Coactivation at the ankle joint is not sufficient to estimate agonist and antagonist mechanical contribution.

The aim of this study was to assess, via an electromyographic (EMG) biofeedback method, the mechanical contribution of both agonist and antagonist muscles during maximal voluntary contraction (MVC). We compared this original method with the MVC-EMGmax ratio and the torque/EMG relationship method, both of which are commonly used to estimate antagonist torque. The plantarflexion (PF) and dorsiflexion (DF) MVCs were measured simultaneously with EMG activity of triceps surae (TS) and tibialis anterior in 15 young adults (mean age 23 years). Antagonist torques obtained from the torque/EMG relationship and EMG biofeedback methods appeared to be similar. TS antagonist torque had a major mechanical impact on DF MVC ( approximately 42%). EMG coactivation is significantly different than normalized antagonist torque. TS antagonist torque is not negligible when maximal DF is assessed, and the EMG biofeedback method is a simple method to estimate antagonist torque.

Age-related relative increases in electromyography activity and torque according to the maximal capacity during upright standing.

The aim of this study was to assess the relative torque (a percentage of the maximal capacity of torque production) at the ankle joint in young and elderly adults during different postural tasks of increasing difficulty. Seven young (approximately 22 years old) and seven older (approximately 80 years old) men took part in this investigation. Maximal agonist torque was estimated from resultant and antagonist torques in both populations in plantar-flexion (PF) and dorsi-flexion (DF). The sum of PF and DF maximal agonist torques was considered as the maximal capacity of torque production. The centre of pressure (CoP) displacement was analysed during Normal Quiet Stance, Romberg and One Leg Balance. During maximal contractions and postural tasks, the electromyographic (EMG) activity was simultaneously recorded on the triceps surae and tibialis anterior muscles. We observed that the maximal capacity of torque production was negatively correlated with the CoP displacement, whatever the population and the postural tasks. The relative torque during all postural tasks was positively correlated with the CoP displacement in both populations. Moreover, older adults needed more EMG activity than young adults to produce the same torque. From this knowledge, one can assume that increasing strength in the muscles of the ankle joint may improve postural stability in older adults; this might have implications in the prevention of falls in elderly persons and in rehabilitation programs for elderly people who have already fallen.

Comparison between voluntary and stimulated contractions of the quadriceps femoris for growth hormone response and muscle damage.

This study aimed to compare voluntary and stimulated exercise for changes in muscle strength, growth hormone (GH), blood lactate, and markers of muscle damage. Nine healthy men had two leg press exercise bouts separated by 2 wk. In the first bout, the quadriceps muscles were stimulated by biphasic rectangular pulses (75 Hz, duration 400 mus, on-off ratio 6.25-20 s) with current amplitude being consistently increased throughout 40 contractions at maximal tolerable level. In the second bout, 40 voluntary isometric contractions were performed at the same leg press force output as the first bout. Maximal voluntary isometric strength was measured before and after the bouts, and serum GH and blood lactate concentrations were measured before, during, and after exercise. Serum creatine kinase (CK) activity and muscle soreness were assessed before, immediately after, and 24, 48, and 72 h after exercise. Maximal voluntary strength decreased significantly (P < 0.05) after both bouts, but the magnitude of the decrease was significantly (P < 0.05) greater for the stimulated contractions (-22%) compared with the voluntary contractions (-9%). Increases in serum GH and lactate concentrations were significantly (P < 0.05) larger after the stimulation compared with the voluntary exercise. Increases in serum CK activity and muscle soreness were also significantly (P < 0.05) greater for the stimulation than voluntary exercise. It was concluded that a single bout of electrical stimulation exercise resulted in greater GH response and muscle damage than voluntary exercise.

Difficult memory task during postural tasks of various difficulties in young and older people: a pilot study.

