Isotonic Knee Extension Research Articles

Voluntary activation of human knee extensors during isotonic shortening contractions.

The interpolated twitch technique (ITT) is often used to assess voluntary activation during isometric contractions; however, this may have limited relevance to dynamic contractions. Although the ITT has been applied to relatively slow isokinetic contractions (< 150°/s), it has received limited consideration during unconstrained velocity (i.e., isotonic) contractions, despite their relevance to natural movements. Here, we explored the ITT during isotonic knee extension contractions using a modified dynamometer. Young males (n = 6) and females (n = 4) performed isometric and isotonic knee extension contractions of sub-maximal and maximal intensities with doublet (150 Hz) muscle belly stimulations to assess voluntary activation. Following each voluntary isotonic contraction (velocity range ~ 35°/s to ~ 275°/s), resting potentiated doublets were evaluated during passive joint rotation at the same angular velocity achieved during voluntary efforts, to account for force-velocity characteristics. Correlations between voluntary activation and the proportion of maximal torque or power were evaluated for isometric and isotonic contractions, respectively. Isometric voluntary activation was strongly correlated with increasing torque output (r = 0.96, p < 0.001). Doublet torque during passive joint rotation displayed a hyperbolic relationship with increasing angular velocity (r = 0.98, p < 0.001). Isotonic voluntary activation was strongly correlated with increasing power output (r = 0.89, p < 0.001). During maximal effort contractions, no differences were observed in voluntary activation between isometric and isotonic conditions (89.4% vs. 89.2%, p = 0.904). The ITT is a valid approach to evaluate voluntary activation during an isotonic contraction using a modified dynamometer. Participants were able to achieve a similar high level of voluntary activation during isometric and isotonic contractions.

Trade-Off Between Maximal Power Output and Fatigue Resistance of the Knee Extensors for Older Men.

This study investigated associations of fatigue resistance determined by an exercise-induced decrease in neuromuscular power with prefatigue neuromuscular strength and power of the knee extensors in 31 older men (65-88 years). A fatigue task consisted of 50 consecutive maximal effort isotonic knee extensions (resistance: 20% of prefatigue isometric maximal voluntary contraction torque) over a 70° range of motion. The average of the peak power values calculated from the 46th to 50th contractions during the fatigue task was normalized to the prefatigue peak power value, which was defined as neuromuscular fatigue resistance. Neuromuscular fatigue resistance was negatively associated with prefatigue maximal power output (r = -.530) but not with prefatigue maximal voluntary contraction torque (r = -.252). This result highlights a trade-off between prefatigue maximal power output and neuromuscular fatigue resistance, implying that an improvement in maximal power output might have a negative impact on neuromuscular fatigue resistance.

Frailty Status Improvement after 5-Month Multicomponent Program PROMUFRA in Community-Dwelling Older People: A Randomized Controlled Trial.

A study was made of the effect of the PROMUFRA multicomponent frailty program upon physical frailty, kinanthropometry, pain and muscle function parameters in frail and pre-frail community-dwelling older people. Eighty-one participants were randomly allocated to the intervention group (IG) or control group (CG). The IG performed PROMUFRA for 20 weeks, using six strength exercises with three series of 8–12 repetitions until muscular failure, and seven myofascial exercises, with one set of 10 repetitions. The CG continued their routine. The frailty criteria number (FCN), kinanthropometric parameters and muscle function were measured at baseline and after the program. Between-group differences were found in the interaction for FCN, muscle mass, fat mass, skeletal muscle mass index, knee flexion range of motion (ROM), hip flexion with knee straight ROM, maximum isometric knee extension, maximum isotonic knee extension, maximum leg press and hand grip strength., and also on post-intervention frailty status. The IG showed a statistical trend towards decreased pain. In conclusion, the PROMUFRA program is a potential training approach that can bring benefits in physical frailty status, body composition, ROM and muscle function among frail or pre-frail community-dwelling older people.

