Biomedical subjects

G Kamen

Publications and source records attributed to G Kamen.

At least 19 recordsLinked to original sources

PubMed2005

Superficial motor units are larger than deeper motor units in human vastus lateralis muscle.

Previous studies have suggested that regionalization may occur for human motor units, whereby smaller motor units are located in deeper parts of the muscle and larger motor units are located in more superficial portions. We examined this possibility in the human vastus lateralis muscle using macro-EMG (electromyography) to estimate motor unit size. The sample consisted of nine individuals from whom 114 motor units were recorded at forces ranging between 5% and 60% MVC. Peak-to-peak macro-EMG amplitude was well correlated with macro area (Spearman rho = 0.96). There was a statistically significant inverse relationship between recording depth and macro peak-to-peak amplitude (rho = -0.402, p < 0.001). We conclude that there is a nonrandom distribution of motor units in human muscle, with larger motor units located in more superficial regions and smaller units located in deeper regions. Clinicians who monitor motor unit activity need to recognize that a representative sample of motor unit recordings should include motor units from both deeper and more superficial regions of muscle.

Action Potentials↗

PubMed2001

Adaptations in maximal motor unit discharge rate to strength training in young and older adults.

Six young (mean = 23 years) and 6 older (mean = 76 years) adults participated in isometric resistance training 5 days/week for 6 weeks. The task involved isometric fifth finger abduction. Maximal motor unit discharge rates (MUDRs) were obtained from the abductor digiti minimi of each hand at 0, 2, 14, and 42 days of training using a quadrifilar needle electrode and automatic spike recognition software. In agreement with previous findings, maximal MUDR at baseline was significantly lower in older adults (P < 0.001), averaging 51.5 (+/-17.13) HZ in young and 43.3 (+/-14.88) HZ in older adults. In response to resistance training, maximal voluntary force increased 25% in young and 33% in older subjects (P < 0.001). Maximal MUDR increased significantly (11% young, 23% older) on day 2 [F(3,36) = 2.58, P < 0.05], but in older subjects returned to baseline levels thereafter. These adaptations in abductor digiti minimi MUDR suggest a two-part response to strengthening fifth finger abduction: early disinhibition followed by altered MU activation.

Action Potentials↗

PubMed2001

Adaptations in muscular activation of the knee extensor muscles with strength training in young and older adults.

The purpose of this study was to compare the extent of muscular activation during maximal voluntary knee extension contractions in old and young individuals and to examine the effects of resistance training on muscular activation in each group. The interpolated twitch technique was used to estimate muscular activation during two pre-training baseline tests, and after two and six weeks of resistance training. Throughout the study, the older group was 30% less strong than the young group (p=0.02). The training protocol was effective in both groups with overall isometric strength gains of 30 and 36% in the older (p=0.01) and young (p<0.01) groups, respectively. 10-RM training loads increased by 66% in the old group (p<0.01) and by 77% in the young group (p<0.01) throughout training. At the first baseline test, a 2% difference in muscular activation between groups (p=0.3) did not explain the large disparity in strength. Muscular activation increased by 2% in both groups throughout training (p<0.01). Despite considerably less muscular strength in the older group, muscular activation was greater than 95% of maximum and appears to be equal in both young and older individuals. Both groups demonstrated similar but small increases in muscular activation throughout training.

Adult↗

PubMed2001

Effect of ketoprofen on muscle function and sEMG activity after eccentric exercise.

PURPOSE: This study examined whether ketoprofen, a nonsteroidal anti-inflammatory drug, attenuated muscle soreness (SOR), improved maximal isometric force (MIF) recovery, and/or altered myoelectric activity after high-force eccentric exercise. METHODS: 48 subjects were randomly assigned to one of four groups: CON: no exercise/no drug (N = 12); PLA: exercise + placebo (N = 12); TRT-100: exercise + 100 mg oral ketoprofen (N = 12); and TRT-25: exercise + 25 mg oral ketoprofen (N = 12). PLA, TRT-100, and TRT-25 were administered in a double-blind fashion. Baseline measurements of SOR, MIF, and surface electromyographic (EMG) amplitude were taken, and PLA, TRT-100, and TRT-25 performed 50 maximal eccentric contractions of the elbow flexors; 36 h later, subjects reporting moderate soreness were given ketoprofen or placebo and SOR measures were taken hourly for 8 h. EMG amplitude was assessed during MIF before dosing and again 8 h later and during submaximal contractions of 5%, 10%, and 20% of MIF before dosing and hourly for 8 h. RESULTS: Eccentric exercise increased myoelectric activity during submaximal force measurements in PLA, TRT-100, and TRT-25 in all conditions. Ketoprofen had no effect on reducing this increase in EMG activity. Ketoprofen attenuated perceived SOR (P < 0.05) and enhanced MIF recovery (P < 0.05) compared with placebo. TRT-100 and TRT-25 demonstrated 10% and 19% reductions in SOR, respectively, and 16% and 9% increases in MIF, respectively, whereas PLA demonstrated a 1% increase in SOR and 9% decrease in MIF over 8 h. CONCLUSION: Ketoprofen treatment after muscle damaging exercise reduces muscle soreness and improves force recovery.

