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PubMed2001

Motor potentials evoked by transcranial magnetic stimulation during isometric and dynamic masseter muscle contraction in humans.

The facilitation of muscle motor potentials evoked by transcranial magnetic stimulation (TMS) has been demonstrated convincingly during both isometric and dynamic activity in the limbs but not in the jaw muscles. An experimental design involving TMS, surface electromyography and controlled muscle-activity was employed to investigate the motor response of the human masseter during voluntary isometric and dynamic voluntary conditions. During the isometric condition, an increase in muscle facilitation resulted in a progressive increase in motor-evoked potential (MEP) amplitude that was consistently greater on the side contralateral to that subjected to TMS (P < 0.05). No difference in MEP amplitude or laterality of response was revealed for the two dynamic conditions. The sample size may have been too small to reveal any differences. The modulation of MEPs during isometric activity was probably due to cortical and brainstem mechanisms. Putative variation in masseter MEPs during dynamic conditions cannot be discounted.

Adult↗

PubMed1998

Optimisation of isometric strength training intensity.

The aim of this study was to determine the optimal intensity for isometric strength training. Isometric elbow flexor strength of 51 subjects was measured before and after a six-week training period. Subjects were randomly assigned to target training intensities between 0 and 100 per cent of a maximal voluntary contraction, and undertook supervised isometric elbow flexor training at the allocated target intensity three times each week for six weeks. Strength increases at the trained angle of the trained limb were linearly or near linearly related to training intensity, with the greatest increases in strength occurring at maximal or near-maximal intensities. These findings suggest that the optimal intensity for isometric strength training is maximal or near maximal.

Journal Article↗

PubMedDate not supplied

Isometric grip endurance of healthy elderly men and women.

Grip endurance is one of the important prerequisites for good performance of the upper extremity and hence it is important to evaluate it correctly. However, we know little about normal grip endurance, especially for elderly people. The main objective of this study was to examine the age-related changes in grip isometric endurance of healthy community-dwelling elderly people. Two-hundred and eighty-six subjects aged between 60 and 90, were randomly selected from the electoral list of the city of Sherbrooke, Quebec, Canada. Grip relative isometric endurance was evaluated using the Jamar dynamometer. Isometric grip endurance was estimated by the number of seconds a subject could sustain 50% of his/her maximal voluntary grip strength. Contrary to other upper extremity sensorimotor parameters, no grip isometric endurance difference was found with age, in spite of a slight tendency towards a decrease. The women tended to have better scores than the men and endurance of dominant hand is better than the non-dominant. The stability with age could be explained by muscle fiber composition modifications.

Journal Article↗

PubMed1975

The relationship of body fat content to deep muscle temperature and isometric endurance in man.

1. The relationship between body fat content, isometric endurance and deep muscle temperature was assessed in eight male and female volunteers trained to static effort. 2. Muscle temperature, measured in the belly of the brachioradialis muscle, was found to be directly related to the subject's body fat content. 3. Associated with a reduction in body fat content, there was a marked decrease in deep muscle temperature and an increase in isometric endurance of the hand-grip muscles; no change in strength was noted. 4. Conversely, an increase in body fat content resulted in an increase in deep muscle temperature and a decrease in isometric endurance. 5. When muscle temperature was stabilized before and after weight loss by immersing the forearm in water at 37 degrees C, a reduction in body fat no longer influenced endurance. 6. It was concluded that the change in isometric endurance associated with either loss or gain of weight can be accounted for entirely by the changes in muscle temperature.

Adipose Tissue↗

PubMed1999

The effect on nasal resistance of an external nasal splint during isometric and isotonic exercise.

The now commonplace wearing of external nasal splints by sportsmen and athletes has never been scientifically evaluated. The present study looks into the effect of both isometric and isotonic exercise on nasal resistance and examines if this is altered by the wearing of an external nasal splint. Twenty subjects who did not suffer from rhinitis were tested. Nasal resistance measurements were recorded using an anterior rhinomanometer before and after exercise, with and without an external nasal splint. Pulse and blood pressure were measured using a Criticare Inc. model 508 physiological monitor before and after exercise. Significant changes were observed in pulse (P < 0.001) and both systolic (P < 0.002) and diastolic (P < 0.001) blood pressure in response to isotonic exercise and pulse (P < 0.0001) and diastolic blood pressure (P < 0.0006) in isometric exercise. Significant differences were seen in nasal resistance when the splint was applied before (P < 0.001) and after exercise in both groups (P < 0.003). No significant difference was observed between the post-isotonic exercise groups with and without the splint (P = 0.167) but significant differences were seen in the isometric group (P < 0.0001). External nasal splints decrease nasal resistance at rest but are of little proven value when performing isotonic exercise however significantly reduce nasal resistance during isometric exercise.