Does Inosine Monophosphate Formation Facilitate the Bioenergetic Response to Incremental Contractions in Human Skeletal Muscle In Vivo?

Traditionally, it has been accepted that working muscle will endeavor to maintain [ATP] near its resting level in order to maintain a favorable free energy of ATP hydrolysis (∆GATP). Recent reports of significant declines in [ATP] during maximal muscular contractions in vivo spark new interest in this concept. ATP is hydrolyzed to adenosine diphosphate (ADP) in the creatine kinase reaction to provide energy directly to working ATPases. As needed, additional energy can be provided by the breakdown of ADP to adenosine monophosphate (AMP), which is rapidly cleared by the formation of inosine monophosphate (IMP), with accompanying loss of total adenine nucleotides. IMP formation has been thought to occur only during very intense muscular work, wherein ATP production does not keep pace with ATP consumption. Here, we propose a role for IMP in managing both ∆GATP and flux through a key energetic pathway during contractions. To test our hypothesis that IMP formation mitigates the accumulation of ADP, 8 males (28.4 + 3.5 yr, mean + SD) performed 8 min of incremental, isotonic knee extension contractions (1 every 2 s, starting at 6% peak torque and incrementing by 3% every 2 min) on a magnetic resonance (MR)‐compatible ergometer in a 3‐Tesla MR system. 31Phosphorus MR spectra were obtained at rest (60‐s average) and throughout the contraction protocol (10‐s temporal resolution). Peaks corresponding to ATP, phosphocreatine (PCr), inorganic phosphate (Pi), and phosphomonoesters (PME) were fit in jMRUI, and concentrations for these metabolites were determined assuming resting [ATP] = 8.2 mM. [ADP], [AMP], [IMP], and the rate of ATP (mM∙s‐1) production by non‐oxidative glycolysis (i.e., wherein the fate of pyruvate is lactate; ATPgly) were also calculated. As expected, [PCr] decreased and [Pi] increased approximately linearly throughout the incremental contraction protocol. Surprisingly, [ATP] decreased and [IMP] increased (to 7.3 + 0.4 mM and 0.8 + 0.56 mM, respectively) steadily from the onset of contractions until the final stage, at which point no further changes were observed. In contrast, both [ADP] and [AMP] increased linearly from the start of contractions until the final stage, when they accumulated more rapidly; ultimately to 109.9 + 52.1 uM and 1.0 + 0.91 uM, respectively. The glycolytic contribution to ATP production was consistent for the first 3 stages, after which ATPgly increased rapidly to 0.102 + 0.14 mM·s‐1 at 480 s. Collectively, the timing and magnitude of these metabolic changes in working muscle suggest a sequence of responses designed to maintain cytosolic [ADP] sufficiently low as to mitigate changes in ∆GATP, but at a level sufficient to stimulate ATPgly as contraction intensity increases. A novel observation in this study was the decrease in [ATP] (and thus increase in [IMP]) from the onset of the contraction protocol, which then leveled off at the highest contraction intensity. The utility and consequences of these responses remain to be determined, but this study supports the concept that IMP formation is important in maintaining the cellular energy state as well as in directing the bioenergetic response to a series of incremental muscular contractions in vivo.

Effect of different eccentric tempos on hypertrophy and strength of the lower limbs.

