Isometric Knee Extension Contractions Research Articles

Short‐term unilateral resistance training affects the agonist–antagonist but not the force–agonist activation relationship

In this study we investigated the contribution of neural adaptations to strength changes after 4 weeks of unilateral isometric resistance training. Maximal and submaximal isometric knee extension contractions were assessed before and after training. Surface electromyography (EMG) data were collected from the agonist and antagonist muscles and normalized to evoked maximal M-wave and maximal knee flexor EMG, respectively. The interpolated twitch technique (ITT) was also used to determine activation at maximum voluntary force (MVF). MVF increased in the trained (+20%) and untrained (+8%) legs. Agonist EMG at MVF increased in the trained leg (+26%), although activation determined via the ITT was unchanged. In both legs the position of the force-agonist EMG relationship was unchanged, but antagonist coactivation was lower for all levels of agonist activation. Strength gains in the trained leg were due to enhanced agonist activation, whereas decreased coactivation may have affected strength changes in both legs.

Older Adults Underestimate RPE and Knee Extensor Torque as Compared with Young Adults

The purpose of this study was to examine knee extensor muscle recruitment, torque, and perceived exertion between healthy young and old adults. During the first experimental session, subjects performed 6-s isometric knee extension contractions to intensities ranging from 10% to 90% (10% increments, random order) of three maximal voluntary contractions (MVC), during which RPE were obtained. During the second session, subjects performed 6-s isometric contractions to perceived exertion intensities corresponding to 1, 2, 3, 4, 5, 6, 7, 8, 9, and "maximal" on a modified Borg category-ratio scale, in a random order, to obtain the produced torque. Surface EMG were recorded from the vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) muscles. Perceived exertion was significantly (P < 0.05) greater for young than for old adults across 10%-90% MVC. Significantly (P < 0.05) greater VL and RF muscle EMG than the VM across 10%-90% MVC was observed for the young adults, whereas the old adults demonstrated no significant differences between the muscles. The produced torque across the perceived exertion levels was significantly (P < 0.05) greater in old than that in young adults. VL and RF muscle EMG was significantly greater than the VM muscle across the perceived exertion levels in the young adults. Recruitment of the VM, VL, and RF muscles was not significantly different across the perceived exertion levels in the old adults. The major findings demonstrated that healthy older adults underestimate voluntary knee extensor torque as compared with young adults, which may be driven by differential patterns of muscle recruitment.

Skeletal Muscle Properties in Rheumatoid Arthritis Patients

Disability in patients with rheumatoid arthritis (RA) is a multifactorial process involving various unaccounted factors. Loss of lean body mass plays an important role in impaired physical function, and exercise studies in RA have shown promising results in restoring muscle mass, strength, and function. However, no comprehensive assessment of the muscle characteristics has been undertaken to determine whether qualitative changes in muscle also contribute to RA disability. This study explores the physiological muscle properties of a community-based population with stable RA. Vastus lateralis (VL) force and physiological cross-sectional area (PCSA), voluntary muscle activation capacity, and contractile properties were assessed in 23 patients with stable RA (age = 60 ± 2 yr (mean ± SEM); 16 women) and age- and sex-matched healthy controls (age = 60 ± 3 yr). Measurements with EMG were obtained during maximal isometric knee extension contractions, with resting and superimposed electrical stimulations. Concentric knee extension contractions were also assessed. Pennation angle and VL volume were measured with ultrasound to determine fiber fascicle length and PCSA. Muscle-specific force was calculated (VL force/VL PCSA). Body composition using dual-energy x-ray absorptiometry and objective physical function were also measured. The patients displayed typical features of RA with reduced physical function (P = 0.001-0.09), a trend toward lower appendicular lean mass (P = 0.09) and increased total body fat (P < 0.05) relative to controls. However, there were no differences in specific force, contractile properties, voluntary activation capacity, and contraction velocity (P = 0.41-0.99). VL PCSA was reduced (P < 0.05) with minor architectural changes in patients with RA. Physiological properties of muscle that determine specific force are not compromised in patients with stable RA despite deficits in physical function.

