Effects of Low-Intensity Fatigue on Motor Unit Firing Properties During Maximal Voluntary Isometric Contractions

Rehabilitation from injuries impairing the neuromuscular system is often difficult to quantify due to task specificity of the affected or immobilized limb or limbs. Often, exercise regimens with clinical practitioners are designed to progress motor performance to a capacity that reflects muscle force production to pre-injury capacity. Modulation of force production from these tasks are regulated from various mechanism in the central and peripheral nervous system. The neural activation of these systems can be recorded as electrical impulses using several non-invasive techniques. Recently, the ability to examine these during fatiguing exercise has provided further insight into activation patterns and firing properties in central (i.e., motor and pre-motor cortex) and peripheral locations (i.e., skeletal muscle). Tracking motor neuron activation from these affords opportunities to investigate the regulation and modulation of task specific performances and can improve the capacity to derive performance criteria for rehabilitation. PURPOSE: The purpose of this investigation was to examine the effects of low-force isometric fatiguing contractions on motor unit firing properties during maximal voluntary contractions (MVCs). METHODS: Following 3 MVCs, four lower-body resistance trained males (23 yrs. ± 3, ht.176cm ±6., 89kg wt.­­ ±16) performed 60 second submaximal (30% MVC) isometric ramp contraction of the knee extension exercise. Knee extensions were performed on a custom-built seat using an S-beam load-cell to measure isometric force production of the quadriceps muscle group. During the fatiguing contractions, participants were encouraged to perform as many trapezoidal ramp contractions (i.e., 30%) as possible, until they could no longer sustain the required force production. Fatigue was established when the participant could no longer maintain the contraction force within 10% for no less that 3 seconds during the isometric hold. Surface electromyography signals were collected from the vastus lateralis of the right leg and were decomposed into their constituent motor unit action potential (MUAP) trains for further analysis. Paired samples t-tests were used to compare the changes in slope and y-intercepts of the MUAP amplitude vs mean firing rate relationships before (PRE) and after (POST) fatigue. RESULTS: There were significant differences between the PRE and POST slopes and y-intercepts of the MUAP amplitude vs mean firing rate relationships (p \u3c 0.05). CONCLUSION: The low-intensity fatiguing contractions elicited responses in motor unit firing properties that are consistent with previous findings. Although there were fewer motor units identified in the POST MVCs, the accuracy in the recordings following decomposition analysis were able to accurately validate a quantifiable amount of motor units for further processing techniques and statistical analysis. These advanced technologies and techniques in identifying local (peripheral) responses in skeletal muscle fatigue will continue to provide robust information in the modulation of force production of motor performance

Bilateral Comparison of Cross-Sectional Area of the Vastus Lateralis Among Rodeo Athletes Who Use a Lead Leg While Competing

Skeletal muscle morphology commonly occurs in athletes who train for specific adaptions to meet the demands of the performance task. For rodeo athletes, specific performance tasks require the use of unilateral dominance. Specifically, the use of a “lead leg” when competing may display adaptations when compared to an underutilized opposite limb. PURPOSE: The purpose of this study was to bilaterally compare the cross-sectional area (CSA) of the vastus lateralis (VL) among rodeo athletes who use a lead leg during their performance versus those who do not. METHODS: Thirty-seven male and female (± 1yrs; 66.38 ± 3.94 cm; 156.89 ± 38.82 kg) rodeo athletes visited the Human Performance Laboratory for testing. Using ultrasonography, each athlete laid supine for CSA measure of the VL from both their lead and non-lead leg. Panoramic images were obtained from half the distance from the anterior superior iliac crest to the lateral patella, and were externally measured using ImageJ software to determine CSA (cm3). Groups were determined by those athletes who predominately use their lead leg (n=9) for their performance versus those who do not (n=28). A two-way mixed factorial ANOVA (group [uses vs doesn’t use] x leg [lead leg vs non-lead leg] was used to compare CSA of the VL. RESULTS: The results of the 2-way ANOVA determined no significant groups x leg interaction (p=0.894), nor main effects for CSA between legs (p=0.306). CONCLUSION: To our knowledge, there are no reports regarding performance measures nor morphological comparisons in rodeo athletes. The small number of athletes that use their lead leg during their respective performance were likely to have influenced the outcome of the present investigation. However, this present study does present opportunities to biomechanically assess performance measures in order to assist in the assessment of task requirements for the separate events of a rodeo athlete

Monitoring Firing Behaviors of Motor Unit Action Potentials During Low-Intensity Fatiguing Contractions

