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Recruitment of the plantar intrinsic foot muscles with increasing postural demand

Discussion in 'Biomechanics, Sports and Foot orthoses' started by NewsBot, Aug 26, 2011.

  1. NewsBot

    NewsBot The Admin that posts the news.

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    Recruitment of the plantar intrinsic foot muscles with increasing postural demand.
    Kelly LA, Kuitunen S, Racinais S, Cresswell AG.
    Clin Biomech (Bristol, Avon). 2011 Aug 22. [Epub ahead of print]
     
  2. Gray and Basmajian already told us pretty much that in 1968: Electromyography and cinematography of leg and foot (“normal” and flat) during walking. The Anatomical Record Volume 161, Issue 1, pages 1–15, May 1968
    Abstract
    Electromyography with fine-wire electrodes and special equipment for synchronized motion pictures were used to study six muscles of the leg and foot during walking in five different ways in ten “normal” and ten flatfooted subjects. Detailed analyses and comparisons of the two groups are described and discussed.

    Tibialis Anterior has two peaks of activity at heel-strike and toe-off of the stance phase; is inactive during mid-swing and middle of the stance phase; is active at full-foot in flatfooted subjects, and generally more active during toe-out and toe-in walking. Tibialis posterior is inactive through the swing phase. In flatfooted persons it becomes activated at heel-strike and more active at full-foot during level walking. The toe-out position reduces its activity. Flexor hallucis longus is most active in mid-stance; during toe-out walking, activity increases in both phases, generally being more active in “normal” persons. Peroneus longus is most active at mid-stance and heel-off and generally more active in flatfooted persons. Abductor hallucis and Flexor digitorum brevis are generally more active in flatfooted persons. An important regular pattern of inversion and eversion during the walking cycle is described. Contingent arch support by muscles rather than continuous support is the rule, muscles being recruited to compensate for lax ligaments and special stresses during the walking cycle.
     
  3. pod29

    pod29 Active Member

    Some of our findings are a confirmation of what Gray and Basmajian found many years ago, however the the main focus of the paper relates to posture rather than gait. Regardless, it's certainly beneficial to repeat and replicate some of these early EMG studies, given the limitations in EMG recordings that were reported at the time.

    Happy to send a PDF of the article if your interested in reading it in full, in order to make further comments,

    Regards
     
  4. Griff

    Griff Moderator

  5. pod29

    pod29 Active Member

    Hi Ian

    Check you inbox

    Cheers:drinks
     
  6. I should take pleasure in reading your paper in its entirety. If you could send a copy to skspooner@blueyonder.co.uk I should be happy to make further comments.

    In the meantime, perhaps a self-critique of the methodology employed could prove helpful? You are right though, despite the "limitations in EMG recording" which Gray and Basmajian must have been up against in the late 1960's, their work which collected data during gait is really not comparable to your paper in which the subjects stood upon one leg. How has EMG changed since the late 1960's?
     
  7. Luke:

    I would love a copy also. I'll try to read it this weekend and offer comments.

    Good job on getting your paper published.:drinks

    kevinakirby@comcast.net
     
  8. Luke, I think it might be helpful for those of us who are not using EMG regularly these days if you could give us all a brief overview on how it works. For instance, when you place the electrode within the muscle, how do you read the signal to differentiate between concentric, eccentric or isometric muscle contraction? Or measure the force developed by the contraction? How do you get around the old problem of the needle electrode only picking up the signal from the muscle fibres directly adjacent to it? etc.

    I'm sure when I've read your paper, there will be more questions than answers.
     
  9. pod29

    pod29 Active Member

    Hi Simon and Kevin

    Thanks for your interest in the publication. I've forwarded on the PDF for your reading.

    Simon, in the mean time I have tried to answer your questions....

    Luke, I think it might be helpful for those of us who are not using EMG regularly these days if you could give us all a brief overview on how it works.