OBJECTIVE: This study examined the effects of a difficult and individually tailored additional cognitive task on postural stability and electromyographic (EMG) activities of the ankle dorsi- and plantar-flexors, in young and older individuals performing postural tasks of varying difficulties. METHODS: Eight young (mean age=24 years) and eight older (74 years) men took part in the investigation. Centre of pressure velocity and surface EMG of ankle joint muscles were both examined during various postural conditions. RESULTS: The main findings suggested that high levels of muscle activity were a characteristic of age-related declines in postural stability. Moreover, during the complex posture, the postural instability, as well as the EMG activity of the ankle joint muscles, was decreased in older adults when the difficult memory task was added. Regarding young participants, the performance in the cognitive task was significantly improved during the complex posture compared to the easy one. CONCLUSIONS: These findings showed that the execution of a second task would make it possible to improve the performance in the original task. SIGNIFICANCE: This pilot study seemed to show that, depending on age, the task of highest priority would be the cognitive one for young adults and the postural one for older people.

Single skeletal muscle fiber elastic and contractile characteristics in young and older men.

The current investigation was designed to: (a) assess the impact of aging on elastic characteristics of single skeletal muscle fibers from young (N = 6) and older men (N = 6); and (b) correlate the potential changes, with the fiber contractile properties. Chemically skinned single muscle fibers (n = 235) from vastus lateralis muscle were maximally activated. Maximal force and cross-sectional area were measured, and specific force calculated. The slack test was used to measure maximal unloaded shortening velocity. A quick release of 0.15% of fiber length was applied to determine instantaneous stiffness. The myosin heavy chain isoform composition of each single fiber was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Aging induces changes in both fiber elasticity (i.e., increased instantaneous stiffness) and contractility (i.e., reduced specific force and unloaded shortening velocity) in type I and IIa fibers. However, the changes in fiber stiffness may not directly influence contractile characteristics alterations.

Is backward disequilibrium in the elderly caused by an abnormal perception of verticality? A pilot study.

OBJECTIVE: We hypothesised that backward disequilibrium (BD), defined by a posterior position of the centre of mass with respect to the base of support, could be caused by a backward tilt in the perception of verticality. METHODS: The relationship between BD, the perception of verticality, and the history of falls in 25 subjects aged 84.5+/-7.4 years was analysed. An original ordinal scale, the BD scale (BDS), was used to quantify BD. Postural (PV) and haptic verticals (HV) were measured in sagittal plane. RESULTS: BDS scores closely correlated with the number of falls (r = 0.81, p =10(-5)). The more the PV was tilted backward, the greater the BDS scores (r = -0.95, p<10(-6)), with a huge backward tilt of about 15 degrees in 4 subjects with severe BD. In these subjects, the tilt in perception of verticality was transmodal since a severe backward HV tilt was also found. CONCLUSIONS: This transmodality suggested high-order cognitive disruption in the construction of the subjective vertical used in postural control by subjects showing BD, which confirmed our hypothesis. SIGNIFICANCE: This study clearly shows that perception and action with respect to gravity are closely related and brings a new insight about fall mechanisms in the elderly.

Effects of joint angle and age on ankle dorsi- and plantar-flexor strength.

This study aimed at examining the effects of joint angle and age on the maximal voluntary contraction (MVC) torque, for the agonist and antagonist muscle groups around the ankle, i.e., the dorsi- and plantar-flexors. To this aim, neural and muscular factors were investigated in two groups of healthy men: 11 young (mean age, 24 years) and 18 older (mean age, 78 years). Plantar-flexion (PF) and dorsiflexion (DF) isometric MVC torques were measured in three different ankle joint angles and surface electromyographic activities of the triceps surae and of the tibialis anterior muscles were recorded. The main findings were that the DF-to-PF MVC torque ratio varied with joint angle and age, indicating that aging affected at different rates the two muscle groups: this ratio was always higher in older adults because of the PF strength decline with aging. Furthermore, the DF MVC torque-angle relationship appeared to be especially explained by neural factors, whereas the relationship in PF seemed to be mainly due to muscular parameters. These relationships would not be a discriminating factor between the two age groups. As a consequence, measurements at one ankle joint angle, whatever the angle, are thus enough to examine the differences within age groups and to perform a rapid assessment of the imbalance at the ankle joint.

Muscular performances at the ankle joint in young and elderly men.