Muscular coactivation (CA) around the knee reduces power production in elderly women

The main objective of this paper was to study the influence of muscular CA around the knee on the rate of force development (RFD) in older women. After personal invitation, 12 older women, participated in the study. Volunteers performed maximal voluntary isometric knee extension and flexion contractions (MVC) where the RFD was determined (considering the first 50, 100 and 200 ms after the force production onset; also, the time necessary to achieve 30% and 50% MVC was considered). Electromyography data from quadriceps and hamstrings were used to determine different CA indexes (CAIs). A correlation test was applied between CAI and RFD; different methods to determine CAI were also compared (Wilcoxon test). The CAI determination methods were not different one from another ( p > 0.05) and were significantly correlated with RFD ( p < 0.05) in special, when time to achieve 30% MVC and 50% MVC were considered (| r| = 0.89 and r 2 = 0.79; | r| = 0.93 and r 2 = 0.86; p < 0.0001). These results clearly state the CA influence over the capacity to produce force in a short period of time in elderly women, what can increase falling risk, since it is essential to recover to gait disturbances and to unbalance.

Power Deficits and Lower Motor Unit Discharge Rates in Persons with Multiple Sclerosis

Both central and peripheral mediators of force production could contribute to muscle weakness in persons with MS. It also may be that these factors have a differential impact, depending on the type of contraction. However, central and peripheral sources of muscle weakness have not been fully examined in this population. PURPOSE: To determine whether muscle weakness in MS is 1) dependent on contraction mode, and 2) associated with decreased motor unit discharge rates (MUDR) or muscle size (cross-sectional area, mCSA). METHODS: Six persons moderately impaired by MS (1 M, 5 F; aged 50 ± 5 years; EDSS 3.5-6) and 6 non-MS participants (1 M, 5 F; aged 43 ± 7) performed maximal voluntary isometric (MVIC; 90° knee angle) and isovelocity (range of motion = 70°, starting at 90°, velocities of 30 to 300°·s-1) knee extension contractions on a Biodex dynamometer. Peak isometric torque (Nm) and power (W) at each velocity were determined. Intramuscular electromyography (25-gauge needle electrode) was performed in the vastus lateralis during MVIC to determine maximal MUDR (pps). Serial proton T1-weighted axial magnetic resonance images of the thigh were acquired in a 1.5T MRI system, and maximal mCSA (cm2) was quantified. To investigate whether isometric torque and power differences were due to muscle size differences, specific strength (Nm·cm-2) and specific power (W·cm-2) were calculated. RESULTS: MVIC (non-MS: 158 ± 61 Nm, MS: 112 ± 33 Nm, p=0.14), mCSA (MS: 46.6 ± 6.9 cm2, non-MS: 53.8 ± 16.6 cm2, p=0.35) and specific strength (non-MS: 2.9 ± 0.8 Nm·cm-2, MS: 2.4 ± 0.6 Nm·cm-2, p=0.28) did not differ across groups. Power over the range of velocities tested was lower in MS compared to non-MS (p<0.0001); and peak power (non-MS: 359 ± 159 W, MS: 207 ± 85 W, p=0.07) and specific power (non-MS: 6.7 ± 2.1 W·cm-2, MS: 4.4 ± 1.5 W·cm-2, p=0.06) tended to be lower in MS. Maximal MUDR tended to be lower in MS (22.4 ± 9.2 pps) than non-MS (31.5 ± 6.9; p=0.08). DISCUSSION: Despite the lack of isometric strength and muscle size deficits in this group of persons with MS, we observed lower power and MUDR compared to non-MS. These results suggest a contraction-mode specificity to muscle weakness in MS that may be mediated more by alterations in central motor drive than by muscle atrophy. Supported by ACSM FRG (LHC) and NMSS RG/3974A2/1 (RVE).