The purpose of this study was to evaluate the effects of altering the duration of the eccentric phase in isotonic contractions on muscle hypertrophy and strength of the quadriceps femoris. Ten healthy young adults (8 men and 2 women: Height: 173.3 ± 9.6 cm: Body mass: 69.84 ± 10.88 kg; Body fat: 19.47 ± 8.42%; Age: 25.3 ± 4.8 years) performed unilateral isotonic knee extension exercise, whereby each leg was randomly allocated to perform the eccentric phase of movement with a duration of either 2 seconds (G2S) or 4 seconds (G4S). Both conditions carried out the concentric phase of each repetition at a 1 second duration with no rest in the transition phases. Each condition performed 5 sets using 70% of 1 repetition maximum until muscle failure with 3 minutes of rest between sets for 8 weeks. The change in muscle strength was assessed by 1RM knee extension and muscle thickness was assessed by A-mode ultrasound. For each outcome variable, linear mixed-effects models were fit using restricted maximum likelihood. Hedges’ g effect sizes were calculated to provide insights into the magnitude of effects. Results showed all muscles increased in size over time; mean effects were similar in all muscles except for the vastus medialis, which favored the G4S condition. Conversely, only a trivial and highly variable effect was observed between interventions for strength gain. Our results suggest different eccentric durations produce similar increases in hypertrophy of the vastus lateralis and rectus femoris; however, the vastus medialis showed greater growth from the slower eccentric duration. Eccentric duration did not differentially affect strength-related adaptations.

Examination Of High-velocity Blood Flow Restricted Resistance Exercise Versus High-load Resistance Exercise

Muscle strength gains following blood flow restricted (BFR) resistance training are consistently lower than those observed after HL training and this may be due to differences in neuromuscular activation. Integrating high-velocity muscle contractions and BFR resistance exercise may increase neuromuscular activation. PURPOSE: To compare torque and neuromuscular activation before and after sessions of HL knee extension exercise and high-velocity BFR knee extension exercise in young and older adults. METHODS: Ten young males and females (20.3+1.5 years, 1.73+.06 m, 69.2+10.4 kg) and 10 older males and females (72.6+4.7 years, 1.70+.12 m, 74.3+14.6 kg) performed randomized sessions of HL (80% 1-RM for 3 sets of 10 isotonic knee extension repetitions) and low-load, high-velocity BFR (30% 1-RM coupled with a vascular restriction for one set of 30 knee extension repetitions and 3 sets of 15 repetitions completed as quickly as possible). Knee extension isometric torque was assessed before and after each session of exercise and neuromuscular activation of the vastus lateralis (VL) was quantified with surface electromyography (EMG). RESULTS: The young participants had higher isometric torque than the older participants (213.4+54.8 vs 160.1+56.3 Nm; P=.01). There was an average decrement in torque of 31.4+9.2% among all participants from baseline to post exercise (P<.01). The magnitude of this decrement was similar between conditions (P=.19), age (P=.12), and the interaction of time, condition and age (P=.16). EMG amplitude normalized to pre isometric torque in the first 5 knee extension repetitions of the HL condition averaged 106+57% and 90+33% in the BFR high-velocity condition. In the last 5 repetitions, EMG amplitude averaged 117+56% and 113+46 in the HL and BFR conditions respectively. This was a significant increase in EMG amplitude (P<.01) but there were no interactions or main effects of condition or age (P>.05). CONCLUSION: Combing high-velocity contractions with BFR resistance exercise results in decrements in torque and heightened neuromuscular activation similar to HL exercise in both young and older adults. Future studies should evaluate the training adaptations from high-velocity BFR resistance training. Supported by University of New Hampshire Grimes Family Fund.

Ability of Isokinetic Dynamometer to Predict Isotonic Knee Extension 1-Repetition Maximum

Resistance training exercise prescription is often based on exercises performed at a percentage of a 1-repetition maximum (1RM). Following knee injury, there is no consensus when a patient can safely perform 1RM testing. Resistance training programs require the use of higher loads, and loads used in knee injury rehabilitation may be too low to elicit gains in strength and power. A maximum isometric contraction can safely be performed during early stages of knee rehabilitation and has potential to predict an isotonic knee extension 1RM. To determine whether a 1RM on an isotonic knee extension machine can be predicted from isometric peak torque measurements. Descriptive laboratory study. University research laboratory. A total of 20 (12 males and 8 females) healthy, physically active adults. An isokinetic dynamometer was used to determine isometric peak torque (in N·m). 1RM testing was performed on a knee extension machine. Linear regression was used to develop a prediction equation, and Bland-Altman plots with limits of agreement calculations were used to validate the equation. There was a significant correlation (P < .001, r = .926) between peak torque (283.0 [22.6]N·m) and the knee extension 1RM (69.1 [22.6]kg). The prediction equation overestimated the loads (2.3 [9.1]kg; 95% confidence interval, -15.6 to 20.1kg). The results show that isometric peak torque values obtained on an isokinetic dynamometer can be used to estimate 1RM values for isotonic knee extension. Although the prediction equation tends to overestimate loads, the relatively wide confidence intervals indicate that results should be viewed with caution.

