Alex R. Ward

Electrical Stimulation Using Kilohertz-Frequency Alternating Current

Transcutaneous electrical stimulation using kilohertz-frequency alternating current (AC) became popular in the 1950s with the introduction of “interferential currents,” promoted as a means of producing depth-efficient stimulation of nerve and muscle. Later, “Russian current” was adopted as a means of muscle strengthening. This article reviews some clinically relevant, laboratory-based studies that offer an insight into the mechanism of action of kilohertz-frequency AC. It provides some answers to the question: “What are the optimal stimulus parameters for eliciting forceful, yet comfortable, electrically induced muscle contractions?” It is concluded that the stimulation parameters commonly used clinically (Russian and interferential currents) are suboptimal for achieving their stated goals and that greater benefit would be obtained using short-duration (2–4 millisecond), rectangular bursts of kilohertz-frequency AC with a frequency chosen to maximize the desired outcome.

Sensory, motor, and pain thresholds for stimulation with medium frequency alternating current.

OBJECTIVES To investigate the effect of frequency of alternating current on the sensory, motor, and pain thresholds in normal subjects, and to establish the optimal frequency for motor stimulation with minimal subject discomfort. DESIGN A repeated measures design using two groups of 11 subjects. SETTING A laboratory setting was used. PARTICIPANTS Participants were volunteers who met the inclusion criteria. INTERVENTIONS Alternating current with 20 different carrier frequencies between 1 and 35 kHz, all modulated at 50 Hz, was applied to each subject on two separate occasions. For half the subjects, the frequency was increased or decreased sequentially (reversed on second occasion), and for the other half, it was applied in a different random order on each occasion. MAIN OUTCOME MEASURES The voltage at the sensory threshold was recorded for each applied frequency. This was subsequently repeated for motor and pain thresholds. RESULTS Sensory, motor, and pain thresholds decreased with increasing frequency between 1 kHz and approximately 10 kHz. Above 10 kHz, the thresholds increased. The ratio pain threshold/sensory threshold increased systematically with increasing frequency over the range examined. By contrast, the ratio pain threshold/motor threshold showed a distinct maximum at a frequency of 10 kHz. Marked accommodation to motor and pain fiber stimulation was observed. CONCLUSIONS For comfortable sensory stimulation, a high frequency of alternating current is preferable. Discrimination between pain and motor stimulation is maximal at a frequency of approximately 10 kHz. This suggests that the optimal frequency for comfortable motor stimulation, one that is least likely to elicit pain fiber stimulation, is close to 10 kHz.

Variation in torque production with frequency using medium frequency alternating current

OBJECTIVES To investigate the effect of frequency of alternating current on electrically induced torque in healthy subjects, and to establish an optimum frequency for motor stimulation. DESIGN A repeated-measures design using 12 subjects (part 1) and 3 subjects (part 2). SETTING A laboratory setting was used. PARTICIPANTS Participants were volunteers who met the inclusion criteria. INTERVENTIONS Alternating current with carrier frequencies between 1 and 15kHz, modulated at 50Hz, was applied to each subject on two separate occasions. The frequencies were applied in a different random order on each occasion and different random orders were used between subjects. For part 1 of this study, six frequencies were used: 1, 2, 4, 7, 10, and 15kHz. For part 2, three frequencies were used: 1, 4, and 10kHz. MAIN OUTCOME MEASURES Part 1: The maximum electrically induced torque, defined as the torque induced at the pain threshold, was recorded for each applied frequency. Part 2: The variation of torque with increasing stimulus intensity was recorded for each applied frequency. RESULTS Highest electrically induced torque was produced at the lowest frequency examined (1kHz). Torque decreased systematically with increasing frequency over the frequency range examined. The rate of increase in torque with applied stimulus intensity was also found to decrease systematically with increasing frequency in a way that depended strongly on the skinfold thickness of the subject. CONCLUSIONS Electrical stimulators using an alternating current waveform typically use a carrier frequency in the range of 2kHz to 4kHz for motor stimulation. This study shows that carrier frequencies in this range, modulated at 50Hz, are a compromise between comfort and maximum torque production. For maximum comfort with a low torque, a frequency close to 10kHz is indicated. For maximum torque, a lower frequency of alternating current (1kHz or less) is preferable.