Exercise↗

PubMedDate not supplied

An in vivo equine forelimb model for short-term recording of peak isometric force in the superficial and deep digital flexor muscles.

OBJECTIVE: To develop and test an experimental model for in vivo short-term recording of peak isometric forces of the digital flexor muscles in the forelimb of adult horses. STUDY DESIGN: In vivo experimental study. SAMPLE POPULATION: Four healthy, anesthetized, adult Thoroughbred horses (3 to 7 years old; 527 +/- 87 kg) METHODS: In dorsal recumbency, ulnar and median nerves were exposed and instrumented with insulated bipolar cuff stimulation electrodes for later connection to an electrical stimulator. In left lateral recumbency, a biplanar fixator was applied to the right humerus and a custom-made, rigid, aluminum frame connected to it, to allow loading of muscles distal to the fixator. Threaded transfixation pins through the radial and metacarpal condyles were clamped to the rigid frame so that the humerus, radius, ulna, and metacarpus were fixed in position. Each digital flexor muscle insertion tendon was transected just above the metacarpophalangeal joint, extracted from the carpal canal, and secured in a metal clamp positioned at the distal myotendinous (MT) junction. Distally, the clamp was connected in series to a load cell and a pneumatic actuator to record force and to maintain muscle length during nerve stimulation. A linear potentiometer was connected in parallel to the actuator to record MT junction position. Initial trials were conducted to identify median and ulnar nerve stimulation variables to achieve maximal muscle contraction. Isometric contractions were performed at different muscle lengths and peak forces registered during 3 seconds of supramaximal dual (ulnar and median) nerve stimulation. RESULTS: A stimulation voltage of 2.5 to 5.0 V at 50 Hz usually produced maximal force for both the superficial digital flexor (SDF) and deep digital flexor (DDF) muscles. Single ulnar and median nerve stimulation elicited force development not only in the DDF muscle but also in the SDF muscle. At voltages higher than 1 V, normalized force was greatest with combined median and ulnar nerve stimulation for both the DDF and SDF muscles; however, normalized force was greater for median nerve stimulation than ulnar nerve stimulation in the DDF muscle, and the opposite relationship was observed for the SDF muscle. Final recording of dual supramaximal nerve stimulation of SDF and DDF muscles resulted in peak isometric forces of 716 +/- 192 N and 1,577 +/- 203 N, respectively. CONCLUSIONS: The instrumentation technique and experimental protocol enabled recording of peak isometric forces in the SDF and DDF muscles of anesthetized adult horses. CLINICAL RELEVANCE: Studies using this model will improve knowledge of SDF and DDF muscle mechanics with insight to functional implications of the complex architecture of these muscles. Knowledge of the dynamic performance of the SDF and DDF muscles would also be useful for the development of new treatment strategies for flexor deformities and tendon injuries in horses.

Animals↗

PubMed1995

Direct vs indirect blood pressure at rest and during isometric exercise in normal subjects.

Systolic and diastolic blood pressures were measured by intraarterial means and by auscultation. Comparisons were made with simultaneously determined intraarterial catheter and auscultation measurements. Five healthy males were measured at rest and during handgrip and deadlift isometric exercises, utilizing small and large muscle mass. The data suggest that indirect systolic blood pressure is highly correlated with the direct method at rest, during handgrip and deadlift (0.8, 0.9 and 0.91 respectively) isometric exercises. Indirect diastolic blood pressure correlates well with intraarterial at rest (0.7) and during the isometric handgrip bout which utilized small muscle mass (0.8). As for the deadlift manoeuvre, the correlation coefficients between the indirect and direct methods were low. These results suggest that when utilizing large muscle mass during isometric deadlift exercise, the indirect method is not valid for assessment of diastolic blood pressure.