Test-retest reliability of knee extensor rate of velocity and power development in older adults using the isotonic mode on a Biodex System 3 dynamometer.

Isotonic testing and measures of rapid power production are emerging as functionally relevant test methods for detection of muscle aging. Our objective was to assess reliability of rapid velocity and power measures in older adults using the isotonic mode of an isokinetic dynamometer. Sixty-three participants (aged 65 to 82 years) underwent a test-retest protocol with one week time interval. Isotonic knee extension tests were performed at four different loads: 0%, 25%, 50% and 75% of maximal isometric strength. Peak velocity (pV) and power (pP) were determined as the highest values of the velocity and power curve. Rate of velocity (RVD) and power development (RPD) were calculated as the linear slopes of the velocity- and power-time curve. Relative and absolute measures of test-retest reliability were analyzed using intraclass correlation coefficients (ICC), standard error of measurement (SEM) and Bland-Altman analyses. Overall, reliability was high for pV, pP, RVD and RPD at 0%, 25% and 50% load (ICC: .85 - .98, SEM: 3% - 10%). A trend for increased reliability at lower loads seemed apparent. The tests at 75% load led to range of motion failure and should be avoided. In addition, results demonstrated that caution is advised when interpreting early phase results (first 50ms). To conclude, our results support the use of the isotonic mode of an isokinetic dynamometer for testing rapid power and velocity characteristics in older adults, which is of high clinical relevance given that these muscle characteristics are emerging as the primary outcomes for preventive and rehabilitative interventions in aging research.

Sports Physical Therapy Section Poster Presentations (Abstracts SPO1-SPO166).

These abstracts are presented here as prepared by the authors. The accuracy and content of each abstract remain the responsibility of the authors. In the identification number above each abstract, SPO designates a Sports Physical Therapy Section poster presentation. J Orthop Sports Phys Ther 2018;48(1):A203-A276. doi:10.2519/jospt.2018.48.1.A203.

Reliability of muscle blood flow and oxygen consumption response from exercise using near-infrared spectroscopy.

What is the central question of this study? Continuous-wave near-infrared spectroscopy, coupled with venous and arterial occlusions, offers an economical, non-invasive alternative to measuring skeletal muscle blood flow and oxygen consumption, but its reliability during exercise has not been established. What is the main finding and its importance? Continuous-wave near-infrared spectroscopy devices can reliably assess local skeletal muscle blood flow and oxygen consumption from the vastus lateralis in healthy, physically active adults. The patterns of response exhibited during exercise of varying intensity agree with other published results using similar methodologies, meriting potential applications in clinical diagnosis and therapeutic assessment. Near-infrared spectroscopy (NIRS), coupled with rapid venous and arterial occlusions, can be used for the non-invasive estimation of resting local skeletal muscle blood flow (mBF) and oxygen consumption (mV̇O2), respectively. However, the day-to-day reliability of mBF and mV̇O2 responses to stressors such as incremental dynamic exercise has not been established. The aim of this study was to determine the reliability of NIRS-derived mBF and mV̇O2 responses from incremental dynamic exercise. Measurements of mBF and mV̇O2 were collected in the vastus lateralis of 12 healthy, physically active adults [seven men and five women; 25 (SD 6)years old] during three non-consecutive visits within 10days. After 10min rest, participants performed 3min of rhythmic isotonic knee extension (one extension every 4s) at 5, 10, 15, 20, 25 and 30% of maximal voluntary contraction (MVC), before four venous occlusions and then two arterial occlusions. The mBF and mV̇O2 increased proportionally with intensity [from 0.55 to 7.68mlmin-1 (100ml)-1 and from 0.05 to 1.86mlO2 min-1 (100g)-1 , respectively] up to 25% MVC, where they began to plateau at 30% MVC. Moreover, an mBF/mV̇O2 muscle oxygen consumption ratio of ∼5 was consistent for all exercise stages. The intraclass correlation coefficient for mBF indicated high to very high reliability for 10-30% MVC (0.82-0.9). There was very high reliability for mV̇O2 across all exercise stages (intraclass correlation coefficient 0.91-0.96). In conclusion, NIRS can reliably assess muscle blood flow and oxygen consumption responses to low- to moderate-intensity exercise, meriting potential applications in clinical diagnosis and therapeutic assessment.