Motor Units (MU) are systematically activated from increasing size in accordance to the size principle. As muscle fatigue sets in, MU firing rate increases in response to sustaining whole-body force production during a low-loaded sustained contraction with respect to MU size and MU interspike intervals (ISI). PURPOSE: The purpose of this study is to compare the motor unit firing behaviors during low-force isometric fatiguing contractions. Methods: Following 3 MVCs, eight lower-body resistance trained males (23.38 ± 2.88 yrs.; 178.38 ± 7.85 cm; 86.10 ± 11.88 kg) performed 60 second submaximal (30% MVC) isometric ramp contraction of the knee extension exercise. Knee extensions were performed on a custom-built seat using an S-beam load-cell to measure isometric force production of the quadriceps muscle group. During the fatiguing contractions, participants were encouraged to perform as many trapezoidal ramp contractions (i.e., 30%) as possible, until they could no longer sustain the required force production. Fatigue was established when the participant could no longer maintain the contraction force within 10% for no less than 3 seconds during the isometric hold. Surface electromyography signals from the vastus lateralis were collected and decomposed into constituent motor unit action potential (MUAP) trains for further analysis. Three separate one-way ANOVAs were used to compare the number of MUs, MFR, and ISI during the first, middle, and last (time) contractions. RESULTS: There were no significant main effects between contractions for the number of MUs, MFR, and ISI (p\u3e0.05). CONCLUSION: The results of this investigation are likely due to the increased recruitment of higher threshold MUs as the muscle fatigued through the protocol. The size (i.e., amplitude) of the action potential, and its respective relationships with the recruitment threshold at which these MU initially fired, are preeminent indicators of MU firing behaviors during sustained force production to fatigue

Maintenance of Muscle Size and Function Following In-Season Training of Division I Football Athletes

During in-season training for collegiate athletes, there are fewer metabolically demanding resistance training sessions compared to other seasons of the programmed macrocycle. The shift of training duration and intensity for the in-season mesocycle may hinder an athlete’s ability to maintain gains from prior training cycles. In-season training commonly focuses on maintaining muscle mass and reducing the probability of atrophy. PURPOSE: The purpose of this study is to examine the potential adaptations following in-season training on muscle size and function in Division I (DI) athletes METHODS: Fifteen DI football athletes (mean ± sd; age, 20 ± 2 years; height, 189.6 ± 11.6 cm; weight, 114.1 ± 19.5 kg) completed two separate visits to the Human Performance Laboratory immediately before and after in-season training. Using an S-beam load cell, peak force (PF) was measured from the right leg during maximal voluntary contractions of the isometric knee extension exercise. Muscle CSA was obtained from the rectus femoris and vastus lateralis using ultrasonography. Paired samples t-test were used to compare pre and post measures of strength and size (i.e. PF, CSA). RESULTS: The results from a dependent t-test revealed no significant difference between preseason and postseason PF (684.87 ± 213.95 N vs 805.87 ± 325.77 ; p = 0.22), as well as no significant difference between CSA of the right leg VL (37.09 ± 16.67 cm3 vs 40.84 ± 8.01 cm3; p = 0.42) and RF (19.35 ± 7.41 cm3 vs 21.82 ± 3.63 cm3; p= 0.28) CONCLUSION: The Division I football athletes maintain muscle size and strength during in-season and pre-season training. This data set indicates the effectiveness of resistance programming during in season training

Electromyographic Activation of the Quadriceps During the Concentric Phase of the Squat

Mechanical muscle properties such as length-tension relationship in conjunction with electromyography (EMG) allow greater insight into the overall motor coordination. The different morphologies of the quadriceps and their force generating capacities can be quantified through the differences in EMG amplitude. PURPOSE: To examine quadriceps activation during the squat exercise under various intensities. METHODS: Eleven participants (age 18-35; height: 181.1 ± 8.9cm; weight: 97.7 ± 19.1kg) were recruited for this study. Subjects completed three visits, one familiarization and two testing days separated by 48 hours. Ultrasound images were obtained from the three primary leg extensor muscles (vastus lateralis (VL), vastus medialis (VM), rectus femoris (RF)) prior to testing. Subjects then performed a randomized order of squats and knee extensions at 25%, 50%, and 75% of their maximal contraction (MVC). Three separate surface electromyography sensors were used to record the activation from the VL, VM, and RF from a .25 second time window at the time the peak force (N) occurred during the concentric portion of the squat. A 2-way mixed factorial ANOVA (muscle [VL v VM v RF] x intensity [25% v 50% v 75%]) was used to analyze EMG activation under each condition. RESULTS: There was no significant muscle x intensity interaction (p\u3e.05); however, there was a significant main effect for muscle (p=.003). When collapsed across intensity, EMG amplitude of the RF was lower than the VL (p=.025) and VM (p=.000). CONCLUSION: The lower EMG amplitude in the biarticular RF is in line with previous findings revealing that morphological differences affect motor coordination during the squat performance. Thus, as the RF is shortening at one portion it is lengthening at the other with little change in length throughout the movement whereas the VL and VM have a greater net length change

Effects of Inducing Post Activation Potentiation on Leg Extensors using Isotonic Movements