Adult↗

PubMed1986

Electromyographic and force production characteristics of leg extensor muscles of elite weight lifters during isometric, concentric, and various stretch-shortening cycle exercises.

Electromyographic and force production characteristics of leg extensor muscles of elite weight lifters (n = 14) were examined during isometric, concentric, and various stretch-shortening cycle exercises. The data showed that utilization of stored elastic energy of leg extensor muscles was observable in countermovement jumping (CMJ) conditions performed at various loads. These greater (P less than 0.01-0.001) vertical jumping heights obtained in CMJ conditions in comparison to the respective squat jumps (SJ) took place at all loads examined from 0 up to 180 kg with no differences in the average IEMG activity between the concentric phases of the performances. In various dropping jumps (from 20 to 100 cm), the average jumping heights were not, however, greater than in the squat jump. The jumping heights both in SJ and CMJ correlated negatively (r = -0.56 and -0.58; P less than 0.05) with the time of isometric force production. Maximal isometric force correlated (r = 0.61; P less than 0.01) with the thigh girth but not (r = 0.22; NS) with the mean muscle fiber area of the vastus lateralis muscle. The jumping heights in SJ and CMJ performed with higher (100-140 kg) loads correlated more significantly (r = 0.76-0.79; P less than 0.01-0.001) than that of maximal isometric force (r = 0.47-0.67; P less than 0.05-0.01) with the results in the Olympic weight lifting. The present findings indicate the important role of effective recoil of the elastic energy in the muscle during the stretch-shortening cycle.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphatases↗

PubMed1990

Concurrent augmented feedback and isometric force generation during familiar and unfamiliar muscle movements.

To evaluate the effect of concurrent augmented feedback on isometric force output during familiar and unfamiliar muscle movements, 18 men and 21 women, 18 to 23 years of age, completed two isometric exercises: flexion of the thumb (a familiar muscle movement) and abduction of the fifth digit (an unfamiliar movement). The exercises consisted of 10 maximum voluntary isometric contractions lasting 10 s each and separated by 10-s intertrial rest intervals. Concurrent visual feedback was provided during alternate contractions. The order of exercises and trials for feedback was randomly assigned and balanced over subjects. Peak force output during abduction of the fifth digit was significantly (p less than or equal to .01) greater with (4.4 +/- 0.29 kg) than without feedback (4.1 +/- 0.26 kg). Feedback did not influence (p greater than .05) peak force output during thumb flexion (23.2 +/- 1.09 kg vs 22.5 +/- 1.05 kg). Muscular fatigue was more pronounced during thumb flexion without feedback (18.4 +/- 1.17%) than when feedback was provided (11.8 +/- 1.36%). These data suggest that fatigue may increase the effect of feedback on force generation during familiar muscular movements. To obtain maximal isometric force measures during strength testing, augmented feedback should be provided.

Adolescent↗

PubMedDate not supplied

Serum creatine kinase activity after isometric exercise in premenopausal and postmenopausal women.

The study was designed to examine the response of women of different ages to exercise-induced muscle damage. Twenty-four women were placed into 3 groups: a postmenopausal group (POST) of 6 women aged 52.8 (2.1) years [mean (SD)] who were at least 2 years without menses; a premenopausal group (PRE) of 10 women aged 43.6 (2.2) years with regular menstrual cycles of 27 (2) days; and a young (YNG) group of 8 women aged 24.6 (3.5) years with regular cycles of 31 (4) days. Serum 17 beta-estradiol measures were taken to confirm menopausal status. Three-month diaries were used to determine regular cycles. Subjects performed 40 maximal isometric knee extensions with a work:rest ratio of 10:20 seconds. Serum creatine kinase activity (CK), muscle soreness (MS), and maximal isometric strength (MVC) were assessed 24 hours prior to, immediately prior to, and 6, 24, 48, and 72 hours after exercise. Subjects experienced significant (p less than .01) increases in CK and MS, and significant decrements in MVC, with peak changes at 24 or 48 hours. Thus, isometric exercise caused slight muscle damage and muscle weakness. No significant differences among the groups were found for any measure (p greater than .05). Thus, estrogen does not seem to serve as a factor in protecting the muscle from damage due to moderate isometric exercise.