Lean muscle volume of the thigh has a stronger relationship with muscle power than muscle strength in women with knee osteoarthritis

BackgroundThigh lean muscle and intramuscular fat have been implicated in the impairment of physical function observed in people with knee osteoarthritis. We investigated the relationships of quadriceps and hamstrings intramuscular fat fraction and lean muscle volume with muscle power and strength, controlling for neuromuscular activation, and physical performance in women with knee OA. MethodsWomen (n=20) 55years or older with symptomatic, radiographic knee osteoarthritis underwent a 3.0T magnetic resonance imaging scan of the thigh of their most symptomatic knee. Axial fat-separated images were analyzed using software to quantify intramuscular fat and lean muscle volumes of the quadriceps and hamstrings. To quantify strength and power of the knee extensors and flexors, participants performed maximum voluntary isometric contraction and isotonic knee extensions and flexions, respectively. Electromyography of the quadriceps and hamstrings was measured. Participants also completed five physical performance tests. FindingsQuadriceps and hamstrings lean muscle volumes were related to isotonic knee extensor (B=0.624; p=0.017) and flexor (B=1.518; p=0.032) power, but not knee extensor (B=0.001; p=0.615) or flexor (B=0.001; p=0.564) isometric strength. Intramuscular fat fractions were not related to isotonic knee extensor or flexor power, nor isometric strength. No relationships were found between intramuscular fat or lean muscle volume and physical performance. InterpretationMuscle power may be more sensitive than strength to lean muscle mass in women with knee osteoarthritis. Thigh lean muscle mass, but neither intramuscular nor intermuscular fat, is related to knee extensor and flexor power in women with knee osteoarthritis.