    For anyone who would like more in depth information about the principles of Electromyography, I suggest reading Merletti and Parker - Electromyography: physiology, engineering, and noninvasive applications. I’m certainly not an engineer like these guys will give my Podiatrist understanding your questions.


    For instance, when you place the electrode within the muscle, how do you read the signal to differentiate between concentric, eccentric or isometric muscle contraction?

    Why would you want to read the signal to determine whether a contraction is eccentric, concentric or isometric? If you want to determine whether a dynamic contraction is eccentric, concentric and isometric, there are now some really neat (published and validated) methods using tracking of muscle fibre length using ultrasound in a dynamic situation. I’m happy to forward the links to these if you would like to read them. There are certainly instances in EMG studies where it is important to maintain an isometric contraction, ie when trying to determine a relationship between force output and muscle activity (fatigue or single motor unit studies etc).

    Or measure the force developed by the contraction?
    Force of contraction obviously can’t be determined from the EMG signal itself, as EMG signal amplitude is effected by many other factors (distance of electrode from motor point, skin properties, sweat etc). However in controlled isometric contractions you can relate EMG signal qualities (RMS amplitude, motor unit recruitment and rate coding etc) to the isometric force output of that contraction.

    How do you get around the old problem of the needle electrode only picking up the signal from the muscle fibres directly adjacent to it? etc.

    We actually used bi-polar fine wire electrodes for this study. I don’t think it would be too pleasant to have a concentric needle electrode in you foot while standing ;-). However, crosstalk from nearby muscles can certainly be a problem in fine wire EMG studies too, but more so when using surface EMG. The risk of cross-talk from nearby muscles can be reduced by altering the size of the recording area (area of fine wire with insulation removed) on the fine wire electrode. By reducing the size of the area you will pick up a smaller amount of motor units recorded from a motor unit pool that is closer to the electrode. The other advantage that we have now for use with our EMG studies is being able to use real time ultrasound to identify the muscle of interest and insure that the electrode is placed in this muscle and not in nearby muscles. This is a very important feature that we have now, that the pioneers in EMG like Basmajian were not able to use. Personally, experience tells me that that it would be near impossible to accurately record EMG from these muscles without u/s, even if you did write the anatomy text! The angle and depth that you need to extend the EMG delivery needle to get access these muscles is so variable in different subjects (due to different arch morphology), therefore there is no real cookbook recipe to insert the wires in each muscle, like you may have for larger muscles like soleus.
     
  10. Hi Luke nice one re the paper.

    I wonder how much of this change in activity is in direct response to tension in the Gastroc/sol Complex ?

    ie as the tension of the Gastroc/sol complex increases the activity levels of the plantar intrinsics.

    kind of wondering out loud
     
  11. pod29

    pod29 Active Member

    Hi Mike

    This is a really good question. Theoretically, increased tension in the gastroc soleus complex ( ie during anterior sway) may lead to increased dorsiflexion forces through the midfoot and thus an increased need for the intrinsics to provide active support to the MLA. Our results found no correlation though between antero-posterior sway and activation in any of the three muscles we recorded.

    BUT, it is also possible increased tension in the Gastroc-soleus complex may lead to a medial deviation in Centre of Pressure via an increase in eversion moment. This could potentially be related to our finding that increased activity in these muscles is associated with medial COP deviations.

    Hopefully this makes some sense???
     
  12. Luke did not expect that, tis a tangled web!
     
  13. Luke:

    Good paper...thanks for sending it my way. A few questions are in order.

    First of all, I was very impressed with your ability to do EMG on the plantar intrinsics, flexor digitorum brevis (FDB), quadratus plantae (QP) and abductor hallucis (AH) muscles. There have been very few good studies on the EMG activity of these muscles within the worldwide medical/scientific literature and it appears in reading your paper you were able to get it right. One question is how painful was this for the subjects? Did they feel the electrodes within their feet while doing the experimental protocols? Was there any pain or post-experimental infections?