Adult↗

PubMed2026

Characteristics of post-exercise responders versus non-responders following aerobic or isometric exercise in physically inactive adults of African and South Asian descent with high-normal blood pressure or grade I hypertension.

OBJECTIVE: To investigate interindividual variability in post-exercise hypotension (PEH) and to characterise cardiovascular and autonomic differences between responders and non-responders following aerobic and isometric exercise in adults of African and South Asian descent with elevated blood pressure (BP). METHODS: Physically inactive adults of African and South Asian descent living in Suriname (18-65 years) with high-normal BP or grade I hypertension participated in a randomised controlled crossover trial. In this randomised cross-over trial, 47 adults (50.1 ± 10.8 years; 38% male) with high-normal blood pressure or grade I hypertension completed three conditions: aerobic exercise (30 min at 40-60% heart rate reserve), isometric handgrip exercise, and a non-exercise control. Ambulatory BP was assessed over 24 h. PEH was defined as the net effect: (post-exercise - pre-exercise) - (post-control - pre-control). Participants were classified as responders if daytime BP decreased ≥5 mmHg. Arterial stiffness, cardiac, and autonomic parameters were assessed. RESULTS: Following aerobic exercise, 46% of participants were classified as systolic responders compared with 28% after isometric exercise. No baseline differences were observed in demographic or clinical characteristics between responders and non-responders, suggesting that PEH variability may reflect underlying physiological rather than clinical differences. Aerobic responders demonstrated greater reductions in aortic augmentation index (-19.4% vs. -10.9%, p = 0.05), larger increases in stroke volume (+8.1 vs. -5.3 mL, p = 0.05) and cardiac output (+1.54 ± 1.89 vs. +0.58 ± 1.60 L/min, p = 0.009), and more favourable autonomic recovery. Among all variables, only the change in cardiac output was associated with PEH magnitude (r = -0.46, p = 0.006). No consistent physiological differences were observed following isometric exercise. CONCLUSION: PEH following aerobic exercise is characterised by a distinct responder phenotype associated with greater reductions in aortic augmentation index and favourable cardiac adaptations. These findings highlight substantial interindividual variability in BP responses and support the need for individualised exercise strategies in hypertension management.

Adolescent↗

PubMed1998

Isometric muscle strength is an indicator of self-reported physical functional disability in patients with rheumatoid arthritis.

The objective of our study was to examine (1) whether isometric muscle strength contributes to the explanation of the physical functional disability of a rheumatoid arthritis (RA) patient population after accounting for other disease parameters and demographic variables and (2) whether change in isometric muscle strength is an indicator of change in physical functional disability. Sixty-five consecutive patients fulfilling the American Rheumatism Association 1987 revised criteria for RA were included in the study. Isometric muscle strength was measured with a validated Muscle Strength Index (MSI) calculated as the mean score of standardized isometric extension and flexion strength of the knee and elbow joints. Physical functional disability was measured with the physical dimension of the Health Assessment Questionnaire (HAQ). For 56 patients, we could obtain 1 yr follow-up data. Cross-sectionally, there was a significant correlation (r = -0.51, P < 0.01) between the MSI and the HAQ. Muscle strength remained a significant determinant of the HAQ in multivariate analysis accounting for disease and demographic variables. Longitudinally, change in MSI explained additional variance of change in HAQ after accounting for change in pain, the most important correlate of change in HAQ.

Adult↗

PubMed1996

Cross-sectional and longitudinal uses of isoinertial, isometric, and isokinetic dynamometry.