The effect of aging on mechanical and electromyographic characteristics of ankle joint muscles was investigated in 11 young (mean age 24 years) and 12 elderly (mean age 77 years) males. Maximal and submaximal isometric voluntary torques were measured during ankle plantarflexion and dorsiflexion. Electromyographic activities of triceps surae and tibialis anterior muscles were recorded. The elderly group developed equal maximal dorsiflexion torques (42 vs 45 N.m, p >.05), but in plantarflexion, the elderly group was weaker (80 vs 132 N.m, p <.001) and presented a decreased twitch amplitude (11 vs 16 N.m) and lower coactivation (8% vs 15%) than that of the young adults. We established a linear relationship between the percentage of coactivation and developed resultant torque. Our results showed that dorsiflexor muscles were not affected by aging, contrary to plantarflexors, in which the decline in torque was partly explained by changes intervening at the peripheral level.

Gender differences in human muscle and joint mechanical properties during plantar flexion in old age.

Muscle and joint mechanical properties during plantar flexion were investigated in 13 elderly women (EW) (age range 73-83 years) and 15 elderly men (EM) (age range 74-81 years). Maximal torque, at several angular velocities, was measured to construct torque-velocity relationship. This led to the calculation of an index of maximal shortening velocity (VI(max)) at 10% of maximal isometric torque. Two methods were then used to calculate musculotendinous (quick-release movements) and musculoarticular (sinusoidal perturbations) stiffness. In both cases, stiffness was linearly related to torque, and the slope was defined as a stiffness index (SI): SI(MT) and SI(MA), respectively. Maximal torques as well as VI(max) (p <.05) were lower in EW compared with EM. Furthermore, SI(MT) and SI(MA) values were higher for EW than for EM (p <.05). These results are interpreted in terms of possible differences in the muscle mass, fiber-type distribution, and tendons. They may also have implications for daily motor behavior.

Effect of quadriceps femoris muscle length on neural activation during isometric and concentric contractions.

The effect of muscle length on neural drive (here termed "neural activation") was investigated from electromyographic activities and activation levels (twitch interpolation). The neural activation was measured in nine men during isometric and concentric (30 and 120 degrees /s) knee extensions for three muscle lengths (35, 55, and 75 degrees knee flexion, i.e., shortened, intermediate, and lengthened muscles, respectively). Long (76 degrees ), medium (56 degrees ), and short (36 degrees ) ranges of motion were used to investigate the effect of the duration of concentric contraction. Neural activation was found to depend on muscle length. Reducing the duration of contraction had no effect. Neural activation was higher with short muscle length during isometric contractions and was weaker for shortened than for intermediate and lengthened muscles performing 120 degrees /s concentric contractions. Muscle length had no effect on 30 degrees /s concentric neural activation. Peripheral mechanisms and discharge properties of the motoneurons could partly explain the observed differences in the muscle length effect. We thus conclude that muscle length has a predominant effect on neural activation that would modulate the angular velocity dependency.

Is eccentric exercise-induced torque decrease contraction type dependent?

PURPOSE: This study was designed to determine whether torque decrease following an acute eccentric exercise is contraction type dependent. METHODS: Ten active males performed an exercise session consisting of five sets of ten maximal eccentric muscle actions of the elbow flexors. Before and immediately after the exercise, maximal voluntary eccentric (-60 degrees.s-1; Ecc60), isometric (0 degrees.s-1; Iso) and concentric (60 degrees.s-1; Con60 and 240 degrees.s-1; Con240) torque were measured. In order to distinguish central from peripheral factors involved in torque decrement, activation level (twitch interpolation technique), myoelectrical activity (RMS) of biceps brachii, as well as electrically evoked M-wave and peak twitch torque (Pt) were recorded. RESULTS: The eccentric exercise induced a significant torque reduction (P < 0.01), whatever the muscular contraction type [mean (SD): -22.3 (8.1)% for Ecc60; -20.8 (11.2)% for Iso; -18.5 (6.1)% for Con60 and -12.5 (8.9)% for Con240]. Relative torque decrement was however significantly less for Con240 compared with Ecc60, Iso, and Con60 (P < 0.05). Torque decreases were associated with a reduction of both M-wave amplitude (P < 0.01) and Pt (P < 0.001), probably related to an impairment of the excitation-contraction coupling. Concurrently, activation level was reduced (P < 0.01), therefore indicating the occurrence of central fatigue, as also confirmed by RMS decreases for all the conditions (P < 0.05), except Con240. DISCUSSION: An acute eccentric exercise induced a significant voluntary maximal torque reduction during eccentric, isometric, and concentric muscle actions ascribed to both peripheral and central failure of force production capacity. It can be concluded that eccentric exercise-induced torque decrease is not contraction type dependent.