Adult↗

PubMed2000

Adaptations in motor unit discharge activity with force control training in young and older human adults.

Six young (aged 18-22 years) and six older (aged 66-76 years) healthy humans participated in a visually guided isometric force modulation training program designed to improve accurate control of force during ankle dorsiflexion. Isometric force and the discharge activity of motor units (MU) supplying the tibialis anterior muscle were sampled concurrently at the beginning of the study, following 2 weeks of force modulation training and again after a 4 week retention period which followed immediately. The initial maximal voluntary force (MVC) and MU discharge rates were similar between young and older adults at 10-60% MVC while MU discharge rates during maximal effort were significantly reduced in older adults. Following the 2 weeks of force modulation training, both young and older adults demonstrated significant improvements in force accuracy (44% young, 48% older) and significantly reduced MU discharge rates at 30%, 40%, and 60% MVC. Young adults also demonstrated increased MVC force (11%), while older adults demonstrated significantly increased (30%) maximal MU discharge rate. Thus, following 2 weeks of force modulation training, young and older individuals demonstrated similar MU discharge rates at all force levels. The MU discharge rate adaptations were retained after the 4 week retention period. In young adults, improved force accuracy and increased MVC force were accompanied by significantly reduced MU recruitment thresholds. In the older subjects, improved force accuracy was accompanied by an increase in the difference between the recruitment-derecruitment force threshold and significantly reduced antagonist co-contraction. Age-related alterations in force regulation and MU discharge activity cannot be explained solely on the basis of contractile changes in senescent muscle. Rather, reliance on compensatory neuromuscular changes including antagonist muscle co-contraction is suggested.

Adolescent↗

PubMed2000

Motor unit synchronization in young and elderly adults.

In an effort to determine whether aging might alter the manner in which pairs of motor units are coactivated, the extent of motor unit synchronization was assessed in the first dorsal interosseous (FDI) muscle in seven young (mean 28 years) and eight older adults (mean 75 years). During constant-force isometric contractions at either 50% or 100% of maximal effort, motor units were recorded using a four-wire needle electrode and a multi-channel recording technique. Customized software was utilized to identify the occurrences of motor unit action potentials from the resultant three-channel signals. The magnitude of motor unit synchronization was determined using six variables that have been described previously in the literature. The extent of motor unit synchronization was similar in both young and older adults, occurring in nearly every motor unit pair observed. During the 50% maximum voluntary contractions, the CIS synchronization measure (the number of synchronized discharges per unit time) averaged 2.5 in the young subjects and 2.4 in the older individuals. The intensity of motor unit synchronization was somewhat greater during maximal force isometric contractions. We conclude that motor unit synchronization in the FDI muscle is a ubiquitous phenomenon that is not affected by the aging process.

Action Potentials↗

PubMed1999

Independence of motor unit recruitment and rate modulation during precision force control.

The vertebrate motor system chiefly employs motor unit recruitment and rate coding to modulate muscle force output. In this paper, we studied how the recruitment of new motor units altered the firing rate of already-active motor units during precision force production in the first dorsal interosseous muscle. Six healthy adults performed linearly increasing isometric voluntary contractions while motor unit activity and force output were recorded. After motor unit discharges were identified, motor unit firing rates were calculated before and after the instances of new motor unit recruitment. Three procedures were applied to compute motor unit firing rate, including the mean of a fixed number of inter-spike intervals and the constant width weighted Hanning window filter method, as well as a modified boxcar technique. In contrast to previous reports, the analysis of the firing rates of over 200 motor units revealed that reduction of the active firing rates was not a common mechanism used to accommodate the twitch force produced by the recruitment of a new motor unit. Similarly, during de-recruitment there was no tendency for motor unit firing rates to increase immediately following the cessation of activity in other motor units. Considerable consistency in recruitment behavior was observed during repeated contractions. However, firing rates during repeated contractions demonstrated considerably more fluctuation. It is concluded that the neuromuscular system does not use short-term preferential motor unit disfacilitation to effect precise regulation of muscular force output.