    Second, if you have the papers from Basmajian on pdf, please send them my way since I would like to put them up, along with your paper, on my website where others can download them.

    Third, it would be hard to judge anything more than the difference between single and double leg standing from your study as to the EMG activity of a few of the medial plantar intrinsic muscles. Was there a reason why you did not study walking gait, where there would likely be more EMG activity than even single leg standing? The EMG activity of the plantar intrinsics during walking activity would be a landmark study for the medical literature....I hope you have plans since this type of study is sorely needed within the medical literature.

    Fourth, I was a tad bit disappointed that you only mentioned in your paper about the plantar intrinsics muscle function to help maintain balance in the foot. I would have mentioned that they also help support the longitidinal arches (i.e. stiffen the medial and lateral longitudinal arches), reduce the tensile force on the plantar fascia and plantar ligaments, and may help decelerate medial arch flattening in early stance and may help accelerate medial arch raising in late stance. Of course, we will need more evidence to prove these plantar intrinsic muscle functions. However, these functions of the plantar intrinsics certainly seems quite likely considering their anatomical location within the plantar foot and stance phase EMG activity.

    Great work!!
     
  14. pod29

    pod29 Active Member

    Hi Kevin

    Luke:

    Good paper...thanks for sending it my way. A few questions are in order.

    Thanks Kevin, your input, as always is greatly appreciated

    First of all, I was very impressed with your ability to do EMG on the plantar intrinsics, flexor digitorum brevis (FDB), quadratus plantae (QP) and abductor hallucis (AH) muscles. There have been very few good studies on the EMG activity of these muscles within the worldwide medical/scientific literature and it appears in reading your paper you were able to get it right.

    It wasn’t an easy process, about 12 months of pilots with some help from an advisor who has vast experience in fine wire EMG.

    One question is how painful was this for the subjects? Did they feel the electrodes within their feet while doing the experimental protocols? Was there any pain or post-experimental infections?

    Kevin, generally most needle insertions are relatively painless, a 25 gauge delivery needle isn’t too bad. Maybe I’m a little desensitized after practicing on myself countless times, but I think by the time we collected data we were pretty good at it The electrodes that remain in the muscle belly are very small in diameter and are quite flexible. They are slightly larger in size than a hair on your leg. The level of discomfort experienced was minimal and most people reported not really noticing them at all. There was one subject where we accidentally touched on a nerve (Sorry CraigT!!!), he had some pain at the time and a little discomfort for about a week after. No infections or post testing complications and all of the subjects have volunteered for our upcoming project (still working on CraigT though ;-) ).

    Second, if you have the papers from Basmajian on pdf, please send them my way since I would like to put them up, along with your paper, on my website where others can download them.

    I’ll send them across, it’s quite incredible what they did given the technology at the time

    Third, it would be hard to judge anything more than the difference between single and double leg standing from your study as to the EMG activity of a few of the medial plantar intrinsic muscles. Was there a reason why you did not study walking gait, where there would likely be more EMG activity than even single leg standing? The EMG activity of the plantar intrinsics during walking activity would be a landmark study for the medical literature....I hope you have plans since this type of study is sorely needed within the medical literature.

    You make a very valid point here and that is why the focus of the study is related to posture and balance (which is quite a large research field in itself). We needed to keep the protocol for this study quite simple, in order to prove to ourselves (and reviewers) that it is possible to accurately record the activity of these muscles. We could possibly have reported data from a few subjects during walking. However we felt that we should assess that as a separate study, where we could “do it properly” and incorporate motion analysis and synchronise all our gait events effectively. Also, using a tethered (cabled) EMG system during walking and running to record fine wire EMG can be problematic due to motion artifact caused by cables etc. The good news is that we now have a totally wireless EMG system and piloting is in process to conduct the next experiment in the very near future.