Brain BDNF levels elevation induced by physical training is reduced after unilateral common carotid artery occlusion in rats.

We investigated the contribution of blood flow elevation in the cerebrovasculature to physical training-induced brain-derived neurotrophic factor (BDNF) levels elevation in the brain. Brain-derived neurotrophic factor protein levels were measured in the motor cortex 24 h after the last session of a forced treadmill walking (30 minutes a day, 18 m/minute for 7 consecutive days). Unilateral common carotid artery occlusion and modulation of exercise intensity (0 versus -10% inclination of the treadmill) were used as strategies to reduce the (normal) elevation of flow in the cerebrovasculature occurring during exercise. Administration of N-nitro-L-arginine methyl ester (L-NAME, 60 mg/kg before each exercise sessions) and genetic hypertension (spontaneously hypertensive rats) were used as approaches to reduce stimulation of nitric oxide production in response to shear stress elevation. Vascular occlusion totally and partially abolished the effect of physical training on BDNF levels in the hemisphere ipsilateral and contralateral to occlusion, respectively. BDNF levels were higher after high than low exercise intensity. In addition, both genetic hypertension and L-NAME treatment blunted the effects of physical training on BDNF. From these results, we propose that elevation of brain BDNF levels elicited by physical training involves changes in cerebral hemodynamics.

Ankle muscle strength discriminates fallers from non-fallers.

It is well known that center of pressure (CoP) displacement correlates negatively with the maximal isometric torque (MIT) of ankle muscles. This relationship has never been investigated in elderly fallers (EF). The purpose of this study was thus to analyze the relationship between the MIT of ankle muscles and CoP displacement in upright stance in a sample aged between 18 and 90 years old that included EF. The aim was to identify a threshold of torque below which balance is compromised. The MIT of Plantar flexors (PFs) and dorsal flexors (DFs) and CoP were measured in 90 volunteers: 21 healthy young adults (YA) (age: 24.1 ± 5.0), 12 healthy middle-aged adults (MAA) (age: 50.2 ± 4.5), 27 healthy elderly non-fallers (ENF) (age: 75.5 ± 7.0) and 30 EF (age: 78.8 ± 6.7). The MIT of PF and DF were summed to obtain the overall maximal ankle muscle strength. Body weight and height were used to normalize MIT (nMIT) and CoP (nCoP), respectively. nCoP correlated negatively with nMIT. 90% of EF generated an nMIT <3.1 N·m·kg(-1), whereas 85% of non-fallers generated an nMIT >3.1 N·m·kg(-1). The relationship between nMIT and nCoP implies that ankle muscle weakness contributes to increased postural instability and the risk of falling. We observed that below the threshold of 3.1 N·m·kg(-1), postural stability was dramatically diminished and balance was compromised. Our results suggest that measuring ankle torque could be used in routine clinical practice to identify potential fallers.

Effects of electromyostimulation versus voluntary isometric training on elbow flexor muscle strength.

The purpose of this study was to determine whether 7 weeks of standardized (same number and duration of repetitions, sets and rest strictly identical) electromyostimulation training of the elbow flexor muscles would induce strength gains equivalent to those of voluntary isometric training in isometric, eccentric and concentric contractions. Twenty-five males were randomly assigned to an electromyostimulated group (EMS, n=9), a voluntary isometric group (VOL, n=8), or a control group (CON, n=8). Maximal voluntary isometric, eccentric and concentric strength, electromyographic (EMG) activity of the biceps and triceps brachii muscles, elbow flexor muscle activation (twitch interpolation technique) and contractile properties were assessed before and after the training period. The main findings were that the isometric torque gains of EMS were greater than those of VOL after the training period (P<0.01) and that the eccentric and concentric torque gains were equivalent. In both groups, we observed that the mechanical twitch (Pt) was increased (P<0.05) and that torque improvements were not mediated by neural adaptations. Considering the respective intensities of the training programs (i.e., submaximal contractions for EMS versus maximal for VOL), it can be concluded that electromyostimulation training would be more efficient than voluntary isometric training to improve both isometric and dynamic strength.

