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Biomedical subjects

Katrina S Maluf

Publications and source records attributed to Katrina S Maluf.

10 recordsLinked to original sources
PubMed2005

Muscle activation and time to task failure differ with load type and contraction intensity for a human hand muscle.

Time to failure for sustained isometric contractions of the elbow flexors is briefer when maintaining a constant elbow angle while supporting an inertial load (position task) compared with exerting an equivalent torque against a rigid restraint (force task). Our primary purpose was to determine whether the effects of load type on time to task failure exist when motor unit recruitment cannot be enhanced during a sustained submaximal contraction of an intrinsic hand muscle. A second purpose was to determine whether a greater reserve remains in the muscle after early failure of the position task. Two groups of 10 strength-matched men performed the force and position tasks at either 20% or 60% of maximal force (MVC) with the first dorsal interosseus, followed by a second force task at the same relative intensity. The rate of increase in surface EMG was greater (P = 0.002) and time to failure was briefer (P = 0.005) for the position task (593 +/- 212 s) compared with the force task (983 +/- 328 s) at 20% MVC, whereas there were no task differences in these variables at 60% MVC (P >or= 0.200). Time to failure for the second force tasks did not differ at either contraction intensity (P>or=0.743). These results demonstrate that previously observed effects of load type generalize to a hand muscle, although only for low-intensity contractions. For the position task at low forces, muscle activity increased more rapidly and no additional reserve remained in the muscle at failure compared with the force task. We propose that the briefer time to failure for the position task during sustained, low-intensity contractions is due to earlier recruitment of the motor unit pool.

Adult↗
PubMed2005

Prolonged vibration of the biceps brachii tendon reduces time to failure when maintaining arm position with a submaximal load.

Vibration reduces the amplitudes of the tendon jerk response and the Hoffmann and stretch reflexes in the muscle exposed to the vibration, yet does not alter the time to task failure when the task involves exerting a submaximal force against a rigid restraint. Because the amplitude of the stretch reflex is greater when a limb acts against a compliant load than a rigid restraint, the purpose was to determine the influence of prolonged tendon vibration on the time to failure when maintaining limb position with the elbow flexor muscles. Twenty-five healthy men performed the fatiguing contraction by maintaining elbow angle at 1.57 rad until failure while supporting a load equal to 20% of maximal voluntary contraction (MVC) force. The fatiguing contraction was performed on 3 separate days with different levels of vibration applied to the biceps brachii tendon: no vibration, subthreshold for a tonic vibration reflex (TVR), and suprathreshold for a TVR. MVC force before the fatiguing contraction was similar across the three sessions (mean of 3 sessions: 313 +/- 54 N, P = 0.83). Despite a similar decline in MVC force after the fatiguing contraction across conditions (-18.0 +/- 8.0%, P > 0.05), the time to task failure was 3.7 +/- 1.4 min for the suprathreshold TVR condition, 4.3 +/- 2.1 min for the subthreshold TVR condition, and 5.0 +/- 2.2 min for the no-vibration condition (P < 0 0.001). The average EMG of the elbow flexor muscles was similar (P = 0.22) during the fatiguing contractions. However, the fluctuations in limb acceleration at task onset were greater for the suprathreshold TVR condition (P < 0.01), but were not different between the subthreshold TVR and no-vibration conditions (P > or = 0.22). Furthermore, the difference in the SD of limb acceleration between the no-vibration and vibration conditions was correlated with the difference in time to failure for the no-vibration and subthreshold TVR conditions (P = 0.03; r2 = 0.22), but not for the no-vibration and suprathreshold TVR conditions (P = 0.90; r2 = 0.001). These findings indicate that prolonged vibration reduced the time to failure of a sustained contraction when subjects maintained limb position, suggesting that peripheral inputs to the motor neuron pool play a significant role in sustaining a contraction during tasks that require active control of limb position.

Adaptation, Physiological↗
PubMed2005

Variability of motor unit discharge and force fluctuations across a range of muscle forces in older adults.