Post Activation Potentiation (PAP) has been documented in previous studies as evoking greater muscle activation and velocities in submaximal contractions. Literature has additionally shown that isotonic contractions have increased motor unit activation when compared to isokinetic. PURPOSE: Induce PAP using different interventions and evaluate PAP differences between interventions in the vastus lateralis (VL), rectus femoris (RF), vastus medialis (VM) and evaluate velocity and muscle activation differences. METHODS: 8 recreationally active males (n=8, age = 20.6 ± 1.8, height = 178.2 ± 12.1 cm, weight = 83.2 ± 48.3 kg) with no previous lower extremity injuries completed this study. Following a familiarization visit, subjects completed three randomized visits separated by 24 hours. Each visit consisted of one of the following interventions to induce PAP: 2 sets of 6 isotonic leg extensions at 50% MVC, 4 isometric MVCs, or two sets of 6 isotonic extensions at 60% MVC. Following each intervention subjects performed 20 isotonic knee extensions at 30% MVC. Peak velocity was recorded during these contractions in the familiarization day as well as subsequent visits using a robotic dynamometer (Biodex System 4). Three 4-pin Surface electromyography (EMG) sensors were placed over the VL, RF, and VM to record muscle activation. After collecting EMG signals root mean square (RMS) values were calculated for each muscle. 3 separate one-way analyses of variance (ANOVA) were completed to compare RMS values for each muscle across each visit. 1 separate one-way ANOVA was run to compare peak velocity across each visit. A 2-way repeated measures ANOVA (Muscle [VL v RF v VM] x Intervention [2 sets of 6 isotonic contractions at 50% MVC v four isometric MVCs v 2 sets of 6 isotonic contractions at 60% MVC]) was used to compare RMS values across visits. RESULTS: There were no significant differences (p\u3e0.05) in RMS values the VL, or the RF across visits in the one-way ANOVAs comparing muscular differences. The one-way ANOVA for peak velocity also showed no significant differences across visits (p\u3e.05). However, a main effect was found between visits in the VM (p =.014). CONCLUSION: PAP intervention has been shown in previous studies to increase velocity; however, this was not found to a significant effect. A larger sample size could result in this being induced to a larger degree. Additionally, intrasubject differences could also have contributed. The VM having increased activation between visits may be caused by PAP evoking a larger response in this muscle compared to the other leg extensors. Further research should be performed to examine these findings. Additionally, motor unit behavior analysis could reveal additional differences between the interventions and the effects on each of the leg extensors

Test Re-Test Reliability of Motor Unit Action Potential Relationships During Isokinetic Knee Extension

Decomposition electromyography (EMG) is now capable of examining relationships between motor unit (MU) firing behaviors during dynamic movements. However, reliability across testing sessions has yet to be established within the quadriceps group during controlled isokinetic contractions. PURPOSE: To examine test re-test reliability of MU identification and firing behaviors of the knee extensor muscles during isokinetic contractions. METHODS: Nine (22.67 ± 1.17 yrs., 172.47 ± 5.43 cm, 77.75 ± 10.72 kg) recreationally active individuals randomly performed ten continuous knee extension contractions at 60°/s and 180°/s on two separate visits. Surface EMG was used to record activation in the vastus medialis (VM), rectus femoris (RF), and vastus lateralis (VL) between visits. Locations were marked for accurate replacement of the sensors between visits. Signals from each location during each visit were decomposed and validated for statistical analysis. Interclass correlation coefficients (ICC, 2,1) were used to assess the reliability of linear coefficients from the MFR vs MUAP relationships for the validated MUs between visits. RESULTS: Test re-test reliability of the MFR vs MUAP slopes for the VM at 60°/s and 180°/s displayed moderate reliability (ICC = 0.681 - 0.727). For the VL, MFR vs MUAP slopes and y-intercepts during both velocities showed poor to moderate reliability (ICC = 0.098 – 0.452). Slope and y-intercepts for the RF at 60°/s displayed poor reliability (ICC = 0.407, 0.492, respectively), but good and moderate reliability at 180°/s (ICC = 0.920, 0.647, respectively) DISCUSSION: These findings suggest that the velocity of the contraction may elicit variations in MU identification, decomposition, and relationships in the knee extensor muscles activation during isokinetic contractions. Many of the moderate to poor reliability classifications may have been due to the chosen velocities and repetitions performed. Future investigations will consider these to best identify future recommendations in decomposition EMG during dynamic movements

Effects of Diverting Activities on Recovery from Repeated Maximum Voluntary Contractions