The purposes of this investigation were to assess whether maximal isoinertial (triceps pushdown [TP] and triceps extension [TE]), isometric and isokinetic (1.04, 2.08, 3.14, 4.16, and 5.20 rad.s-1) forearm extension strength measures: 1) presented statistical generality when they were correlated prior to and following 4, 8, and 12 wk of resistance training; 2) were similarly affected by training; and 3) presented statistical generality when their changes as a consequence of training were intercorrelated. Fifteen men (11 experimental and 4 controls) without a history of resistance training participated in the study. Training involved four sets of 8-12 repetitions, each followed by 90-s recovery, at 70-75% one repetition maximum (1RM), three times a week, for 12 wk. Training incorporated the TP, close-grip bench press, and triceps kickback exercises. Prior to and after 4, 8, and 12 wk of training, the intercorrelations among the TP, isometric, and isokinetic indices almost always achieved statistical generality (i.e., r2 > 0.5). It was concluded that the strength measures generally discriminated similarly between subjects. However, the sensitivity of the strength measures to the effects of training were dissimilar. While all strength indices increased with the training, the timing (isoinertial prior to isometric and isokinetic adaptations) and magnitude (TP > TE > isometric > isokinetic) of the adaptations varied greatly. None of the intercorrelations between changes in the strength indices achieved statistical generality. Furthermore, factor (F)-analyses on these changes indicated that in the initial and later stages of training, there were three and four discrete factors, respectively, accounting for strength development. These factors were thought to reflect differential effects of training on the structural, neural (including learning), and mechanical mechanisms underpinning each strength index. Possible applications of this research design in better understanding strength development were also canvassed.

Adaptation, Physiological↗

PubMed2000

Comparison of sympathetic nerve responses to neck and forearm isometric exercise.

PURPOSE: Although the autonomic and cardiovascular responses to arm and leg exercise have been studied, the sympathetic adjustments to exercise of the neck have not. The purpose of the present study was twofold: 1) to determine sympathetic and cardiovascular responses to isometric contractions of the neck extensors and 2) to compare sympathetic and cardiovascular responses to isometric exercise of the neck and forearm. METHODS: Muscle sympathetic nerve activity (MSNA), mean arterial pressure (MAP), and heart rate were measured in nine healthy subjects while performing isometric neck extension (INE) and isometric handgrip (IHG) in the prone position. After a 3-min baseline period, subjects performed three intensities of INE for 2.5 min each: 1) unloaded (supporting head alone), 2) 10% maximal voluntary contraction (MVC), and 3) 30% MVC, then subjects performed two intensities (10% and 30% MVC) of IHG for 2.5 min. RESULTS: Supporting the head by itself did not significantly change any of the variables. During [NE, MAP significantly increased by 10 +/- 2 and 31 +/- 4 mm Hg and MSNA increased by 67 +/- 46 and 168 +/- 36 units/30 s for 10% and 30% MVC, respectively. IHG and INE evoked similar responses at 10% MVC, but IHG elicited higher peak MAP and MSNA at 30% MVC (37 +/- 7 mm Hg (P < 0.05) and 300 +/- 48 units/30 s (P < 0.01) for IHG, respectively). CONCLUSIONS: The data indicate that INE can elicit marked increases in MSNA and cardiovascular responses but that it evokes lower peak responses as compared to IHG. We speculate that possible differences in muscle fiber type composition, muscle mass, and/or muscle architecture of the neck and forearm are responsible for these differences in peak responses.

Adult↗

PubMed2001

Muscle vibration sustains motor unit firing rate during submaximal isometric fatigue in humans.

1. In keeping with the 'muscular wisdom hypothesis', many studies have documented that the firing rate of the majority of motor units decreased during fatiguing isometric contractions. The present study investigated whether the application of periodic muscle vibration, which strongly activates muscle spindles, would alter the modulation of motor unit firing rate during submaximal fatiguing isometric contractions. 2. Thirty-three motor units from the lateral head of the triceps brachii muscle were recorded from 10 subjects during a sustained isometric 20 % maximal voluntary contraction (MVC) of the elbow extensors. Vibration was interposed on the contraction for 2 s every 10 s. Twenty-two motor units were recorded from the beginning of the fatigue task. The discharge rate of the majority of motor units remained constant (12/22) or increased (4/22) with fatigue. Six motor units demonstrated a reduction in discharge rate that later returned toward initial values; these motor units had higher initial discharge rates than the other 16 motor units. 3. In a second series of experiments, four subjects held a sustained isometric 20 % MVC for 2 min and then vibration was applied as above for the remainder of the contraction. In this case, motor units initially demonstrated a decrease in firing rate that increased after the vibration was applied. Thus muscle spindle disfacilitation of the motoneurone pool may be associated with the decline of motor unit discharge rate observed during the first 2 min of the contraction. 4. In a third set of experiments, seven subjects performed the main experiment on one occasion and repeated the fatigue task without vibration on a second occasion. Neither the endurance time of the fatiguing contraction nor the MVC torque following fatigue was affected by the application of vibration. This finding calls into question the applicability of the muscular wisdom hypothesis to submaximal contractions.