Wrist Extensor Torque Production and Discomfort Associated With Low-Frequency and Burst-Modulated Kilohertz-Frequency Currents

Background and Purpose. A randomized controlled trial to compare 2 forms of monophasic pulsed currents with 2 forms of burst-modulated, kilohertz-frequency alternating current (“Russian current” and “Aussie current”) was conducted to establish whether different amounts of wrist extensor torque were produced and whether discomfort varied with stimulus type. Subjects. The 32 subjects were adults who were healthy and were drawn from a population of staff and students at La Trobe University. Methods. Each subject received all 4 currents. Maximal electrically induced torque (MEIT) of the wrist extensors was measured for each stimulus type. Relative discomfort of stimulation also was assessed. Results. Russian current elicited lower mean torque than those elicited by Aussie current and monophasic pulsed currents. The Russian and Aussie currents elicited significantly less discomfort than the 2 monophasic pulsed currents. Discussion and Conclusion. When force production and relative discomfort were jointly used as the criteria, Aussie current was found to be more effective than either of the monophasic pulsed currents or Russian current stimulation.

Variation in motor threshold with frequency using kHz frequency alternating current

We investigated the frequency dependence of motor thresholds over the frequency range 1 kHz to 25 kHz. Alternating current (AC), ramped in intensity, was applied transcutaneously, and the induced wrist extensor torque was measured. Plots of log torque versus stimulus voltage were used to accurately determine thresholds. Three kinds of sinusoidal AC stimuli were compared: continuous, 10 ms bursts at 50 Hz, and 50‐Hz single‐cycle. Differences were attributed to summation of subthreshold depolarizations. The variation in relative thresholds (continuous/single‐cycle and burst/single‐cycle) indicates that summation occurs more efficiently at higher kHz frequencies. The observed frequency and waveform dependence provides evidence for high‐frequency nerve fiber firing rates and fiber dropout when continuous or modulated AC is used, with the effects increasing with AC frequency. The form of the motor response evoked at high frequencies has features that suggest that frequencies above 10‐kHz have little or no useful clinical role in rehabilitation procedures.

Longwave Ultrasound Reviewed and Reconsidered

Summary Longwave ultrasound (45 kHz) is now available clinically, but there is little independent and scientifically acceptable support for using this frequency. This paper reviews the physical properties of different frequencies of ultrasound, and existing laboratory and clinical studies of 45 kHz ultrasound, and reconsiders their implications for clinical practice. Both physical theory and laboratory studies indicate that the effects of longwave ultrasound are quite superficial. The sole clinical study of 45 kHz ultrasound identified is not convincing. Additional laboratory and clinical studies are required. In the meantime, physiotherapists should continue to use MHz frequency ultrasound for treating deep tissues.

Secondary school science predictors of academic performance in university bioscience subjects

In 2009 the Faculty of Health Sciences at La Trobe University in Melbourne, Australia is introducing a common first year for 11 different undergraduate courses in the faculty. Current prerequisite science entry requirements vary with course and range from none to at least two science or mathematics subjects and from ∼50 to 99 in Equivalent National Tertiary Entrance Rank (ENTER) scores. Under the previous structure, students in different courses completed a variety of different subjects at first year. Concern about the ability of such disparate groups to complete a common first year led to the current investigation of the relationships between year 12 (final year of secondary school) science subjects and performance in first year university bioscience subjects. Year 12 results for all science‐related units and ENTER scores were obtained for all Victorian students enrolling in a first year course in the Faculty of Health Sciences in 2005 and 2006. Regression and other analyses were conducted for five first year bioscience subjects. The ENTER score was the best predictor of academic performance in all units except regional anatomy. Performance in many secondary school science subjects was highly predictive of performance in physiology, combined systematic physiology and anatomy and biomechanics units, but again not for regional anatomy units. It appears that year 12 performance in science subjects and ENTER scores may be important predictors of success in physiology, but not regional anatomy subjects at university. It is possible that regional anatomy is an entirely new subject area that requires new types of learning unrelated to year 12 science subjects. Anat Sci Ed 2:113‐118, 2009.