Mechanical properties of the patellar tendon in adults and children

It is not currently known how the mechanical properties of human tendons change with maturation in the two sexes. To address this, the stiffness and Young's modulus of the patellar tendon were measured in men, women, boys and girls (each group, n=10). Patellar tendon force ( F pt) was calculated from the measured joint moment during a ramped voluntary isometric knee extension contraction, the antagonist knee extensor muscle co-activation quantified from its electromyographical activity, and the patellar tendon moment arm measured from magnetic resonance images. Tendon elongation was imaged using the sagittal-plane ultrasound scans throughout the contraction. Tendon cross-sectional area was measured at rest from ultrasound scans in the transverse plane. Maximal F pt and tendon elongation were (mean±SE) 5453±307 N and 5±0.5 mm for men, 3877±307 N and 4.9±0.6 mm for women, 2017±170 N and 6.2±0.5 mm for boys and 2169±182 N and 5.9±0.7 mm for girls. In all groups, tendon stiffness and Young's modulus were examined at the level that corresponded to the maximal 30% of the weakest participant's F pt and stress, respectively; these were 925–1321 N and 11.5–16.5 MPa, respectively. Stiffness was 94% greater in men than boys and 84% greater in women than girls ( p<0.01), with no differences between men and women, or boys and girls (men 1076±87 N/mm; women 1030±139 N/mm; boys 555±71 N/mm and girls 561.5±57.4 N/mm). Young's modulus was 99% greater in men than boys ( p<0.01), and 66% greater in women than girls ( p<0.05). There were no differences in modulus between men and women, or boys and girls (men 597±49 MPa; women 549±70 MPa; boys 255±42 MPa and girls 302±33 MPa). These findings indicate that the mechanical stiffness of tendon increases with maturation due to an increased Young's modulus and, in females due to a greater increase in tendon cross-sectional area than tendon length.

Relationships between the Mechanical Properties of Patellar Tendons and Quadriceps Strength in Humans

The mechanical and morphological properties of patellar tendons were in vivo measured during isometric knee extension contractions in ten male subjects using ultrasonography. The subjects were accustomed to various exercise and training. The stiffness, cross-sectional area, and tangent modulus of the patellar tendons were positively correlated with the quadriceps strength. The enhancement of contractile forces induced by exercise and training increases the quadriceps strength. Since the primary role of patellar tendons is the transmission of contractile forces from quadriceps to tibia, the contractile forces are directly applied to the patellar tendons. Therefore, in the subjects with the larger quadriceps strength, the forces applied to the patellar tendons during exercise and training may be larger, and the force enhancement may induce the increases in the stiffness, cross-sectional area, and tangent modulus of the patellar tendons.

Locomotor exercise induces long-lasting impairments in the capacity of the human motor cortex to voluntarily activate knee extensor muscles

Muscle fatigue is a reduction in the capacity to exert force and may involve a "central" component originating in the brain and/or spinal cord. Here we examined whether supraspinal factors contribute to impaired central drive after locomotor endurance exercise. On 2 separate days, 10 moderately active individuals completed a locomotor cycling exercise session or a control session. Brief (2 s) and sustained (30 s) isometric knee extension contractions were completed before and after locomotor exercise consisting of eight, 5-min bouts of cycling at 80% of maximum workload. In the control session, subjects completed the isometric contractions in a rested state. Twitch responses to supramaximal motor nerve stimulation and transcranial magnetic stimulation were obtained to assess peripheral force-generating capacity and voluntary activation. Maximum voluntary contraction (MVC) force during brief contractions decreased by 23 +/- 6.3% after cycling exercise and remained 12 +/- 2.8% below baseline 45 min later (F(1,9) > 15.5; P < 0.01). Resting twitch amplitudes declined by approximately 45% (F(1,9) = 28.3; P < 0.001). Cortical voluntary activation declined from 90.6 +/- 1.6% at baseline to 80.6 +/- 2.1% after exercise (F(1,9) = 28.0; P < 0.001) and remained significantly reduced relative to control 30-45 min later (80.6 +/- 3.4%; F(1,9) = 10.7; P < 0.01). Thus locomotor exercise caused a long-lasting impairment in the capacity of the motor cortex to drive the knee extensors. Force was reduced more during sustained MVC after locomotor exercise than in the control session. Peripheral mechanisms contributed relatively more to this force reduction in the control session, whereas supraspinal fatigue played a greater role in sustained MVC reduction after locomotor exercise.