Previous investigations have demonstrated that diverting activities (e.g., contralateral exercise and pinching one’s fingers together) can minimize the severity of muscle fatigue that occurs during exercise. In addition to their ability to resist fatigue during exercise, diverting activities may help subjects recover between work bouts. The purpose of this study was to examine the effects of mental and physical diverting activities on recovery from fatiguing isometric muscle actions. On three separate occasions, twelve men (mean ± SD age = 22 ± 1 years) and nine women (age = 22 ± 2 years) performed ten repeated, ten-second isometric maximum voluntary contractions (MVCs) of the dominant leg extensors with ten seconds of rest between each attempt (i.e., ten seconds “on,” ten seconds “off”). Following this fatiguing protocol, the subjects performed math problems (mental diverting activity), five, 20-second isometric muscle actions with the non-dominant leg extensors at a force corresponding to 50% MVC (physical diverting activity), or rested quietly (control). Immediately following this intervention period, the subjects performed a final three-second MVC, which assessed how well the leg extensors recovered from the fatiguing protocol (Post-Recovery MVC). These three data collection trials were randomly performed, and separated by at least 48 hours of rest. A two-way (time [Pre-Fatigue MVC, MVC #1-10, Post-Fatigue MVC, Post-Recovery MVC] × intervention [math problems, contralateral, and control]) repeated measures analysis of variance was used to examine the isometric force data. The mean ± SD Post-Recovery MVC values were as follows: mental diverting activities =765 ± 210 N; physical diverting activities = 797 ± 235 N; control = 790 ± 258 N. There was no significant time × intervention interaction (partial eta squared = .072), no main effect for intervention, but there was a main effect for time. The marginal mean pairwise comparisons indicated: Pre-Fatigue MVC \u3e MVC#1 – Post-Fatigue MVC; MVC #4 – Post-Fatigue MVC \u3c Post-Recovery MVC. In contrast to the results from previous studies, our results indicated that performing mental and physical diverting activities did not help the subjects recover from fatigue

Relationship Between One Repetition Maximum Strength and Peak Power Output for the Free-Weight Bench Press Exercise

Strength and conditioning coaches are often interested in improving power output in their athletes. As many coaches are aware that power is the product of force and velocity, an emphasis is often placed on training with loads that correspond to peak mechanical power output (e.g., 30-50% of the one repetition maximum [1RM]). The purpose of this study was to examine the relationship between 1RM strength and peak power output during the free-weight bench press exercise. Twenty-one healthy, recreationally resistance-trained men (mean ± SD age = 24 ± 3 years; body mass = 90.5 ± 14.6 kg; 1RM bench press = 125.4 ± 18.4 kg) volunteered for this investigation. A minimum of 48 hours following a maximal strength testing and familiarization session, the subjects performed a single repetition of the bench press using 50% of the 1RM. For each repetition, the subjects were instructed press the weight explosively throughout the concentric portion of the range of motion. A Tendo Weightlifting Analyzer was used to assess peak power output for each repetition. A linear regression analysis was used to examine the relationship between 1RM strength and peak power output. The results indicated that the coefficient of determination (R2) was 0.697, suggesting that 1RM strength explained roughly 70% of the variance in peak power output. The linear slope coefficient was 5.94 W/kg, and the y-intercept was 110.2 W. These findings demonstrated that 1RM strength and peak power output were highly correlated. Although explosive training with light loads is often stressed in strength and conditioning programs, for the free-weight bench press exercise, a relatively small degree of variance in peak power output can be explained by factors other than 1RM strength

Laboratory Measurements of Division 1 College Baseball Players to Predict Field Base Performance

Cross-sectional area (CSA) of skeletal muscle has been commonly used as a predictor of force production. Peak force (PF) production during maximal contractions of the lower body can be a primary indicator of an athlete’s potential to perform on the field. PURPOSE: The purpose of this study was to use CSA for the VL and RF and PF to predict 60-yard dash times. METHODS: Fifteen male athletes (20.64 yrs. ±1.54, 182.89 cm. ±5.74, 196.58 kg ±21.2.) participated in this investigation. Sixty-yard dash times were recorded from each athlete prior to laboratory testing. Skeletal muscle CSA were taken from the VL and RF of the right leg using ultrasonography. PF was obtained during a maximal voluntary contraction of the isometric knee extension exercise using a custom-built seat and an S-beam load-cell. A multiple regression analysis was used to determine the influence of muscle CSA for the VL and RF during peak force.2 maximal isometric voluntary contractions (MVC) of the right knee extension exercise. RESULTS: This data was assessed using a forward stepwise multiple regression and found no statistical significance (p \u3e 0.05) for the prediction model of 60-yard dash times using the PF, VL and RF CSA. These variables were only able to account for 16.3% of the variance in 60 dash yard times. The resulting prediction equation was as follows: 60-yard dash time = 7.453 + (-.001x PF) + (-.014x RF) + (.004xVL; R2 0.163). CONCLUSION: This is likely due to the low number of participants and potential error in familiarization of laboratory testing and measurement as it relates to field-based testing measures for specific populations