Action Potentials↗

PubMed1999

Maximal motor unit discharge rates in the quadriceps muscles of older weight lifters.

UNLABELLED: Although the existence of "neural factors" is regularly cited as an important contributor to muscular strength, we have little specific knowledge regarding the existence of such neural factors or how they contribute to the expression of muscular force. PURPOSE: The present investigation sought to assess maximal motor unit discharge rates in older, highly resistance-trained adults to determine whether maximal motor unit discharge rates might be one such neural contributor to maximal strength production. METHODS: Subjects consisted of seven well-trained older weight lifters (ages 67-79 yr) and five untrained age-matched older adults. While subjects performed 50 and 100% maximal voluntary knee extensor contractions (MVC), recordings from groups of motor units were obtained from the rectus femoris muscle by using an indwelling electrode. Off-line analysis was performed to identify individual motor unit firing occurrences and to compute maximal motor unit discharge rates. RESULTS: As expected, knee extension strength in the trained weight lifters (367.0 +/- 72.0 N) was significantly greater than that in the control subjects (299.9 +/- 35.9 N; P < 0.05). Motor unit discharge rates were similar in the two subject groups at the 50% MVC force level (P > 0.05), but maximal (100% MVC) motor unit discharge rate in the weight lifters (23.8 +/- 7.71 pps) was significantly greater than that in the age-matched controls (19.1 +/- 6.29 pps; P < 0.05). CONCLUSION: Motor unit discharge rates may comprise an important neural factor contributing to maximal strength in older adults.

Action Potentials↗

PubMed1999

Adverse events associated with eccentric exercise protocols: six case studies.

PURPOSE: Rhabdomyolysis is a condition characterized by muscle damage and degeneration of muscle cells after strenuous, overexertion exercise. Although the incidence of severe rhabdomyolysis is rare, this condition can be dangerous and even fatal. Eccentric exercise protocols are currently being used to induce and study mild forms of muscle damage. However, serious adverse events can occur in these laboratory investigations. The purpose of this report was to expose some of the adverse events resulting from performance of eccentric exercise protocols to study muscle damage in humans. METHODS: The following case studies involved an eccentric exercise protocol where two sets of 25 maximal eccentric actions of the elbow flexors were performed, separated by a 5-min rest period. RESULTS: Case reports are presented that reveal prolonged losses in the ability of the muscle to generate force lasting 43-47 d, extreme swelling of the exercised arm lasting several weeks, and greatly elevated serum creatine kinase levels. CONCLUSIONS: Although adverse events resulting from eccentric exercise are rare, our laboratory has observed a 3% incidence rate during the past year. Investigators should be knowledgeable of the sequelae of events that are associated with muscle damage after high-force eccentric exercise and take appropriate precautions.

Arm↗

PubMed1998

Detecting balance deficits in frequent fallers using clinical and quantitative evaluation tools.

OBJECTIVE: This investigation sought to determine whether older idiopathic frequent fallers could be distinguished from healthy older adults on the basis of balance and movement coordination tests. A secondary objective was to determine the relationships among clinical balance test scores, balance performance data obtained by accelerometry, and quantitative motor coordination tests. DESIGN: Two group comparison designs. SETTING: A motor control research laboratory in a university setting. PARTICIPANTS: The 16 subjects recruited for this study included eight healthy older subjects and eight age-matched idiopathic fallers. MEASUREMENTS: Each participant's balance performance was assessed by accelerometry, as well as by coordination and clinical tests. Accelerometry scores, obtained by 1g accelerometers placed at the hip and on the head, were made with eyes open or closed, either standing on the floor or on a wedge of compliant foam. Clinical balance scores were obtained using variants of Romberg's test and the functional reach test. Motor coordination tests obtained included the heel-to-toe transition and rapid stepping tests. MAIN RESULTS: Statistically significant differences were obtained between groups for all accelerometry variables except root mean square. All accelerometry variables were successful in discriminating between head and hip sites. Moreover, the amplitude of sway obtained from accelerometry data identified significant differences among the four test conditions. The Romberg test, using right leg alone with eyes open, showed a significant difference between fallers and healthy older subjects. Walking velocity was significantly faster for normal older subjects than for fallers (1.10 m/sec vs 0.80 m/sec). No significant between-group differences were obtained using the functional reach test. Coordination skills yielded significant between-group differences using the rapid stepping test but no significant differences between groups with the heel-toe transition test. CONCLUSION: Accelerometry is an inexpensive and clinically useful technique that can distinguish between healthy older people and idiopathic frequent fallers. In conjunction with clinical procedures and commercially available tests to assess motor coordination, these techniques can identify older individuals susceptible to frequent falls.