Variability of motor unit discharge is a likely contributor to the greater force fluctuations observed in old adults at low muscle forces. We sought to determine whether the variability of motor unit discharge rate underlies the fluctuations in force during steady contractions across a range of forces in young (n = 11) and old (n = 14) adults. The coefficient of variation (CV) for discharge rate and force were measured during a force-matching task as the first dorsal interosseous muscle performed isometric contractions. The recruitment thresholds of the 78 motor units ranged from 0.04% to 34% of maximal voluntary contraction (MVC) force. The CV for discharge rate ranged from 7.6% to 46.2% and was greater (P < 0.05) for old adults (21.5% +/- 7.7%) than young adults (17.3% +/- 8.1%). Although the CV for force was similar for young and old subjects (2.53% +/- 1.6%) across all target forces, it was greater for old adults at the lowest forces. Furthermore, there was a positive relation (r2 = 0.20, P < 0.001) between the CV for force and the CV for discharge rate across the range of recruitment thresholds. This relation was significant for old adults (r2 = 0.30, P < 0.001), but not for young adults (r2 = 0.06, P > 0.05). Thus, the normalized variability (CV) of motor unit discharge was greater in old adults and was related to the amplitude of force fluctuations across a broader range of forces than previously examined. These findings underscore the contribution of variability of motor unit activity to motor output in normal human aging.

Action Potentials↗
PubMed2005

Task failure during fatiguing contractions performed by humans.

By comparing the physiological adjustments that occur when two similar fatiguing contractions are performed to failure, it is possible to identify mechanisms that limit the duration of the more difficult task. This approach has been used to study two fatiguing contractions, referred to as the force and position tasks, which differed in the type of feedback given to the subject and the amount of support provided by the surroundings. Even though the two tasks required a similar net muscle torque during submaximal isometric contractions, the duration that the position task could be sustained was consistently much briefer than that for the force task. The position task involved a greater rate of increase in EMG activity and more marked changes in motor unit recruitment and rate coding compared with the force task. These observations are consistent with the hypothesis that the motor unit pool was recruited more rapidly during the position task. The difference in motor unit behavior appeared to be caused by variation in synaptic input, likely involving heightened sensitivity of the stretch reflex during the position task. Upon repeat performances of the two fatiguing contractions, some subjects were able to increase the time to failure for the force task but not the position task. Furthermore, the time to failure for the position task could be influenced by the postural demands associated with maintaining the position of the limb, and the difference in the two durations was enhanced when the postural activity evoked a pressor response. These observations indicate that the difference in the duration of the two fatiguing contractions was attributable to differences in the control strategy used to sustain the tasks and the magnitude of the associated postural activity.

Adaptation, Physiological↗
PubMed2005

Relationship between changes in activity and plantar ulcer recurrence in a patient with diabetes mellitus.

BACKGROUND AND PURPOSE: Although pressure-reducing interventions have been effective in the healing of neuropathic foot ulcers, these ulcers frequently recur in people with diabetes mellitus (DM). This case report illustrates how sudden changes in weight-bearing activity may have affected ulcer recurrence in a patient with DM and how the physical stress theory (PST) relates to ulcer recurrence for this patient. CASE DESCRIPTION: The patient was a 66-year-old man with a history of DM, peripheral neuropathy, and recurrent plantar ulcers. His plantar ulcer healed after total contact casting. OUTCOME: Despite relatively low peak plantar pressure (9.3 N/cm(2)), the patient's ulcer recurred within 4 weeks of healing. Plantar pressure assessment and activity monitoring suggested that a rapid and sudden increase in weight-bearing activity (steps per day) contributed to cumulative plantar tissue stress that was 3.3 times higher on the day of ulcer recurrence than his average value. Although his cumulative plantar stress was high compared with his usual value, the cumulative value was similar to the amount of daily stress of individuals without a history of recurrent ulcers. DISCUSSION: Within the context of the PST, rapid change in activity level may have an effect on cumulative stress and the risk of ulcer recurrence.

Aged↗
PubMed2004

Influence of amplitude cancellation on the simulated surface electromyogram.

The purpose of the study was to quantify the influence of selected motor unit properties and patterns of activity on amplitude cancellation in the simulated surface electromyogram (EMG). The study involved computer simulations of a motor unit population with physiologically defined recruitment and rate coding characteristics that activated muscle fibers whose potentials were recorded on the skin over the muscle. Amplitude cancellation was quantified as the percent difference in signal amplitude when motor unit potentials were summed before and after rectification. The simulations involved varying the level of activation for the motor unit population, the recording configuration, the upper limit of motor unit recruitment, peak discharge rates, the amount of motor unit synchronization, muscle fiber length, the thickness of the subcutaneous tissue, and the motor unit properties that change with advancing age. The results confirmed a previous experimental report (Day SJ and Hulliger M, J Neurophysiol 86: 2144-2158, 2001) that amplitude cancellation in the surface EMG can reach 62% at maximal activation. A decrease in the range of amplitudes of the motor unit potentials, as can occur during fatiguing contractions, increased amplitude cancellation up to approximately 85%. Differences in the amount of amplitude cancellation were observed across all simulated conditions, and resulted in substantial changes in the absolute magnitude of the EMG signal. The most profound factors influencing amplitude cancellation were the number of active motor units and the duration of the action potentials. The effects of amplitude cancellation were minimal (<5%) when the EMG amplitude was normalized to maximal values, with the exception of variations in peak discharge rate and recruitment range, which resulted in differences up to 17% in the normalized EMG signal across conditions. These results indicate the amount of amplitude cancellation that can occur in various experimental conditions and its influence on absolute and relative measures of EMG amplitude.