Deficits in the way to achieve balance related to mechanisms of dynamic stability control in the elderly

The purpose of this study was to examine the postural corrections related to components of dynamic stability aimed to increase our understanding of successful postural control among the elderly population. This was done by comparing balance behaviour of older adults who were able to recover stability (stable) and others who failed to regain stability (unstable) with a single step after a forward fall. Thirty-eight old male adults (64±3yr, 176±6cm, 78.5±7.8kg) had to recover balance after a sudden induced forward fall. All participants performed maximal isometric ankle plantarflexion and knee extension contractions on a dynamometer. The elongation of the gastrocnemius medialis and the vastus lateralis tendon and aponeuroses during isometric contraction was examined by ultrasonography. There were no differences in leg-extensor muscle strength or tendon stiffness between the two groups showing that the muscle tendon capacities may not be the reason for the observed differences in dynamic stability control. The unstable participants created a higher horizontal ground reaction push-off force of the support limb in the second part (∼260ms after release) of the phase until touchdown leading to an unstable body position at touchdown. The results indicate deficits in the way to achieve balance related to mechanisms responsible for dynamic stability control within the elderly population.

Effects of submaximal and maximal long-lasting contractions on the compliance of vastus lateralis tendon and aponeurosis in vivo

The present study investigated the effects of submaximal sustained and maximal repetitive contractions on the compliance of human vastus lateralis (VL) tendon and aponeurosis in vivo using two different fatiguing protocols. Twelve male subjects performed three maximum voluntary isometric contractions (MVC) of the knee extensors before and after two fatiguing protocols on a dynamometer. The first fatiguing protocol consisted of a long-lasting sustained isometric knee extension contraction at 25% MVC until failure (inability to hold the defined load). The second fatiguing protocol included long-lasting isokinetic (90°/s) knee extension contractions, where maximum moment was exerted and failure was proclaimed when this value fell below 70% of unfatigued maximum isokinetic moment. Ultrasonography was used to determine the elongation and strain of the VL tendon and aponeurosis. Muscle fatigue was indicated by a significant decrease in maximum resultant knee extension moment ( p < 0.05) observed during the MVCs after both long-lasting contractions. No significant ( p > 0.05) differences in elongation and strain of the VL tendon and aponeurosis were found, when compared every 300 N (tendon force) before and after the fatiguing protocols. The present data indicate, that the VL tendon and aponeurosis in vivo do not suffer from changes in the compliance neither after long-lasting static mechanical loading (strain ∼3.2%) nor after long-lasting cyclic mechanical loading (strain 6.2–5.5%).

Age-Related Differences in the Force Generation Capabilities and Tendon Extensibilities of Knee Extensors and Plantar Flexors in Men