Accidental Falls↗

PubMed1998

An accelerometry-based system for the assessment of balance and postural sway.

The aim of this investigation was to develop an inexpensive, efficient system for the clinical assessment of static and dynamic balance and postural sway using accelerometry-based measurements. Subjects consisted of 10 young (range 18-32 years) and 10 older (range 69-86 years) individuals screened for polypharmacy and history of cardiovascular, neurological or orthopedic health conditions. A lightweight uniaxial accelerometer and general-purpose microcomputer were used to obtain measurements of postural sway. Customized software was written to acquire the data and provide a real-time display consisting of amplitude and frequency characteristics of the sway profile. Intraclass reliability coefficients greater than R = 0.75 were obtained in both eyes-open and challenging-standing balance tasks. Preliminary results demonstrate that the instrumentation can be used to discriminate among balance tasks and to differentiate healthy older adults from those with a tendency toward frequent falls. Moreover, the technique described yields a simple-to-administer, inexpensive procedure that can be conducted in the home or another natural environment. Accelerometry also allows for balance training and re-learning, using tasks that might ordinarily pose a balance challenge for the older or frail adult.

Adolescent↗

PubMed1996

Age-dependent effects of muscle vibration and the Jendrassik maneuver on the patellar tendon reflex response.

OBJECTIVE: To explore possible effects of aging on the excitability of spinal reflexes. DESIGN: Using a cross-sectional design, the influences of muscle vibration and the Jendrassik maneuver on patellar tendon reflex function were compared between 30 young adults and 15 older adults. SETTING: Motor control research laboratory. SUBJECTS: The young adults were volunteers of college age. The older adults (74.5 +/- 4.14 yr) were volunteers from the local community. All subjects were free of medications and neurological conditions that would affect normal neuromuscular responses. MAIN OUTCOME MEASURES: A force-time curve analysis of the patellar tendon reflex response was used to assess the inhibition and facilitation of spinal reflexes. In the experimental protocol to assess spinal reflex inhibition, 100 Hz vibration was applied to the right quadriceps muscle. In another experimental protocol, spinal reflex facilitation was assessed using the Jendrassik maneuver. To perform the Jendrassik maneuver, subjects were instructed to grasp their hands together and to pull as hard as possible while breathing normally. After a 2-second count, the tendon tap was delivered to the right leg and the subject was instructed to relax. In both experimental protocols, control patellar tendon reflexes were collected. RESULTS: Analysis of variance for reflex peak force revealed a significant 30% reduction in the amount of vibration-induced reflex inhibition with increasing age, and a similar 33% reduction in the amount of Jendrassik maneuver facilitation observed for the older adults as compared with the younger adults. CONCLUSION: These results support the hypothesis that inhibitory and excitatory influences acting on the alpha motoneuron pool are different in young and older adults.

Adult↗

PubMed1996

Changes in spinal reflexes preceding a voluntary movement in young and old adults.

BACKGROUND: Age-related differences in spinal excitability during response preparation were assessed by eliciting either a 50% H-reflex or an Achilles tendon reflex preceding the onset of a right plantar flexion contraction in 20 young adults (23.1 +/- 1.64 yrs) and 20 old adults (68.5 +/- 5.53 yrs). METHODS: On each simple reaction time trial, the test reflex was elicited at a specific test interval during either the foreperiod or the response period. The foreperiod test intervals were 500, 600, 700, 800, 900, and 1000 msec after the presentation of the warning stimulus. The response period test intervals were 50, 100, 150, 200, 250, and 300 msec after the presentation of the response stimulus. Control reflexes were randomly elicited between the simple reaction time trials. RESULTS: Changes in reflex excitability were not observed during the foreperiod in either age group. During the response period, the percentage of H-reflex facilitation as compared to control H-reflexes was similar for the young (68%) and the old (61%) adults, but the magnitude of Achilles tendon reflex facilitation with respect to control reflex responses was greater in the young adults (74%) than in the old adults (38%). The time course of H- and tendon reflex facilitation was delayed in the old group during the response period. CONCLUSIONS: The results indicate that processes underlying the preparation and generation of a motor response are similar in young and old adults. However, these processes occur at a slower rate in old adults.