Antagonist mechanical contribution to resultant maximal torque at the ankle joint in young and older men.

A recorded muscular torque at one joint is a resultant torque corresponding to the participation of both agonist and antagonist muscles. This study aimed to examine the effect of aging on the mechanical contributions of both plantar- and dorsi-flexors to the resultant maximal voluntary contraction (MVC) torques exerted at the ankle joint, in dorsi-flexion (DF) and plantar-flexion (PF). The estimation of isometric agonist and antagonist torques by means of an EMG biofeedback technique was made with nine young (mean age 24 years) and nine older (mean age 80 years) men. While there was a non-significant age-related decline in the measured resultant DF MVC torque (-15%; p=0.06), there was a clear decrease in the estimated agonist MVC torque exerted by the dorsi-flexors (-39%; p=0.001). The DF-to-PF resultant MVC torque ratio was significantly lower in young than in older men (0.25 vs. 0.31; p=0.006), whereas the DF-to-PF agonist MVC torque ratio was no longer different between the two populations (0.38 vs. 0.35; p>0.05). Thus, agonist MVC torques in PF and DF would be similarly affected by aging, which could not be deduced when only resultant torques were examined.

Adaptations to long-term strength training of ankle joint muscles in old age.

The aim of this study was to enquire whether older adults, who continue plantar-flexion (PF) strength training for an additional 6-month period, would achieve further improvements in neuromuscular performance, in the ankle PFs, and in the antagonist dorsi-flexors (DFs). Twenty-three healthy older volunteers (mean age 77.4 +/- 3.7 years) took part in this investigation and 12 of them followed a 1-year strength-training program. Both neural and muscular factors were examined during isometric maximal voluntary contraction (MVC) torques in ankle PF and DF pre-training, post 6 and post 12 months. The main finding was that 6 months of additional strength training of the PFs, beyond 6 months, allowed further improvements in neuromuscular performance at the ankle joint in older adults. Indeed, during the first 6 months of progressive resistance training, there was an increase in the PF MVC torque of 11.1 +/- 19.9 N m, and then of 11.1 +/- 17.9 N m in the last 6-month period. However, it was only after 1 year that there was an improvement in the evoked contraction at rest in PF (+ 8%). The strength training of the agonist PF muscles appeared to have an impact on the maximal resultant torque in DF. However, it appeared that this gain was first due to modifications occurring in the trained PFs muscles, then, it seemed that the motor drive of the DFs per se was altered. In conclusion, long-term strength training of the PFs resulted in continued improvements in neuromuscular performance at the ankle joint in older adults, beyond the initial 6 months.

Changes in muscle and joint elasticity following long-term strength training in old age.

The current investigation was designed (1) to examine the effect of a 48-week strength training on musculotendinous (MT) and musculoarticular (MA) stiffness characteristics in older men and women; and (2) to evaluate the influence of gender on stiffness behaviour in response to such training. The training was performed twice per week and mainly consisted of three series of 10 repetitions of calf-rise at 75% of the 3-repetition maximum. Two methods were used to perform stiffness measurements during plantar flexion: (1) the use of quick-release movements, allowing the calculation of MT stiffness; (2) the application of sinusoidal perturbations to the joint, allowing the calculation of MA stiffness. In each case, stiffness was linearly related to torque, leading to the calculation of a normalized stiffness index (SI) as the slope of this stiffness-torque relationship: SI(MT) and SI(MA), respectively. Results showed a similar decrease in SI(MT) among older men (-27%, P < 0.05) and women (-29%, P < 0.05) following training. A decrease in SI(MA) was only observed among women (-11%, P < 0.05). The results suggest that (1) MT stiffness decreases following training in older individuals, counterbalancing the effect of ageing; and (2) older men and women respond differently to the same resistance-training stimulus in terms of MA stiffness. Gender-related differences in MA stiffness response may originate from passive MA elastic structures.