The relationship between muscle fatigue and physical function in older adults

Background:Aging can inhibit a physical therapist’s ability to halt physical function decline. Although the ability to generate a large force in a very short time [peakmuscle power (PMP) and strength] is important to physical function, several daily living activities require sustained muscle power (muscle fatigue resistance). Therefore, muscle fatigue may be related to physical function. However, the relationship between force generation decrease during a fatigue-inducing isotonic contraction and physical function is unknown. Purpose: To determine the relationship between force generation and physical function decreases and the cause of decreased physical function by investigating muscle ability to produce force output and neuromuscular activity during a fatigue-inducing isotonic contraction. Methods:We compared 15 community-dwelling elderly women [age: 79.6± 6.2 years; body mass index (BMI): 23.0± 3.0 kg/m2; mini-mental state examination score≥24] with high (HO) and low (LO) physical functioning levels based on their Short Physical Performance Battery score (HO> 9; LO≤ 9). Surface electromyograms were recorded from the rectus femoris and vastus lateralis muscles. Torque wasmeasured during three isometricmaximal voluntary contractions (MVCs) of 5-s in knee extension at 90◦. After a 5-min rest, the participants performed 30 fatigue-inducing isotonic knee extensions as fast as possible (from 90 g to terminal knee extensionwith 20%MVC resistance). During this task, we assessed PMP, peak velocity (PV), total power (TP), the rate of change in power (RCP, baseline to peak power), and the rate of change in the rootmean square (RMS, using the average value of the two muscles; baseline: 1–5th). An independent t-test was performed to compare age, BMI, MVC, PMP, PV, and TP. A two-way repeated-measures ANOVA was used to determine the interaction and main effects of RCP and RMS. When ANOVA revealed significant interaction or main effects, Tukey’s test or an independent t-test were performed for post hoc analyses to assess differences. Pearson’s correlation analysis was used to assess associations between RMS and TP. For all analyses, the significance level α was set at p< 0.05. Results: Age, BMI, MVC, PMP, and PV were not different between groups. RCP during the fatigue task decreased significantly in both groups (p< 0.05). However, TP (LO: 1385.4± 547.5WvsHO2068.8± 635.3W) and the 26th–30th RMS (LO: 91.7± 10.3% vs HO: 106.9± 12.1%) were significantly lower in theLOgroup than in theHOgroup (p< 0.05). A significant positive correlation was detected between TP and the 26th–30th RMS (r= 0.585, p< 0.05). Conclusion(s): These results suggest that a low muscle ability to produce force output is related to low physical functioning and that neuromuscular activity decreases physical function, providing evidence for the relationship between muscle fatigue and physical function. Implications:High-intensity exercise is recommended to maintain physical function in the elderly, despite its inherent risks. Our results suggest that it is more appropriate for physical therapists to provide low-intensity, fatigue-inducing exercises that increase neuromuscular activity rather than high-intensity exercises.

膝関節伸展の等張性収縮における力─速度の関係

膝関節伸展の等張性収縮における力─速度の関係

Consistency of the optimized bandwidth in filter-based fatigue index

In this study, we evaluated the cut-off frequency of the filter-based fatigue index using surface electromyography (EMG) signals from different upper and lower extremity muscles under different exercise conditions. Twenty seven subjects participated in isotonic knee extension exercises, fourteen subjects in isotonic elbow flexion and isokinetic knee extension exercises, and twenty subjects in isometric knee extension, isotonic ankle dorsiflexion, and isotonic elbow extension exercises. EMG signals were obtained from right rectus femoris (RF), vastus medialis (VM), vastus lateralis (VL), biceps brachii (BB), triceps brachii (TC), and tibialis anterior (TA) muscles during exercises. The filter-based fatigue index was compared with sEMG-based fatigue indices such as mean root-mean-square (RMS) values, median frequency, Dimitrov spectral index, and Gonzalez-Izal wavelet index. Results showed that optimized cut-off values for all muscles and all exercises approximated to 350 Hz. This implies that the cut-off frequency of 350 Hz is appropriate for general use, showing better determination coefficient with biomechanical fatigues such as joint torques and powers than the other fatigue indices in different muscles during different exercises.

Muscle activation and knee biomechanics during squatting and lunging after lower extremity fatigue in healthy young women

Muscle activations and knee joint loads were compared during squatting and lunging before and after lower extremity neuromuscular fatigue. Electromyographic activations of the rectus femoris, vastus lateralis and biceps femoris, and the external knee adduction and flexion moments were collected on 25 healthy women (mean age 23.5years, BMI of 23.7kg/m2) during squatting and lunging. Participants were fatigued through sets of 50 isotonic knee extensions and flexions, with resistance set at 50% of the peak torque achieved during a maximum voluntary isometric contraction. Fatigue was defined as a decrease in peak isometric knee extension or flexion torque ⩾25% from baseline. Co-activation indices were calculated between rectus femoris and biceps femoris; and between vastus lateralis and biceps femoris. Fatigue decreased peak isometric extension and flexion torques (p<0.05), mean vastus lateralis activation during squatting and lunging (p<0.05), and knee adduction and flexion moments during lunging (p<0.05). Quadriceps activations were greater during lunging than squatting (p<0.05). Thus, fatigue altered the recruitment strategy of the quadriceps during squatting and lunging. Lunging challenges quadriceps activation more than squatting in healthy, young women.