Achilles Tendon↗

PubMed1996

Physiology and interpretation of the electromyogram.

The purpose of this review is to consider some issues in the interpretation of the electromyogram (EMG) and to discuss current areas of controversy regarding use of the EMG. We consider the underlying physiology and origin of the EMG signal and offer an abbreviated discussion of measurement issues and selected factors that affect the characteristics of the EMG signal. We discuss many of the problems affecting interpretation, including normalization, crosstalk, and issues specific to contraction. In the final section, we consider topics of current interest in electromyography, such as muscle fatigue, task specificity, multichannel representations, and muscle fiber conduction velocity. We present, in addition, alternative analysis techniques. This review should interest researchers and clinicians who seek to obtain the valuable information inherent in the EMG while respecting the potential sources of variance and misinterpretation.

Electrodes↗

PubMed1995

Motor unit discharge behavior in older adults during maximal-effort contractions.

A reduction in maximal force production is a common observation in older individuals. In an effort to determine whether aging is accompanied by reductions in central motoneuron drive limiting motor performance, motor unit discharge records were obtained from seven young (21-33 yr) and seven older (> 67 yr) adults. Informed consent was obtained from all subjects. The task required the subject to perform a maximal abduction of the second digit under isometric conditions. Motor unit potentials in the first dorsal interosseous were monitored by using a selective four-wire needle electrode and identified off-line with the aid of a Dantec electromyograph. The maximal discharge rate in the older adults (31.1 impulses/s) was significantly smaller (P < 0.05) than that in the younger subjects (50.9 impulses/s). These findings suggest that reductions in maximal force capability in older adults are partially due to an impaired ability to fully drive the surviving motor units.

Action Potentials↗

PubMed1995

Impairments of the response preparation process in the elderly.

In 20 young adults (23.1 +/- 1.64 yrs) and 20 old adults (68.5 +/- 5.53 yrs), H-reflexes were evoked simultaneously in the right soleus muscle that was preparing to respond to a reaction time stimulus and in the left soleus muscle that was uninvolved in the impending reaction time response. In the one-second period preceding the presentation of the response stimulus, the right H-reflex was inhibited as compared to the left H-reflex in the young adults; whereas, in the old adults, the response preparation profiles for the left and right H-reflexes were similar. Following the presentation of the response stimulus, H-reflex facilitation in the young adults reflected a non-specific tuning response and a specific, movement-related response. The nonspecific H-reflex enhancement was not observed in our older adults. These results indicate that the generalized tuning sequence necessary for movement planning is impaired in the elderly.

Adult↗

PubMed1993

The relationship between body height extremes and the conditioned patellar tendon reflex response.

The purpose of this study was to determine the effects of extreme body heights on the conditioned patellar tendon reflex response in an attempt to differentiate between the contributions of supraspinal and spinal mechanisms on long-latency reflex facilitation. Unilateral and conditioned right patellar tendon reflexes were assessed in 10 extremely short males and females and 10 extremely tall males and females. The conditioning stimulus was a contralateral patellar tendon tap and the conditioning intervals were 10, 15, 25, 50, 60, 75, 100, 150, and 300 ms. There was a long-latency facilitation of quadriceps excitability beginning at the 75 ms conditioning interval regardless of the height of the subject. It is hypothesized that the similar conditioned patellar tendon recovery profiles in the extremely short subjects and the extremely tall subjects reflects the activation of a spinal polysynaptic pathway as the predominant mechanism for our long-latency reflex facilitation.

Adolescent↗

PubMed1992

Lateral dominance and motor unit firing behavior.

Twelve subjects were classified as left-handed (LH) or right-handed (RH) using Annett's hand dominance classification. Motor unit recordings were obtained from the first dorsal interosseous (FDI) muscle of each hand using a quadrifilar needle electrode. Firing occurrences of individual motor units were then identified and the firing rates of all motor units recorded during the contraction were cross-correlated. The results demonstrated significantly greater firing rate cross-correlation scores in the dominant hand than in the non-dominant hand for both LH and RH subjects. This association between hand dominance and the common drive of motor unit firing rates lends credence to the idea that one or more CNS sites may influence conjoint motor unit firing behavior.

Action Potentials↗