Effects of a one-year exercise training program in adults over 70 years old: a study with a control group.

BACKGROUND AND AIMS: Exercise training is known to improve exercise tolerance in elderly subjects. Therefore the present study aimed at investigating the effects of one year of combined endurance and resistance training in healthy older people. METHODS: After baseline evaluation, subjects were assigned to either the training group (n=24, age 77.2+/-3.6) or the control group (n=16, age 76.1+/-4.8). Subjects in the control group did not change anything in their everyday activities, whereas subjects in the training group underwent moderately intensive combined exercise training, 3 hours a week over the course of one year. Breath-by-breath oxygen uptake and heart rate were measured at each workload during the symptom-limited cardiopulmonary exercise test. Performance on the 6-minute (6-MWT) and 200-meter (200-MWT) walk tests was registered and maximal strength was measured on knee extensor and plantar flexor muscles. RESULTS: After training, oxygen uptake was significantly increased, both at the ventilatory threshold (+11.6%, p<0.01) and at the end of exercise (+14.8%, p<0.001). The distance walked in 6 min (+10%, p<0.001), the time required to cover 200 m (-7.3%, p<0.001) and the maximal muscle strength (+15.2% and +17.4% for knee extensors and plantar flexors respectively, p<0.05) also improved after training. All these parameters had not significantly changed in the control group after the one-year period. CONCLUSIONS: The results of the present study show that one year of combined exercise training is well-tolerated and improves aerobic capacity, performance on field tests and muscle strength in healthy subjects over 70 years old.

Strength training in old age: adaptation of antagonist muscles at the ankle joint.

The purpose of this study was to determine whether strength training could reduce the deficit in plantarflexion (PF) maximal voluntary contraction (MVC) torque observed in previous studies in older subjects relative to young adults. Accordingly, the effects of a 6-month strength training program on the muscle and neural properties of the major muscle groups around the ankle were examined. PF and dorsiflexion (DF) isometric MVC torques were measured and surface electromyographic activity of the triceps surae and tibialis anterior muscles was recorded. The strength training program was very effective in improving strength in PF (+24.5%), and it thus reduced the DF-to-PF MVC torque ratio; in addition, it also induced gains in DF (+7.6%). Thus, there must be an improvement in ankle joint stability. In PF, gains were due particularly to a modification of the agonist neural drive; in DF, the gains appeared to be the consequence of a reduction in antagonist coactivation. Our findings indicate that the investigation of one muscle group should always be accompanied by examination of its antagonist muscle group.

Effect of strength training on musculotendinous stiffness in elderly individuals.

The present study deals with the question whether 24-week strength training alters neuromechanical properties of plantar flexors in elderly people (73-83 years). The first purpose of the present study was to evaluate the effect of strength training on musculotendinous (MT) stiffness of the triceps surae (TS). The training was performed twice per week and mainly consisted of three series of ten repetitions of calf-rise and leg-press exercises at 75% of the three-repetition maximum. Using quick-release movements at different levels of submaximal torques performed measurements of MT stiffness. Surface electromyograms (EMG) of each part of the TS and the tibialis anterior were also recorded. A stiffness index (SI), defined as the slope of the angular stiffness-torque relationship (SI(MT-Torque)), was used to quantify changes in MT stiffness. Results showed a significant decrease in SI(MT-Torque) by 25.2% following training (P < 0.05). The second purpose of the study was to evaluate whether neural mechanism has influences on this decrease. Therefore, an activation SI, defined as the slope of the angular stiffness-EMG relationship (SI(MT-EMG)) was used to overcome the influence of changes in agonist activity, and thus to quantify changes in MT intrinsic elastic properties. SI(MT-EMG) only decreased by 11.2% following training (P < 0.05). The present results underlined that MT stiffness decreases following training in elderly individuals, counterbalancing the effect of ageing. These changes seem not only to be due to peripheral but also to neural adaptations.