Tissue oxygenation, strength and lactate response to different blood flow restrictive pressures

This study aimed to determine whether changes in initial restrictive pressures (IRP, tightness of the cuff before inflation with air) affect tissue oxygenation, lactate production and leg strength before, during and after knee extension exercises. The cuff was positioned on the right thigh, and the IRP of either 40-45 or 60-65 mmHg were applied randomly prior to inflating the cuff to the final restrictive pressure (the pressure reached after inflating the cuff with air). Subjects performed four sets (30, 15, 15 and 15 reps) of isotonic knee extensions with 1-min rest between sets. Tissue oxygenation and blood lactate levels were assessed prior to, during and after exercise, and leg strength was assessed pre- and postexercise. There were significant condition by time interactions (P<0·01) and main effects for both condition (P<0·01) and time (P<0·01) for tissue oxygenation, deoxyhaemoglobin, total haemoglobin. Significant main effects were detected for both condition (P<0·01) and time (P<0·01) for leg strength values. There was only a significant time main effect for lactate concentrations. This study is the first to show that a higher IRP had a significant impact on percent tissue oxygenation, leg strength and deoxygenated haemoglobin accumulation during exercise.

Biomechanical changes at the knee after lower limb fatigue in healthy young women

BackgroundThe purpose of this study was to identify changes in knee kinematics, kinetics and stiffness that occur during gait due to lower limb neuromuscular fatigue. MethodsKinematic, kinetic and electromyographic measures of gait were collected on healthy, young women (n=20) before and after two bouts of fatigue. After baseline gait analysis, two bouts of fatiguing contractions were completed. Fatigue was induced using sets of 50 isotonic knee extensions and flexions at 50% of the peak torque during a maximum voluntary isometric contraction. Fatigue was defined as a drop in knee extension or flexion maximum voluntary isometric torques of at least 25% from baseline. Gait analyses were completed after each bout of fatigue. Dynamic knee stiffness was calculated as the change in knee flexion moment divided by the change in knee flexion angle from 3 to 15% of the gait cycle. Co-activations of the biceps femoris and rectus femoris muscles were calculated from 3 to 15% and 40 to 52% of gait. Repeated measures analyses of variance assessed differences in discrete gait measures, knee torques, and electromyography amplitudes between baseline and after each bout of fatigue. FindingsFatigue decreased peak isometric torque. Fatigue did not alter knee adduction moments, knee flexion angles, dynamic knee stiffness, or muscle co-activation. Fatigue reduced the peak knee extension moment. InterpretationWhile neuromuscular fatigue of the knee musculature alters the sagittal plane knee moment in healthy, young women during walking, high intensity fatigue is not consistent with known mechanical environments implicated in knee pathologies or injuries.

Neuromuscular fatigue in young and older men using constant or variable resistance

The purpose of the present study was to determine: (1) if different acute neuromuscular responses occur between constant versus variable external resistance machines, two commonly used resistance machines and (2) whether the potential differences in fatigability between young and older men influence the magnitude of acute response between these resistance machines. Twelve young men (28 ± 5 year) and 13 older men (65.4 ± 4 year) performed 15 × 1 repetition maximum and 5 × 10 repetitions isotonic knee extension resistance loadings with both constant and variable resistance (four loadings in total). Maximum isometric knee extension torque, superimposed twitch, resting twitch torque, maximal M wave properties, electromyograph, and blood lactate concentration measured the effects of loading. Concentric torque reduced to a greater extent during variable 15 × 1 versus constant loading in young men only (P < 0.05). While three out of the four loadings caused decreased voluntary activation in young men, only 15 × 1 using variable resistance caused reductions in older men (P < 0.05). 5 × 10 variable resistance loading significantly increased M wave duration and decreased EMG median frequency, which was not observed following constant resistance loading in both age groups. Acute decreases in force production were significantly greater in young men following all loading protocols (P < 0.05). Both young and older men showed indications of greater fatigue from variable resistance loadings. Differing muscle properties may have led to different magnitudes of fatigue between groups, and older subjects may benefit from specifically tailored training programs.

Effects of Mild Electro-Stimulation Treatment on Healthy Humans Following Exercise Induced Muscle Damage

Exercise-induced muscle damage has been widely reported in healthy humans, and is characterised by morphological changes in muscle, protein leakage to the circulation, acute inflammatory responses, muscle dysfunction and subsequent soreness. PURPOSE: This study investigated plasma protein, inflammatory markers and exercise performance responses to multi-day mild electro-stimulation treatment following a single bout of eccentrically biased exercise. It was hypothesised that mild electro-stimulation would decrease inflammation and muscle damage, and subsequently accelerate recovery. METHODS: Sixteen healthy volunteers (11 M, 5 F) performed exercise trials and provided blood samples pre and for 7 days post muscle damage (45min bench stepping). Exercise trials comprised isotonic knee extension at 180°s, vertical jump, and cycling at 90% VO2 WRpeak to fatigue. Blood analyses included creatine kinase (CK), myoglobin (Mb), lactate dehydrogenase (LDH) and c-reactive protein (CRP). Experimental (EXP) or control (CON) treatment (single blind, randomised) was administered for 20min immediately following the muscle damage step test, and for 7 days thereafter. RESULTS: There was a rapid increase in c-RP above baseline (5-fold at +2 days), following bench stepping in CON (P<0.05), which was systematically higher than EXP each day, with peak 4.4-fold difference at +2 days (P<0.05). CK increased above baseline in both trials (P<0.05), and tended to be greater at +1 day post damage in CON (P=0.08). There was no condition effect for Mb and no time or condition effect for LDH. Leg extension force at 180°s decreased by 3% and 5% respectively for CON & EXP following muscle damage, decreased by 9% in CON on +2 days. Muscle force was re-established by +1 days in EXP (p<0.05). There were no significant differences between trials for vertical jump or 90% VO2 WRpeak to fatigue. CONCLUSIONS: Mild electro-stimulation attenuated the rise in c-RP and CK following exercise-induced muscle damage compared to CON. This suggests decreased inflammation and damage, and accelerated recovery, which may in part be due to increased lymphatic and blood flow. The underlying mechanisms are unclear, and require further investigation. Supported by Bodyflow International

Agonist muscle activity and antagonist muscle co-activity levels during standardized isotonic and isokinetic knee extensions

This study aimed to analyze the effects of the contraction mode (isotonic vs. isokinetic concentric conditions), the joint angle and the investigated muscle on agonist muscle activity and antagonist muscle co-activity during standardized knee extensions. Twelve healthy adult subjects performed three sets of isotonic knee extensions at 40% of their maximal voluntary isometric torque followed by three sets of maximal isokinetic knee extensions on an isokinetic dynamometer. For each set, the mean angular velocity and the total external amount of work performed were standardized during the two contraction modes. Surface electromyographic activity of vastus lateralis (VL), vastus medialis (VM), rectus femoris (RF), semitendinosus (ST) and biceps femoris (BF) muscles was recorded. Root mean square values were then calculated for each 10° between 85° and 45° of knee extension (0° = horizontal position). Results show that agonist muscle activity and antagonist muscle co-activity levels are significantly greater in isotonic mode compared to isokinetic mode. Quadriceps activity and hamstrings co-activity are significantly lower at knee extended position in both contraction modes. Considering agonist muscles, VL reveals a specific pattern of activity compared to VM and RF; whereas considering hamstring muscles, BF shows a significantly higher co-activity than ST in both contraction modes. Results of this study confirmed our hypothesis that higher quadriceps activity is required during isotonic movements compared to isokinetic movements leading to a higher hamstrings co-activity.