Comparison of the Hypoalgesic Efficacy of Low-Frequency and Burst-Modulated Kilohertz Frequency Currents

Background and Purpose: A within-subject randomized controlled trial was conducted to compare monophasic pulsed current (PC) with a frequency of 50 Hz and a phase duration of 500 microseconds with burst-modulated alternating current (BMAC) (1-kHz alternating current, burst modulated at a frequency of 50 Hz with a 20% duty cycle) to establish whether there were differences in hypoalgesic efficacy as assessed by cold pain threshold measurements. Subjects: Twenty-two young adults who were healthy and drawn from a population of students of La Trobe University volunteered to participate in the study. Nineteen subjects (7 male, 12 female) met the inclusion criteria. Method: Each participant experienced monophasic PC and BMAC. Time to cold pain threshold was measured before, during, and after the electrical stimulation intervention. Results: Both PC and BMAC currents were effective at elevating the cold pain threshold, although there was no statistically significant difference between the 2 currents during stimulation. Discussion and Conclusion: Pulsed current and BMAC appear to be equally effective at elevating the cold pain threshold. Analysis indicated that if any real difference exists, it would only be apparent with large (100) subject numbers. Any differences in hypoalgesic efficacy thus are not likely to be clinically important.

Subaqueous ultrasound: 45kHz and 1MHz machines compared

This study compared the tissue heating produced by 45kHz and 1MHz ultrasound machines used subaqueously in metal and in plastic basins. An intervention study with cross-over design was used. The setting was a laboratory. Two cross-sections of nonliving pig tissue were used with thermocouples positioned, in the tissues, at different distances along the ultrasound beam axis. Specimens were each exposed subaqueously to ultrasound produced by a 45kHz machine and a 1MHz machine. Both metal and plastic treatment basins were used (2 machines x 2 basin types). Thermocouples measured temperature variations for 30 minutes with each machine on and 10 minutes with it off. Temperature increases at different tissue depths varied with both the ultrasound machine and the type of basin used. The 45kHz frequency ultrasound machine produced a maximum temperature increase of 0.4 degrees C and an initial rate of heating of 0.05 degrees C per minute. The 1MHz machine consistently produced greater temperature increases. These were further increased by 2.65 degrees C (average) if a metal rather than plastic basin was used. The 45kHz machine, operated at maximum output, produced little heating of the irradiated tissue. By contrast, the 1MHz ultrasound unit produced appreciable heating to almost the full depth of the tissue irradiated. Use of a metal basin, rather than plastic, resulted in a markedly higher maximum temperature increase and a significantly greater initial rate of heating.

Thymosin β4 administration enhances fracture healing in mice

Thymosin β4 (Tβ4) is a regenerative peptide that we hypothesized would promote healing of fractured bone. Mice received a bilateral fibular osteotomy and were given i.p. injections of either Tβ4 (6 mg/kg) or saline. Calluses from saline‐ and Tβ4‐treated mice were analyzed for: (1) biomechanical properties and (2) composition using micro‐computed tomography (µCT) and histomorphometry. Biomechanical analysis showed that Tβ4‐treated calluses had a 41% increase in peak force to failure (p < 0.01) and were approximately 25% stiffer (p < 0.05) than saline‐treated controls. µCT analysis at 21 days post‐fracture showed that the fractional volume of new mineralized tissue and new highly mineralized tissue were respectively 18% and 26% greater in calluses from Tβ4‐treated mice compared to controls (p < 0.01; p < 0.05, respectively). Histomorphometry complemented the µCT data; at 21 days post‐fracture, Tβ4‐treated calluses were almost 23% smaller (p < 0.05), had nearly 47% less old cortical bone (p < 0.05) and had a 31% increase in new trabecular bone area/total callus area fraction compared with controls (p < 0.05). Our finding of enhanced biomechanical properties of fractures in mice treated with Tβ4 provides novel evidence of the therapeutic potential of this peptide for treating bone fractures.