Recently, the number of elderly individuals who participate in sports has increased, thus injuries from overuse are now becoming recognized in the elderly population. Therefore, it is important to determine which muscle groups and tendons are most affected with aging to plan appropriate exercise interventions for elderly individuals. In particular, muscles and tendons in knee extensors and plantar flexors play an important role during locomotion. The purpose of this study was to compare the knee extensor and plantar flexor muscles and tendons. Young (n = 19) and elderly (n = 17) men performed isometric voluntary knee extension and plantar flexion contractions. Muscle thickness and elongation of tendon structures in knee extensors and plantar flexors were measured by ultrasonography. Relative muscle thickness (to limb length) in the elderly group was significantly lower than that in the young group in knee extensors (p <.001), although no significant difference was found between the two groups in plantar flexors (p =.063). Relative muscle strength (to body mass) in the elderly group was significantly lower than that in the young group in both sites (all p <.001). Ratio of muscle strength to muscle thickness in the elderly group was significantly lower than that in the young group in plantar flexors, but not in knee extensors. The elderly group had significantly lower maximal elongation and strain of tendon structures in both sites than the young group had. These results suggest that the age-related weakness in knee extensors may be attributed to muscle atrophy, whereas that in plantar flexors is not, and that elderly persons have less extensible tendon structures in both sites.

Myoelectric manifestations of fatigue in vastus lateralis, medialis obliquus and medialis longus muscles

The purpose of this study was to determine whether surface electromyography (EMG) assessment of myoelectric manifestations of muscle fatigue is capable of detecting differences between the vastus lateralis and medialis muscles which are consistent with the results of previous biopsy studies. Surface EMG signals were recorded from the vastus medialis longus (VML), vastus medialis obliquus (VMO) and vastus lateralis (VL) muscles during isometric knee extension contractions at 60% and 80% of the maximum voluntary contraction (MVC) for 10 s and 60 s, respectively. Initial values and rate of change of mean frequency (MNF), average rectified value (ARV) and conduction velocity (CV) of the EMG signal were calculated. Comparisons between the two force levels revealed that the initial values of MNF for the VL muscle were greater at 80% MVC compared to 60% MVC ( P < 0.01). Comparisons between the vasti muscles demonstrated lower initial values of CV for VMO compared to VL at 60% MVC ( P < 0.01) and lower than VML and VL at 80% MVC ( P < 0.01). In addition, initial values of MNF were higher for VL with respect to both VML and VMO at 80% MVC ( P < 0.01) and initial estimates of ARV were higher for VMO compared to VML at both force levels ( P < 0.01 at 60% MVC and P < 0.05 at 80% MVC). For the sustained contraction at 80% MVC, VL demonstrated a greater decrease in CV over time compared to VMO ( P < 0.05). These findings suggest that surface EMG signals and their time course during sustained isometric contractions may be useful to non-invasively describe functional differences between the vasti muscles.

Diurnal Variation in Maximal and Submaximal Strength, Power and Neural Activation of Leg Extensors in Men: Multiple Sampling Across Two Consecutive Days

This study aimed to compare day-to-day repeatability of diurnal variation in strength and power. Thirty-two men were measured at four time points (07 : 00 - 08 : 00, 12 : 00 - 13 : 00, 17 : 00 - 18 : 00, and 20 : 30 - 21 : 30 h) throughout two consecutive days (day 1 and day 2). Power during loaded squat jumps, torque and EMG during maximal (MVC) and submaximal (MVC40) voluntary isometric knee extension contractions were measured. The EMG/torque ratio during MVC and MVC40 was calculated to evaluate neuromuscular efficiency. A significant time-of-day effect with repeatable diurnal patterns was found in power. In MVC, a significant time-of-day effect was present on day 2, whereas day 1 showed a typical but nonsignificant diurnal pattern. EMG and antagonist co-activation during MVC remained statistically unaltered, whereas neuromuscular efficiency improved from day 1 to day 2. A similar trend was observed in MVC40 neuromuscular efficiency with significant time-of-day and day-to-day effects. Unaltered agonist and antagonist activity during MVC suggests that modification at the muscular level was the primary source for the diurnal variation in peak torque. A learning effect seemed to affect data in MVC40. In conclusion, the second consecutive test day showed typical diurnal variation in both maximum strength and power with no day-to-day effect of cumulative fatigue.

