Physiotherapy July 2000/vol 86/no 7

Letters 391

MAY I applaud the authors of the articleon inhibition taping (Tobin andRobinson, 2000) on a bold attempt toexamine electromyographic behaviourunder dynamic conditions. Dynamic EMGis a challenging area to work in, mostauthors concentrating upon isometriccontractions, which are technically muchless demanding. I would however like tosuggest some areas for improvement intheir study design.

First, the use of a sampling frequency of400 Hz (presumably to convert the rawEMG to digital data, although they do not state at what stage the samplingoccurred) would seriously comprise the quality of data yielded from theirstudy. A variety of authors have describedthe minimum acceptable samplingfrequency to be 1000 Hz, ie at least twicethe highest expected input frequency(Basmajian and de Luca, 1985, page 57;Gilmore and Meyers, 1983, page 7;Merletti, 1994, page 26; Ng et al, 1996,pages 100, 102).

Secondly, they do not state anycalculation for intra-test reliability of their

readings. The number of variablesinvolved in dynamic work, such as velocity,movement artefacts, range, etc, can easilyproduce erratic data (Basmajian and DeLuca, 1985, page 198; Gilmore andMeyers, 1983, page 8). The large values ofstandard deviation shown in table 3 donot suggest consistent data. This may bedue to uncontrolled variables, with orwithout the effect of an inappropriatesampling rate. If the intra-test data are notreliable, meaningful comment aboutinter-test changes cannot be made.

The authors should be able to calculateintra-test reliability from their existingdata. If they have saved the raw(analogue) EMG data, they would also beable to re-sample at a higher rate and re-calculate their results. I would encouragethem to do this and publish the results, asI feel they have made a good attempt inother respects at tackling the problems ofdynamic EMG in a functional setting.

Martin ScottBSc MCSPNottingham

References

Basmajian, J V and De Luca, C J (1985).Muscles Alive, Williams and Wilkins,Baltimore, 5th edn.

Gilmore, K L and Meyers, J E (1983).‘Using surface electromyography inphysiotherapy research’, Australian Journalof Physiotherapy, 29, 1.

Merletti, R (1994). ‘Surfaceelectromyography: Possibilities andlimitations’, Journal of RehabilitationSciences, 7.

Ng, J K-F, Richardson, C A, Kippers, V, Parnianpour, M and Bui, B H (1996).‘Clinical applications of power spectralanalysis of electromyographicinvestigations in muscle function’, ManualTherapy, 2.

Tobin, S and Robinson, G (2000). ‘Theeffects of McConnell's vastus lateralisinhibition taping technique on vastuslateralis and vastus medialis obliquusactivity’, Physiotherapy, 86, 4, 173-183.

I WOULD like to thank the editors forgiving me the opportunity to respond tothese letters. I would also like to thank MrScott for the positive and constructive wayhe makes his criticisms.

Both letters make reference to thesampling frequency I used. The value of400 Hz was a compromise between thesampling frequency and the length oftime needed for the completion of thedynamic functional activity. I understandthe need to sample at a high frequencyand agree with the proposed idealminimum of 1000 Hz.

However, the major consideration of mystudy was the functional task, and for thetime period needed the equipment usedfor data collection had a ceiling of 400 Hz.

With respect to Mr Scott’s point aboutintra-test reliability and Mr Herrington’spoint about expressing the results interms of a percentage change in muscleactivity, I accept these as valid criticisms. Ihope that future studies in this area take

note of these limitations in my researchand will be stronger because of them.

With respect to the rest of MrHerrington’s letter, unfortunately he hasmisinterpreted and misrepresented anumber of issues. Regarding his pointabout the use of EMG signals, I am incomplete agreement with his assertionthat you cannot compare EMG signalsfrom two different muscles or from thesame muscle in two different individuals ifyou want to be able to infer anything from the results. This is why I did not do so. I believe my report makes it quite clearthat what I actually did was to comparethe EMG readings from the same musclein the same individual under different test conditions. This of course is entirelyjustifiable as ‘EMG is an appropriate toolto measure the relative intensity of muscleactivity during exercises or functionalactivities’ (Gryzlo et al, 1994).

I would also like to point out thatnowhere in my report do I state that EMG

activity corresponds to muscle force. Mysole interpretation of the changes in EMGactivity was that it represented changes inmuscle activity. Muscle force is a separateissue and one that quite obviously youwould not use EMG/millivolt values tomeasure.

Mr Herrington then goes on tointroduce the subject of MIVCs. As it iswidely accepted that VMO and VL differconsiderably in their muscle fibrecomposition and their function(Richardson, 1985; McConnell, 1995;Hodges and Richardson, 1993; Karst andJewett, 1993; Hilyard, 1990), using MIVCswould not appear to be the best tool forVMO:VL comparisons. VMO workingoptimally shows tonic activity throughoutthe entire range of knee movement (Karstand Willett, 1995). The predominance oftype I muscle fibres in VMO reflects itstonic role whereas the predominance oftype II a and b muscle fibres in VL revealsit to be a phasic muscle (McConnell,1995). It would appear that besides notbeing functional, MIVCs cannotaccommodate these significant

Room for Improvement in Study Design

Sue Tobin, first author of the article on taping, replies to Mr Herrington and Mr Scott: