in the dominant limb (DOM), as compared with the NON limb (10). Farthing et al. (10) speculated that the reason for the unidirectional effect of cross education was due to less coordination in the NON limb and therefore a greater potential for neural adaptations. Although the cross education of strength has been studied, further understanding of the underlying mechanisms is needed to explain why this phenomenon is occurring.
There are numerous potential mechanisms that have been proposed as the cause of cross education (17). Enoka postulated that cross education is due to a central neural adaptation (7). The central neural mechanisms postulate that during voluntary activation of a single limb, there is a crossover effect of the neural drive occurring at either the motor cortex, pyramidal tract, or somewhere in the spinal cord (17). Furthermore, Carroll et al. (3) identified potential adaptation sites within the spinal cord, cerebral cortex, and subcortical centers within the brain. The humoral mechanism asserts that URT will increase blood-borne factors (hormones), which ultimately will increase contralateral strength (41). Although not necessarily a mechanism of cross education, as it implies the activation of the untrained musculature, the postural hypothesis states that when contracting a limb, the muscles of the contralateral limb will contract to maintain posture and stability (42). All mechanisms of cross education that have been proposed may affect muscle activation, muscle size, or the acute hormonal response to exercise.
Neuromuscular adaptations have been shown to have an important role in generating initial strength improvements during resistance training (1,5,12,13,30) and have been proposed to be the primary mechanism responsible for cross education (24,30). Previous studies have shown a significant increase in maximal muscle activation, measured by electromyography (EMG), during isometric, isokinetic, and a 1 repetition maximum (1RM) test of the knee extensor muscles after 4 (5), 14 (1), and 16 weeks (13) of bilateral resistance training. Another mechanism stimulating increases in muscular strength is an increase in muscle size (28). Increases in muscle size have been seen in as little as 20 days of resistance training (38). Both muscle activation and size have been assessed after URT (9,11,16,20,30,43). A significant increase in maximal muscle activation in the untrained arm has been seen after only 4 (30) or 6 weeks (9) of URT. However, studies have shown no change in muscle activation of the quadriceps musculature after URT (20,43). In terms of muscle size, only 1 study has shown a significant increase in the thickness of the untrained musculature (27); however, this study did not use a control group, and there were minimal changes in muscle thickness (4.2%). No other studies have observed an increase in muscle size in the untrained limb after URT despite increases in the strength of the untrained limb and the muscle size of the trained limb (9,11,30,34). Farthing et al. (9) showed significant increases in strength in the untrained arm with no changes in muscle thickness after 6 weeks of URT, implying that the cross education of strength is primarily due to neuromuscular adaptations rather than hypertrophic gains. Similarly, Moritani and deVries (30) reported significant increases in the strength and maximal muscle activation of the untrained arm, but no change in muscle cross-sectional area after 8 weeks of URT. It is important to note that recent research has shown URT to prevent muscle atrophy during periods of immobilization (11,26,33).
It is important to note that although the adaptations to URT have been thoroughly investigated, the training interventions used in the research focus solely on the investigated musculature. The present research study includes a training intervention designed to target numerous muscle groups within the body. While including a similar unilateral leg press (LP) and leg extension (LE) used by Houston et al. (18), the current training intervention also includes 2 bilateral upper-body exercises. The inclusion of additional muscle groups will increase total training volume, which has been shown to enhance the acute hormonal response (29,39). Previous research has shown bilateral resistance exercise (29) or full-body resistance exercise (35) to stimulate the acute hormonal response to exercise greater than unilateral resistance exercise alone. To the authors' knowledge, no study has been conducted, which examined the strength, muscle activation, muscle size of the untrained limb, and acute hormonal response after a dynamic constant external resistance full-body URT. Therefore, the purpose of this study was to examine the effects of 4 weeks of dynamic constant external resistance full-body URT on strength, neuromuscular activation, and muscle size of the trained and untrained musculature. The secondary purpose of this study was to examine the effect of 4 weeks of training on the acute hormonal response to URT and whether an augmented response contributed to changes in muscle size. We hypothesize that strength, muscle size, and muscle activation will increase in the trained leg in the URT group, whereas only strength and muscle activation will increase in the untrained leg.
Experimental Approach to the Problem
A randomized controlled trial design was used to determine the effects of URT on the muscular strength, size, and activation of the trained and untrained legs. Each participant visited the Human Performance Laboratory for pretraining, training sessions, and posttraining. After completing pretraining, participants were randomly assigned to either the URT (n = 9, 22.89 ± 3.14 years, 1.74 ± 0.08 m, 76.80 ± 14.36 kg) or control (CON; n = 8, 24.00 ± 4.57 years, 1.84 ± 0.05 m, 94.21 ± 16.14 kg) group. Participants in the URT group completed a 4-week URT program. During these 4 weeks, the CON group did not engage in any structured physical activity. After the 4-week intervention period, all participants completed posttraining.
Nineteen previously untrained (no resistance training of any kind within the last year) men were recruited for this study. Age range of subjects was 18–31 years. Two participants in the control group did not complete the study because of attrition; therefore, a total of 17 participants completed the study. Participants were allowed to be currently engaged in cardiovascular training as long as the total volume did not exceed 150 minutes of moderate activity or 75 minutes of vigorous activity per week. Before enrolling in the study, all participants completed a Confidential Medical and Activity Questionnaire, as well as a Physical Activity Readiness Questionnaire, to determine whether they had any physical limitations that would keep them from performing the testing and/or training procedures. Potential participants were excluded from consideration for the study if they had been using any ergogenic nutritional supplement within the last 3 months, such as, but not limited to, protein powders and creatine. Throughout the study, participants were not allowed to use any ergogenic nutritional supplements or engage in any outside structured resistance training program. Informed consent was obtained from all individual participants included in the study. This study was approved by the New England Institutional Review Board for the protection of human subjects.
Pretraining and posttraining sessions occurred during the week, before and after the training period, respectively. Each testing session consisted of 2 testing days. On the first day of testing (T1), each participant completed an ultrasound measurement, maximal voluntary isometric test (MVIC), and 1RM test, in that order. All tests were performed on both legs. Limb dominance was assessed by the Waterloo Footedness Questionnaire (6). Seventy-two hours later (T2), participants completed an acute training protocol, which was modeled after the training sessions. During T2, blood samples were obtained before (BL), immediately following (IP), and 30- (30P) and 60-minute (60P) postexercise.
Ultrasound procedures were adapted from previous work by Scanlon et al. (37). Participants reported to the Human Performance Laboratory for a noninvasive skeletal muscle ultrasound (LOGIQ P5; General Electric, Wauwatosa, WI, USA) examination of the thigh musculature. Participants were asked to lie supine on an examination table with both legs fully extended for 15 minutes. Images of the rectus femoris (RF) were taken midway between the anterior inferior iliac crest and proximal patellar border. Images of the vastus lateralis (VL) were taken midway between the greater trochanter and lateral epicondyle (2). After determining the desired anatomical position, a linear probe coated with transmission gel was positioned on the surface of the skin to collect the ultrasound image. For all ultrasound measurements, the frequency was set at 12 MHz, depth at 5.0 cm, gain at 50 dB, and dynamic range at 72. All ultrasound images were analyzed offline using image analysis software (ImageJ, version 1.45s) available from the National Institutes of Health (NIH, Bethesda, MD, USA). All ultrasound images were taken and analyzed by the same technician.
Muscle thickness (MT) measures were obtained using a longitudinal B-mode image. Three consecutive MT images were captured and analyzed for each muscle and leg, respectively. For each image, MT was measured with a single perpendicular line from the superficial aponeurosis to the deep aponeurosis. The average of the 3 MT measures was used for statistical analyses. Intraclass correlation coefficients (ICCs) for MT were as follows: RF, 0.730 (SEM = 0.165 cm) and VL, 0.490 (SEM = 0.163 cm).
Cross-sectional area (CSA) measures were obtained using a transverse sweep in the extended field of view mode. Three consecutive CSA images were captured and analyzed for each muscle and leg, respectively. For each image, CSA was assessed using the polygon tracking tool to include as much lean tissue as possible while excluding bone and surrounding fascia. The average of the 3 CSA measures was used for statistical analyses. The ICCs were as follows: RF, 0.93 (SEM = 0.581 cm2) and VL, 0.96 (SEM = 0.812 cm2).
One Repetition Maximum Procedures
Maximal strength was determined on 4 different exercises through 1RM testing. The 4 exercises were unilateral (LP) and unilateral (LE) for each leg, as well as bilateral chest press and low row (Power Lift; Conner Athletics Products, Inc., Jefferson, IA, USA). Before beginning the test, each participant completed a general and specific warm-up. The general warm-up consisted of riding a cycle ergometer for 5 minutes at the participant's preferred resistance. The specific warm-up consisted of 10 body weight squats, 10 alternating lunges, 10 walking knee hugs, and 10 walking butt kicks. Each participant performed 2 warm-up trials using a resistance that was approximately 40–60% and 60–80% of their perceived maximum, respectively. The third trial served as the first attempt at the participant's 1RM. If the trial was successfully completed, then weight was added and another trial was attempted. If the trial was not successfully completed, then the weight was reduced and another trial was attempted. The amount of weight that was added or removed was at the discretion of the Certified Strength and Conditioning Specialist (CSCS) supervising the testing session. A 3- to 5-minute rest period was provided between each trial. This process of adding and removing weight continued until a 1RM was reached. Attempts not meeting the range of motion criterion for each exercise, as determined by the trainer after previously established guidelines (15), were discarded. All LP and LE values were reported relative to each participant's body weight.
To assess muscle activation, as measured as millivolts from EMG during the MVIC, a bipolar (4.6 cm center-to-center) surface electrode (Quinton Quick-Prep; Mortara Instrument, Inc., Milwaukee, WI, USA) arrangement was placed over the VL and RF of both legs. Electrode arrangement for each muscle was similar to the configuration previously reported by the Surface Electromyography for the Non-Invasive Assessment of Muscles project (14). For the VL, electrodes were placed at approximately two-thirds of the line between the anterior superior iliac spine and lateral superior aspect of the patella. For the RF, electrodes were placed at the midpoint of the line between the inguinal crease and superior border of the patella. The reference electrode was placed over the lateral epicondyle of femur. The skin beneath the electrodes was shaved and cleaned with alcohol to keep interelectrode impedance below 5,000 Ω. EMG signals were obtained with a differential amplifier (MP150 BIOPAC Systems, Inc., Santa Barbara, CA, USA) sampled at 1,000 Hz. Files were then stored on an external drive for later analysis. EMG signals were band-pass filtered from 10 to 500 Hz and expressed as root mean square (RMS) amplitude values by software (AcqKnowledge v4.2; BIOPAC Systems, Inc., Santa Barbara, CA, USA). ICCs for the average RMS EMG signal ([VL + RF]/2) were as follows: R = 0.87 with a SEM of 115.73 mV.
Maximal Voluntary Isometric Contraction Procedures
After electrode placement, participants completed an MVIC test. Individuals were positioned in an isokinetic dynamometer (S4; Biodex Medical System, Inc., New York, NY, USA) in a seated position with the hip at an angle of 110° and strapped to the machine at the waist and shoulders. Next, the researchers positioned the individual's knee at an angle of 110° of extension (180° representing full extension). Participants were then instructed to exert their maximum strength as quickly as possible when trying to extend the knee. Researchers provided verbal encouragement throughout each trial to motivate participants to perform a maximal contraction. Participants were given 3 attempts on each leg, with each attempt lasting 5 seconds, and there was a 3-minute rest interval between each attempt. The highest peak force (PKF) of the 3 attempts was recorded and expressed relative to the individual's body weight. At the point of PKF, a 1-second segment of the EMG signal was recorded and the highest average ([VL + RF]/2) RMS amplitude value from the 3 attempts was used as the participant's maximum EMG activity for each leg.
Unilateral Resistance Training Procedures
Throughout the 4-week intervention period, each participant in the URT group reported to the Strength and Conditioning Laboratory 3 times per week (Monday, Wednesday, and Friday) for their exercise sessions. If a participant missed a training session, make-up sessions were scheduled with laboratory staff to ensure that 12 total sessions were completed during the 4 weeks while still maintaining appropriate rest periods between training sessions. Before each session, participants completed the same general and specific warm-up used before the 1RM testing. During each training session, participants in the URT group performed a unilateral lower-body and bilateral upper-body resistance training routine. All exercises were completed for 3 sets of 8–10 repetitions at 80% of the participant's previously determined 1RM. In the event that a participant could not complete the minimum amount of repetitions, they were allowed up to 30 seconds to recover and resume the set. If the participant was still unable to complete the required number of repetitions, then the weight was reduced on subsequent sets. The rest interval between each set was 90 seconds. Unilateral lower-body exercises were performed on the DOM leg. The load and number of repetitions for each exercise were recorded in workout logs. All training sessions were supervised by a CSCS. Throughout the 4-week training program, load for each exercise was increased at the discretion of the CSCS to ensure proper overload. The untrained limb remained relaxed throughout the exercise protocol. Participants were able to maintain recreational activities as usual but were not allowed to participate in any structured exercise programs throughout the duration of this study.
Blood Collection Procedures
On T2, all participants from URT and CON groups reported to the Human Performance Laboratory after a 10-hour overnight fast to complete an acute training protocol, which mirrored a training session used during the intervention period. The intensity of the load for each acute training protocol was set at 80% of each participant's pretraining and posttraining 1RM values. Before beginning the acute training protocol, a Teflon cannula was inserted into a superficial antecubital vein. A resting baseline measurement (BL) was taken after a 15-minute equilibration period (participant lying supine). The cannula was secured as to not interfere with the ability to complete the acute training protocol. On completing the acute training protocol, subsequent blood draws were drawn immediately after (IP), 30 minutes after (30P), and 60 minutes (60P) after exercise. Each participant's blood samples were obtained at the same time of day during each session to avoid diurnal variations in circulating hormones. Samples were drawn into serum or EDTA treated Vacutainer tubes (Becton Dickinson, Broken Bow, NE, USA) for further analysis. Whole blood samples were analyzed in duplicate for hematocrit by microcapillary technique and hemoglobin content at each time point. The remaining whole blood was centrifuged for 15 minutes at 1,500g at 4° C. The resulting plasma and serum was aliquoted and stored at −80° C until further analysis. Samples were thawed only once for biochemical analysis.
Blood Analysis Procedures
Plasma concentrations of total testosterone (nmol·ml−1), growth hormone (pg·ml−1), and insulin (μIU·ml−1), as well as serum concentrations of insulin-like growth factor-1 (nanogram per milliliter), were assayed using commercially available ELISA kits (testosterone: KGE010; growth hormone: DGH00; insulin-like growth factor-1: DG100; R&D Systems, Minneapolis, MN, USA; insulin: ELH-Insulin; RayBiotech, Inc., Norcross, GA, USA). The growth hormone ELISA focused on 20- and 22-kDa variants. Assay absorbance was read according to manufacturer specifications on a BioTek Eon Microplate Spectrophotometer (BioTek Instruments, Inc., Winooska, VT, USA). All samples remained frozen until analysis, were thawed only once, and were measured in duplicate. The sensitivity of the testosterone assay was 0.041 ng·ml−1, and the intra-assay coefficient of variation was 5.3%. The sensitivity of the growth hormone assay was 7.18 pg·ml−1, and the intra-assay coefficient of variation was 5.8%. The sensitivity of the insulin assay was 4.00 μIU·ml−1, and the intra-assay coefficient of variation was 4.1%. The sensitivity of the insulin-like growth factor-1 assay was 0.026 ng·ml−1, and the intra-assay coefficient of variation was 3.4%. All assay procedures followed those outlined by the manufacturer. Blood data were analyzed at individual time points during pretraining and area under the curve (arbitrary units) was calculated at pretraining and posttraining for each hormone to assess changes due to training.
All strength, muscle size, muscle activation, and hormone area under the curve data were analyzed using 20 separate 2 × 2 mixed analysis of variance (ANOVA) with group serving as a between-subjects comparison and time serving as a within-subjects comparison. If a significant interaction was observed, post hoc Bonferroni adjusted dependent t-tests (p ≤ [0.05/2]) were conducted to determine differences between groups. Separate analyses were conducted for each leg and muscle, because the combined effect of limb and muscle were not of specific interest for this investigation. In addition, percent change from baseline and 95% confidence intervals were constructed to further illustrate potential leg and group differences. For hormone concentrations, 4 separate 4 × 2 × 2 ANOVA was used to determine differences between groups, at each time point (BL, IP, 30P, and 60P) and pretraining and posttraining. If a significant 3-way interaction or main effect was observed, appropriate post hoc procedures were conducted. For effect size, the partial eta squared (η2) statistic was calculated. Statistical software (SPSS, V. 20.0; SPSS, Inc., Chicago, IL, USA) was used for all analyses. Missing data points (1 participant's ultrasound data missing because of excess body fat; 1 participant's blood data missing because of trypanophobia (needle phobia); and 1 participant's blood data missing because of an inability to finish the workout) were left out of the affected analyses. Results were considered significant at an alpha level of p ≤ 0.05. All data are reported as mean ± SD.
Figure 1A–D displays the percent changes in strength, muscle size, and muscle activation for each individual participant and also displays the mean percent change ±95% confidence intervals.
Relative Strength Measures
Table 1 displays the values for PKF, LP strength, and LE strength for the trained, DOM leg and untrained, NON leg before and after the 4-week intervention period. There was a significant group × time interaction for PKFDOM (F = 5.363; p = 0.035; η2 = 0.263). However, there was no significant main effect for time (F = 0.325; p = 0.577; η2 = 0.021) or group × time interaction (F = 0.026; p = 0.874; η2 = 0.002) for PKFNON. Post hoc tests revealed no significant change in the CON group for PKFDOM (p = 0.492), but a significant increase in the URT group for PKFDOM (p = 0.001).
There was a significant group × time interaction for LPDOM (F = 21.055; p < 0.001; η2 = 0.584), LPNON (F = 9.693; p = 0.007; η2 = 0.393), and LEDOM (F = 20.158; p < 0.001; η2 = 0.573). There was no significant group × time interaction for LENON (F = 0.408; p = 0.533; η2 = 0.026). However, there was a significant main effect for time for LENON (F = 6.828; p = 0.020; η2 = 0.313). Post hoc tests revealed a significant increase in the CON group for LPDOM (p = 0.005), but no significant change in LPNON (p = 0.533) or LEDOM (p = 0.119), but a significant increase in the URT group for LPDOM (p < 0.001), LPNON (p = 0.001), and LEDOM (p < 0.001).
Table 2 displays the ultrasound measures between the DOM and NON legs in the trained and control groups. There was a significant group × time interaction for RF MTDOM (F = 24.993; p < 0.001; η2 = 0.641). There was no significant group × time interaction for VL MTDOM (F = 1.989; p = 0.180; η2 = 0.124), VL MTNON (F = 1.871; p = 0.195; η2 = 0.126), and RF MTNON (F = 0.209; p = 0.654; η2 = 0.015). However, there was a significant main effect for time in VL MTDOM (F = 19.498; p = 0.001; η2 = 0.582) but not for VL MTNON (F = 2.611; p = 0.130; η2 = 0.167) or RF MTNON (F = 0.291; p = 0.598; η2 = 0.020). Post hoc tests revealed no significant changes in the CON group in RF MTDOM (p = 0.108). However, there was a significant increase in the URT group for RF MTDOM (p < 0.001).
There was a significant group × time interaction for VL CSADOM (F = 20.390; p < 0.001; η2 = 0.593) and RF CSADOM (F = 10.352; p = 0.006; η2 = 0.425). There was no significant group × time interaction for VL CSANON (F = 1.171; p = 0.297; η2 = 0.077) and RF CSANON (F = 2.780; p = 0.118; η2 = 0.166). However, there was a significant main effect for time in VL CSANON (F = 6.809; p = 0.021; η2 = 0.327), but no main effect for time in RF CSANON (F = 2.578; p = 0.131; η2 = 0.156). Post hoc tests revealed no significant change in the CON group for VL CSADOM (p = 0.142) and RF CSADOM (p = 0.160). However, there was a significant increase in the URT group for VL CSADOM (p < 0.001) and RF CSADOM (p < 0.001).
Table 1 displays the average muscle activation measures (EMG) in the DOM and NON limbs in both the trained and control groups. No significant group × time interactions were observed for EMGDOM (F = 1.490; p = 0.242; η2 = 0.096) or EMGNON (F = 0.613; p = 0.446; η2 = 0.039).
Figures 2–5 display changes in testosterone, growth hormone, insulin, and insulin-like growth factor-1 concentrations, and area under the curve measures.
There was no significant group × time point × time interaction for testosterone concentrations. However, there was a significant main effect for time point (F = 50.635; p < 0.001; η2 = 0.796) with IP being significantly higher than BL (p < 0.001), 30P (p < 0.001), and 60P (p < 0.001).
Furthermore, 60P was significantly less than 30P (p < 0.001). There was no significant group × time point × time interaction for growth hormone concentrations. However, there was a significant main effect for time point (F = 12.151; p < 0.001; η2 = 0.503) with IP being significantly higher than BL (p = 0.019), 30P (p = 0.028), and 60P (p = 0.017). Furthermore, 60P was significantly less than 30P (p = 0.048).
There were no significant group × time point × time interactions for insulin concentrations. However, there was a significant main effect for time point (F = 5.096; p = 0.005; η2 = 0.282) with 30P being significantly higher than BL (p = 0.038).
There were no significant group × time point × time interactions for insulin-like growth factor-1 concentrations. However, there was a significant main effect for time point (F = 17.808; p < 0.001; η2 = 0.578) with IP being significantly higher than BL (p = 0.004), 30P (p = 0.001), and 60P (p = 0.001).
There were no significant group × time interactions for area under the curve values for testosterone (F = 0.253; p = 0.623; η2 = 0.019), insulin (F = 0.427; p = 0.525; η2 = 0.032), or insulin-like growth factor-1 (F = 2.200; p = 0.162; η2 = 0.145); however, there was a significant group × time interaction for growth hormone (F = 5.189; p = 0.042; η2 = 0.302). Post hoc tests for growth hormone revealed no significant change in the CON group (p = 0.833) or URT group (p = 0.046).
The primary findings of this study were that 4 weeks of URT increased the dynamic strength of both the trained and untrained legs; however, increases in muscle cross-sectional area and thickness were only observed in the trained leg. Furthermore, there were no changes in the maximal activation of either leg, nor were there any differences noted in the hormonal response to the exercise regimen over time in either group. This is the first study to examine the effects of dynamic constant external resistance, full-body URT on muscle size, activation, and the hormonal response to exercise.
In this study, the URT group experienced significant increases in peak isometric force (9.0%), LP strength (55.5%), and LE strength (36.4%) in the trained leg, whereas in the contralateral limb, only LP strength increased (40.4%). The lesser adaptations seen in the untrained limb are in agreement with Zhou et al. (43), who suggested that the strength increase in the untrained limb was only about 60% of the trained limb. Although many studies have examined cross education after URT, limited research has used dynamic constant external resistance training (4,40). Similar to this study, Coburn et al. (4) used untrained young adult men and found no significant increases in LE strength in the untrained leg after 8 weeks of training. However, Tracy et al. (40) observed significant increases in LE strength of the untrained leg after 9 weeks of training in older adults. The study with the most similar training intervention showed increases in isometric and isokinetic strength of the trained and untrained musculature after 10 weeks of dynamic constant external resistance URT (18). The current results indicated a significant increase in LP strength in the untrained leg, but no change in PKF. The disparity in cross education may be due to the specificity of training. Previous research has shown that adaptations to resistance training are specific to the modality in which the musculature is trained (31). Furthermore, studies examining URT have shown the greatest cross education to occur in movements that were used during the training program (16,40). Although training specificity may explain the disparity in cross education between LP and PKF, it does not address the different results observed between LP and LE strength. The primary reason for the inconsistency in cross education of strength during dynamic exercises may be the amount of muscle mass used during the exercise. Because the LP engages additional muscle groups (hamstrings and gluteals) when compared with the LE, it is plausible that there are adaptations occurring in each of these muscle groups, which collectively contribute to an increase in LP strength, but have no effect on LE strength. However, in this study, we only examined the size and activation of the RF and VL, and future studies should examine more of the musculature used during training.
Previous research has established an increase in muscle activation after resistance training (16,25,30). Neuromuscular adaptations have been shown to occur earlier in the training program when compared with changes in muscle hypertrophy (30). However, in this study, there were no significant changes in the activation in either the trained or untrained leg. Previous research regarding cross education and muscle activation is equivocal, with some studies showing increases in muscle activation of the untrained limb (16,25,30), whereas other studies have reported no changes (8,20,43). Most likely, the changes in muscle activation are modality specific and similar to changes in strength (16). Hortobagyi et al. (16) showed increases in muscle activation, but only during the muscle action, which was used during training. In this study, dynamic constant external resistance exercise was used during training, but muscle activation was measured during isometric contractions.
It has been well established that resistance training as a direct stimulus leads to significant increases in muscle size (13,30). The changes in muscle size in response to resistance training have been seen in as little as ∼3 (38) or 4 weeks (30). Similar to previous research (30,38), this study showed that there was a significant increase in the muscle thickness and CSA of the RF and VL of the trained leg after 4 weeks of URT. However, the untrained RF and VL showed no changes in muscle thickness or CSA. Previous research has supported these results, showing cross education to occur without any changes in muscle size (9,11,30,34).
Furthermore, in this study, we observed no changes in testosterone, growth hormone, insulin, or insulin-like growth factor-1 to an acute URT session after 4 weeks of URT. Previous research has shown that resistance training can augment the rise in testosterone in response to a resistance exercise bout (21,23); however, the training programs employed were conducted over a longer duration (6 and 10 weeks, respectively) and included greater musculature (full-body, bilateral) than this study. Considering that the early strength improvements in response to resistance training are primarily due to neurological adaptations (30), the effect of any endocrine influence may not be seen in such a time frame, and may have contributed to the lack of change in size of the NON limb. The current data support previous claims that the hormonal response to exercise is not the primary mechanism responsible for cross education (32). However, the DOM limb did experience an increase in muscle size after the 4-week resistance training program while exposed to the same hormonal environment. The disparity in muscle growth between the trained and untrained musculature may be due to adaptations in receptor content and affinity (22). Therefore, further research should be conducted, examining the mechanical stress required to cause hypertrophy and the underlying adaptations to hormonal receptors. With the current sample consisting of fewer than 10 participants per group, future investigations would also benefit from larger sample sizes that may be more likely to detect change.
Four weeks of dynamic constant external resistance full-body URT resulted in the cross education of strength, specifically in the exercises, which use larger muscle mass that were included in the training program. However, there were no significant changes in the maximal activation or size of the untrained musculature. It may be possible that the nonsignificant improvements in the NON limb (Figure 1) collectively contributed to the cross education of strength observed in this study. Furthermore, there were no changes in the acute training response of testosterone, growth hormone, insulin, or insulin-like growth factor. In this study, only the RF and VL were assessed for activation and size; future studies should investigate more of the musculature used during training to determine where the mechanisms of cross education are occurring. Also, examining maximal muscle activation during the dynamic movements that were used during training may be a more appropriate measure because training adaptations are specific to the modality of the training. Although this study did not detect any changes in the underlying mechanisms related to cross education, there was still an increase in strength in the untrained leg from a 4-week dynamic constant external resistance full-body URT program. Future research should compare full-body vs. single limb URT to determine whether training additional musculature will have an added benefit on the strength, muscle size, and activation of the untrained limb.
Funding for this study was provided by iSatori, Inc., Golden, CO.
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