    Fourth, I was a tad bit disappointed that you only mentioned in your paper about the plantar intrinsics muscle function to help maintain balance in the foot. I would have mentioned that they also help support the longitidinal arches (i.e. stiffen the medial and lateral longitudinal arches), reduce the tensile force on the plantar fascia and plantar ligaments, and may help decelerate medial arch flattening in early stance and may help accelerate medial arch raising in late stance. Of course, we will need more evidence to prove these plantar intrinsic muscle functions. However, these functions of the plantar intrinsics certainly seems quite likely considering their anatomical location within the plantar foot and stance phase EMG activity.

    I agree with all of your above statements and hopefully we can discuss those in depth in our next couple of papers. But, given that the focus of this study is related to balance and posture, we really couldn’t attempt to extend too much further in our interpretations. But, it certainly leaves some discussions for the future.

    Great work!!

    Thanks for your constructive review, it really is appreciated!
     
  15. Luke, thank you for sending me your paper.

    Can you explain for me and anyone else with limited knowledge of this subject how you know that the signal intensity is related to the number of motor units being recruited? Moreover, how we can be sure that the signal intensity is a fair reflection of the overall recruitment pattern within the muscle and not just of the fibres which are adjacent to the electrodes?

    The reasons I asked my questions regarding the force of contraction and type of contraction were related to the different effects these type of contraction might have on power generation and power absorption. Does the type and force of contraction have any implications to the centre of pressure variable and the interpretation of the muscle function provided within your paper?

    What exactly were the variables within the correlational models? You say there was no correlation between anterior-posterior displacement of the centre of pressure and the signal intensity, did you attempt to fit any curvilinear models or was this limited to linear model fitting? Were any scale transforms attempted to improve the linear fit?

    Nice work, and like Kevin I think this is an important study. I look forward to reading your dynamic studies, with and without orthoses in situ, please.

    For those that may be interested here is the seminal paper by Mann and Inman from 1964.
     

    Attached Files:

  16. pod29

    pod29 Active Member

    Luke, thank you for sending me your paper.
    Thanks for taking the time to read and evaluate it!The response is a little long, hopefully you and anyone else reading can stay awake until the end:morning:

    Can you explain for me and anyone else with limited knowledge of this subject how you know that the signal intensity is related to the number of motor units being recruited?
    This is a good question. I’m not quite sure that signal intensity is the right phrase to use, as intensity of EMG signal is usually related to the frequency components of the signal, which we haven’t evaluated in this study. Signal amplitude is probably a reflection of what we have discussed, and I think this is what you are referring to.

    In simplified terms, an EMG signal is the summation of all the motor unit action potentials (MUAP’s) that are recorded by the electrodes. A motor unit consists of a motoneurone and all the muscle fibres that it innervates. The amplitude of the signal will be determined by the number of motor units recruited and also to a lesser extent, the rate at which the motor units are being fired (rate coding). So generally a greater strength of contraction will be caused by a larger number of active motor units, firing at a faster speed. Therefore, a larger signal amplitude is recorded. There are of course some other factors external to the muscle that may effect the amplitude, but we don’t really need to go into those.

    In surface EMG there is usually a very large amount of MUAP's, and therefore the signal amplitude is usually quite large. But, when recording from small muscles there is a significant risk that some of these MUAP’s are actually from different muslces. When using fine wire EMG it is possible to manipulate the size of the recording area on the electrode to reduce the number of motor units detected, thus reducing the risk of crosstalk from the adjacent musclesThis is an advantage in small muscles.