Action Potentials↗
PubMed2004

Foot pressures during level walking are strongly associated with pressures during other ambulatory activities in subjects with diabetic neuropathy.

OBJECTIVE: To assess the relationship between foot pressures measured during level walking and other types of ambulatory activity in subjects with diabetes mellitus (DM) and peripheral neuropathy (PN). DESIGN: Descriptive survey with repeated measures. SETTING: University medical center. PARTICIPANTS: Convenience sample of 16 ambulatory subjects with DM and PN. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Peak pressure and pressure-time integral (PTI) at the heel, great toe, first metatarsal head (MT1), and third metatarsal head (MT3) during level walking, ramp climbing, stair climbing, and turning at a self-selected speed. RESULTS: Peak pressure and PTI during level walking correlated highly with pressures during ramp climbing (r range,.85-.97) and turning (r range,.75-.96) at all regions examined and with pressures during stair climbing at MT1 and MT3 (r range,.84-.91). Correlations between pressures during level walking and stair climbing were moderate at the great toe (r range,.46-.57) and poor at the heel (r range, -.12 to -.06). With few exceptions, pressures during ramp climbing, stair climbing, and turning were less than (P<.008) or equal to pressures during level walking. We found no activity-related differences in peak pressure or PTI independent of the effects of preferred walking speed. CONCLUSIONS: Results support the clinical evaluation of peak pressure during level walking as an efficient method to screen for maximum levels of stress on the foot as patients with DM and PN perform their daily activities.

Activities of Daily Living↗
PubMed2003

Comparison of muscle activity during walking in subjects with and without diabetic neuropathy.

The purpose of this study was to compare muscle activity and joint moments in the lower extremities during walking between subjects with diabetic neuropathy (DN) and control subjects. Tests were performed on nine subjects with DN, and nine age, gender, and weight-matched controls. Onset and cessation times of lower extremity electromyographic (EMG) activity and joint moments were determined. Results demonstrated that subjects with DN had less ankle mobility, slower walking speeds, longer stance phases, and lower peak ankle dorsiflexion, ankle plantar flexion, and knee extension moments than control subjects. Onset times with respect to heel-strike (HS) for the soleus, medial gastrocnemius, and medial hamstring muscles were significantly earlier during the gait cycle (GC) in subjects with DN than in control subjects. The cessation times of soleus, tibialis anterior, vastus medialis, and medial hamstring muscles were significantly prolonged in subjects with DN. Subjects with DN showed more co-contractions of agonist and antagonist muscles at the ankle and knee joints during stance phase compared with control subjects. These gait changes and co-contractions may allow subjects with DN to adopt a safer, more stable gait pattern to compensate for diminished sensory information from the ankle and the foot. The premature activation of soleus and medial gastrocnemius muscles in subjects with DN could be contributing to abnormal forefoot plantar pressure distribution. Additional research is needed to clarify the relationship between the premature activation of triceps surae muscles and the forefoot plantar pressure parameters in subjects with DN.

Aged↗
PubMed2002

Tissue adaptation to physical stress: a proposed "Physical Stress Theory" to guide physical therapist practice, education, and research.

The purpose of this perspective is to present a general theory--the Physical Stress Theory (PST). The basic premise of the PST is that changes in the relative level of physical stress cause a predictable adaptive response in all biological tissue. Specific thresholds define the upper and lower stress levels for each characteristic tissue response. Qualitatively, the 5 tissue responses to physical stress are decreased stress tolerance (eg, atrophy), maintenance, increased stress tolerance (eg, hypertrophy), injury, and death. Fundamental principles of tissue adaptation to physical stress are described that, in the authors' opinion, can be used to help guide physical therapy practice, education, and research. The description of fundamental principles is followed by a review of selected literature describing adaptation to physical stress for each of the 4 main organ systems described in the Guide to Physical Therapist Practice (ie, cardiovascular/pulmonary, integumentary, musculoskeletal, neuromuscular). Limitations and implications of the PST for practice, research, and education are presented.

Adaptation, Physiological↗