Adult↗

PubMed1975

Relationship between isometric endurance and fibre types in human leg muscles.

Relationship between isometric endurance performance at 50% of maximal voluntary isometric contraction (MCV) and skeletal muscle fibre composition has been elucidated in 19 physical education students. This was found to be linear and the equation corresponded to: y=9.35 + 1.093x; r=0.70 (endurance time expressed in seconds and fibre composition as percent slow twitch muscle fibres (ST) of the vastus lateralis muscle). As it is assumed from previous studies that similar isometric tensions preferentially recruit fast twitch muscle fibres (FT) and that the muscle at the point of exhaustion exhibits maximal values for lactate accumulation, it is suggested that lactate formed in FT fibres is released and stored in nonrecruited ST fibres. The ability to sustain similar isometric tension would then be depending on how large the fraction of ST fibres is that can serve as a lactaterecipient for lactate producing FT fibres.

Adult↗

PubMed1974

Blood flow in resting (contralateral) arm and leg during isometric contraction.

1. Blood flow in resting forearm and calf were measured plethysmographically in healthy young men during isometric contraction performed both as a handgrip and as a dorsiflexion of the foot. The isometric contraction was maintained for 2 min at one third maximal voluntary contraction for the handgrip and half maximal voluntary contraction for the dorsiflexion of the foot. In some experiments the possible influence on blood flow of inadvertent muscle activation in the resting limb was checked by recording the e.m.g.2. Both handgrip and dorsiflexion of the foot produced substantial increases in heart rate and mean arterial pressure, the pressure rise being almost linear with time throughout the contraction.3. Isometric contraction produced a rapid increase in forearm blood flow which reached a maximum on average two and a half times the resting value after 1 min and then declined slightly. Similar increases in forearm blood flow were produced by handgrip and dorsiflexion of the foot.4. Unlike in the resting forearm, the blood flow in the resting calf increased only slightly during the first minute of contraction and then decreased again to the resting level.5. The flow increase in the resting limbs could not be ascribed to inadvertent muscle activation as judged from the e.m.g. recordings.6. It is concluded that isometric muscle contraction produces a rapid increase in blood flow in the resting forearm, but only a very slight flow increase in the calf. Since the flow increased faster than the arterial pressure it must to a certain extent be induced by active neurogenic vasodilatation. Similarly, the relative flow decrease during the latter half of the contraction concomitantly with a progressively rising arterial pressure suggests that the neurogenic effect on the resistance vessels changes character, becoming more vasoconstrictive, and this might be related to increased effort in sustaining the contraction.

Adult↗

PubMed1975

Muscular reflex stimuli to the cardiovascular system during isometric contractions of muscle groups of different mass.

1. The cardiovascular responses to voluntary isometric contractions performed by human subjects are determined by the proportion of maximal tension achieved by the muscles contracting, and not by the mass of the contracting muscles, nor by the absolute tension achieved (Lind & McNicol, 1967; confirmed here). When two or more muscle groups contract simultaneously at different relative tensions, the increments in heart rate and blood pressure are the same as when the muscle group at the higher relative tension contracts alone at that tension (Lind & McNicol, 1967). It is known that there are both central and reflex stimuli to the cardiovascular system in exercise, and the present study examines whether the muscular reflex stimuli are related to the proportion of maximal tension achieved or to the mass of contracting muscle. 2. Isometric hind-limb contractions were induced in anaesthetized dogs and cats by stimulation of spinal ventral roots. Pressor responses to contraction of both hind limbs were greater than responses to contractions of either hind limb alone. No differences were observed between heart rate responses to single or combined hind-limb contractions. 3. When human subjects perform isometric contractions, a pressor response can be maintained beyond the conclusion of the exercise by occluding muscle blood flow. This response is generally attributed to a reflex set up in the muscle by the action of chemical factors on afferent nerves. When comparable pressor responses were evoked by comparable proportional efforts with either the whole hand or the little finger, it was found here that the pressor responses remaining during the period of post-exercise occlusion were greater when the greater mass of muscle had been exercised. 4. It is concluded that the muscular reflex drive in isometric exercise is related to the bulk of contracting muscle.

Adult↗