Muscle – tendon unit mechanical and morphological properties and sprint performance

The objective of this study was to determine whether sprint performance is related to the mechanical (elongation – force relationship of the tendon and aponeurosis, muscle strength) and morphological (fascicle length, pennation angle, muscle thickness) properties of the quadriceps femoris and triceps surae muscle – tendon units. Two groups of sprinters (slow, n = 11; fast, n = 17) performed maximal isometric knee extension and plantar flexion contractions on a dynamometer at 11 different muscle – tendon unit lengths. Elongation of the tendon and aponeurosis of the gastrocnemius medialis and the vastus lateralis was measured using ultrasonography. We observed no significant differences in maximal joint moments at the ankle and knee joints or morphological properties of the gastrocnemius medialis and vastus lateralis between groups (P > 0.05). The fast group exhibited greater elongation of the vastus lateralis tendon and aponeurosis at a given tendon force, and greater maximal elongation of the vastus lateralis tendon and aponeurosis during maximum voluntary contraction (P < 0.05). Furthermore, maximal elongation of the vastus lateralis tendon and aponeurosis showed a significant correlation with 100-m sprint times (r = −0.567, P = 0.003). For the elongation – force relationship at the gastrocnemius medialis tendon and aponeurosis, the two groups recorded similar values. It is suggested that the greater elongation of the vastus lateralis tendon and aponeurosis of the fast group benefits energy storage and return as well as the shortening velocity of the muscle – tendon unit.

Age-related deficit in dynamic stability control after forward falls is affected by muscle strength and tendon stiffness

The purpose of the work was to determine whether the age-related muscle weakness diminishes older adults’ ability to use mechanisms responsible for maintaining dynamic stability after forward falls. Nine older and nine younger adults participated in this study. To analyse the capacities of the leg-extensor muscle–tendon units, all subjects performed isometric maximal voluntary plantarflexion and knee extension contractions on a dynamometer. The elongation of the gastrocnemius medialis and the vastus lateralis tendon and aponeuroses during isometric contraction was examined by ultrasonography. Recovery behaviour was determined after a sudden fall from two forward-inclined lean angles. Compared to older adults, younger adults had higher muscle strength and tendon stiffness. Younger adults created a higher margin of stability compared to older, independent of perturbation intensity. The main mechanism improving the margin of dynamic stability was the increase of the base of support. The results, further, demonstrated that the locomotion strategy employed before touchdown affects the stability of the stance phase and that muscle strength and tendon stiffness contributed significantly to stability control. We concluded that, to reduce the risk of falls, older individuals may benefit from muscle–tendon unit strengthening programs as well as from interventions exercising the mechanisms responsible for dynamic stability.

Hamstring antagonist moment estimation using clinically applicable models: Muscle dependency and synergy effects

The purpose of this study was to examine whether prediction of antagonist moment ( M flx) of the hamstrings using clinically applicable models depends on the muscles examined. Nine healthy males performed maximal isometric knee extension and flexion contractions at 0°, 45° and 90° angles. Calibration knee flexion efforts were also performed at different levels of intensity. The resulting electromyographic (EMG) – moment curves were fitted using polynomial equations which were then used to estimate M flx from the antagonist EMG. Analysis of variance designs showed that the M flx predicted using the biceps femoris EMG was not significantly different compared with those predicted using the semitendinosus EMG data ( p > 0.05). Further, prediction of M flx using the EMG of both muscles or a combination of EMGs and architectural properties reduced estimation error but did not provide significantly different predicted values compared with the simpler single-muscle EMG – moment models ( p < 0.05). It appears that M flx prediction using the present EMG - moment model is not muscle specific. Prediction using models which combine EMG data and anatomic parameters of the hamstring muscle components yielded more accurate estimates and therefore their use to examine co-contraction levels is recommended.

Influence of the muscle-tendon unit's mechanical and morphological properties on running economy

The purpose of this study was to test the hypothesis that runners having different running economies show differences in the mechanical and morphological properties of their muscle-tendon units (MTU) in the lower extremities. Twenty eight long-distance runners (body mass: 76.8+/-6.7 kg, height: 182+/-6 cm, age: 28.1+/-4.5 years) participated in the study. The subjects ran on a treadmill at three velocities (3.0, 3.5 and 4.0 m s(-1)) for 15 min each. The V(O(2)) consumption was measured by spirometry. At all three examined velocities the kinematics of the left leg were captured whilst running on the treadmill using a high-speed digital video camera operating at 250 Hz. Furthermore the runners performed isometric maximal voluntary plantarflexion and knee extension contractions at eleven different MTU lengths with their left leg on a dynamometer. The distal aponeuroses of the gastrocnemius medialis (GM) and vastus lateralis (VL) were visualised by ultrasound during plantarflexion and knee extension, respectively. The morphological properties of the GM and VL (fascicle length, angle of pennation, and thickness) were determined at three different lengths for each MTU. A cluster analysis was used to classify the subjects into three groups according to their V(O(2)) consumption at all three velocities (high running economy, N=10; moderate running economy, N=12; low running economy, N=6). Neither the kinematic parameters nor the morphological properties of the GM and VL showed significant differences between groups. The most economical runners showed a higher contractile strength and a higher normalised tendon stiffness (relationship between tendon force and tendon strain) in the triceps surae MTU and a higher compliance of the quadriceps tendon and aponeurosis at low level tendon forces. It is suggested that at low level forces the more compliant quadriceps tendon and aponeurosis will increase the force potential of the muscle while running and therefore the volume of active muscle at a given force generation will decrease.

HIP JOINT POSITION AFFECTS VOLITIONAL KNEE EXTENSOR ACTIVITY POST-STROKE.

Purpose/Hypothesis: Individuals post-stroke often exhibit significant functional limitations due to either inadequate or inappropriate volitional muscle activation. In the lower extremity this is particularly apparent during ‘spastic paretic stiff-legged gait’, which has been attributed to excessive quadriceps activity during the stance to swing transition. The mechanism underlying the altered volitional activation of the knee extensors has yet to be elucidated; however, evidence from animal and human models has demonstrated the importance of hip angle sensors in modulating lower extremity muscle activity. Although the hip's position has become a focus for retraining ‘normal’ walking patterns following stroke, there are as of yet, no quantitative studies which examined the influence of hip angle afferent information during isolated volitional muscle activation after stroke. The purpose of this study was therefore to quantify the role of hip joint positioning on the volitional activation of the uniarticular knee extensor muscles after stroke. It was hypothesized that greater hip flexion would yield greater quadriceps activity. Number of Subjects: Fourteen subjects with chronic (> 1 yr) stroke were recruited for testing. Materials/ Methods: Subjects were positioned on a Biodex dynamometer to record the electromyographic (EMG) activity from the uniarticular knee extensor muscles [vastus lateralis (VL) and vastus medialis (VM)] during maximum voluntary isometric knee extension contractions. The knee was fixed at 60°, while hip position was fixed by altering the subject's posture in random order to include: sitting upright (90°), semi-reclined (45°), and supine (0°). EMG data was full wave rectified and low pass filtered to produce a linear envelope. The outcome variable of interest was the integrated EMG activity around peak torque production. A two-way (muscle and hip position) repeated-measures ANOVA (repeated for hip position) was used to determine the effect of varying hip posture on knee extensor activity. Results: Hip position had a substantial effect on the amount of volitional VL and VM muscle activity produced (p=0.07). With the hip positioned in full extension, both muscles were volitionally activated significantly less than when the hip was positioned in flexion (p=0.03). Conclusions: The angle of the hip joint appears to play a significant role in the volitional activation of the uniarticular knee extensors, although more data is required. Patients with stroke tend to walk with the hips in greater flexion, which may facilitate activation of the knee extensors contributing to ‘stiff knee gait’ pattern. Clinical Relevance: Hip joint angle contributes to the modulation of volitional quadriceps activation and likely influences reflexive/spastic activation contributing to inappropriate muscle activity during functional activities. A strong emphasis should therefore be placed on hip joint posturing during gait retraining to assist with appropriate muscle activation patterns.

Mechanical and morphological properties of different muscle–tendon units in the lower extremity and running mechanics: effect of aging and physical activity

The objectives of this work were (i) to investigate whether chronic endurance running is a sufficient stimulus to counteract the age-related changes in the mechanical and morphological properties of human triceps surae (TS) and quadriceps femoris (QF) muscle-tendon units (MTUs) by comparing runners and non-active subjects at different ages (young and old), (ii) to identify adaptational phenomena in running mechanics due to age-related changes in the mechanical and morphological properties of the TS and QF MTUs, and finally (iii) to examine whether chronic endurance-running exercise is associated with adaptational effects on running characteristics in old and young adults. The investigation was conducted on 30 old and 19 young adult males divided into two subgroups according to their running activity: endurance-runners vs non-active. To analyse the properties of the MTUs, all subjects performed isometric maximal voluntary (MVC) ankle plantarflexion and knee extension contractions at 11 different MTU lengths on a dynamometer. The activation of the TS and QF during MVC was estimated by surface electromyography. The gastrocnemius medialis and the vastus lateralis and their distal aponeuroses were visualized by ultrasonography at rest and during MVC, respectively. Ground reaction forces and kinematic data were recorded during running trials at 2.7 m s(-1). The TS and QF MTU capacities were reduced with aging (lower muscle strength and lower tendon stiffness). Runners and non-active subjects had similar MTU properties, suggesting that chronic endurance-running exercise does not counteract the age-related degeneration of the MTUs. Runners showed a higher mechanical advantage for the QF MTU while running (lower gear ratio) compared to non-active subjects, indicating a task-specific adaptation even at old age. Older adults reacted to the reduced capacities of their MTUs by increasing running safety (higher duty factor, lower flight time) and benefitting from a mechanical advantage for the TS MTU, lower rate of force generation and force generation per meter distance. We suggest that the improvement in running mechanics in the older adults happens due to a perceptual motor recalibration and a feed-forward adaptation of the motor task aimed at decreasing the disparity between the reduced capacity of the MTUs and the running effort.

The relationship between ciliary neurotrophic factor (CNTF) genotype and motor unit physiology: preliminary studies

BackgroundCiliary neurotrophic factor (CNTF) is important for neuronal and muscle development, and genetic variation in the CNTF gene has been associated with muscle strength. The effect of CNTF on nerve development suggests that CNTF genotype may be associated with force production via its influence on motor unit size and firing patterns. The purpose of this study is to examine whether CNTF genotype differentially affects motor unit activation in the vastus medialis with increasing isometric force during knee extension.ResultsSixty-nine healthy subjects were genotyped for the presence of the G and A (null) alleles in the CNTF gene (n = 57 G/G, 12 G/A). They were tested using a dynamometer during submaximal isometric knee extension contractions that were from 10–50% of their maximal strength. During the contractions, the vastus medialis was studied using surface and intramuscular electromyography with spiked triggered averaging to assess surface-detected motor unit potential (SMUP) area and mean firing rates (mFR) from identified motor units. CNTF genotyping was performed using standard PCR techniques from DNA obtained from leucocytes of whole blood samples. The CNTF G/A genotype was associated with smaller SMUP area motor units and lower mFR at higher force levels, and fewer but larger units at lower force levels than G/G homozygotes. The two groups used motor units with different size and activation characteristics with increasing force generation. While G/G subjects tended to utilize larger motor units with increasing force, G/A subjects showed relatively less increase in size by using relatively larger units at lower force levels. At higher force levels, G/A subjects were able to generate more force per motor unit size suggesting more efficient motor unit function with increasing muscle force.ConclusionDifferential motor unit responses were observed between CNTF genotypes at force levels utilized in daily activities.