    Moreover, how we can be sure that the signal intensity is a fair reflection of the overall recruitment pattern within the muscle and not just of the fibres which are adjacent to the electrodes?
    This is also a fair question. In a larger muscle you may also record a channel of surface EMG for comparison, just to ensure that you are recording an accurate reflection of the motor unit population. Larger muscles can comprise of hundreds and hundreds of motor units. Even the small muscles in the hand (dorsal interossei) have up to 300 motor units. Unlike the hand muscles or larger muscles of the leg, the plantar intrinsic muscles appear to have a relatively small number of motor units, on average about 40( range of 20 -120) per muscle. See Johns and Fuglevand, Muscle & Nerve 2011 for more detail. Given that muscle fibres innervated by a single moto-neurone are thought to be dispersed through different regions of a muscle and not in one segmented location (and these muscles are quite small), I think we can be pretty sure that our signal was a good reflection of the entire motor unit population for these muscles. If a signal is too selective (ie not enough motor units) you won’t really have any significant fluctuations in signal amplitude due to the small number of motor units recorded.

    The reasons I asked my questions regarding the force of contraction and type of contraction were related to the different effects these type of contraction might have on power generation and power absorption.
    We can’t really determine the type of contraction in these muscles during standing or walking, unless we can devise a special EMG probe to sit under the foot without obstructing the subject. It can (and has been done) for the gastroc and soleus (see the work of Glenn Lichtwark or Neil Cronin), but as yet is too difficult for the feet. I had similar thoughts in wanting to know whether these muscles are lengthening or shortening at different stages of the gait cycle. We could also determine this with bone pins if you feel like volunteering???

    As for power absorption and generation... Good question, I’m not sure of the answer though.....


    Does the type and force of contraction have any implications to the centre of pressure variable and the interpretation of the muscle function provided within your paper?
    The other interesting factor here arising from our findings, that has also been widely reported in the Gastroc-Soleus, is that activation of these muscles consistently occurs before the peaks in COP displacement. Part of this may be due to electromechanical delay, but it has also been considered that this occurs as a feed forward response to maintain a stable “inverted pendulum” in stance. Ie the muscles are activated centrally in a pro-active manner, rather than a re-active manner. This whole discussion gets a little off topic, but is nevertheless quite interesting (see papers by Ian Loram).

    What exactly were the variables within the correlational models? We cross correlated the EMG channels with each other, as well as cross correlating each EMG channel with that of both COP directions (AP and ML). All signals were filtered, additionally the EMG channels were down-sampled to the same number of points the force signal (which also acts to smooth the signal) in order to allow for the analysis

    You say there was no correlation between anterior-posterior displacement of the centre of pressure and the signal intensity, did you attempt to fit any curvilinear models or was this limited to linear model fitting? Were any scale transforms attempted to improve the linear fit?
    The cross-correlation function applied here is not actually a linear analysis, but is applied commonly in Balance and Posture studies (as well as numerous other engineering applications). Instead of using one variable to directly account for changes in another variable. This type of analysis helps to compare spatial and temporal similarities between the 2 time varying signals. See a review by Nelson-Wong in JOSPT April 2009 for more in depth information. I’m definitely not a mathematician or engineer, so the actual mathematical modeling of this is way over my head:dizzy:

    Nice work, and like Kevin I think this is an important study. I look forward to reading your dynamic studies, with and without orthoses in situ, please.Thanks for the comments, stay posted for the next couple of studies!
     
  17. Griff

    Griff Moderator

    Luke,

    Thanks for your paper. Also thanks for the advanced replies to Simon and Kevin above - for someone like me with very limited knowledge on EMG it's been great to read the discussion. Could I get a copy of the Basmajian paper too if it's not too much trouble please?

    Any plans for dynamic EMG studies of the plantar intrinsics barefoot Vs stability/motion control road running shoes? Some data on that would be super...
     
  18. Athol Thomson

    Athol Thomson Active Member

    G'day Luke,

    Hope you are well mate?

    Well done on the paper!

    Would you mind sending me a copy?

    info@multisportpodiatry.com

    Thanks in advance,
    Athol
     
  19. Definitely, the electromechanical delay can be quite significant (28-41 msec in elbow muscles) [http://www.ncbi.nlm.nih.gov/pubmed/506761] especially when you consider faster moving activities such as running or sprinting where the time spent on the ground for the foot is less than 200 msec. This is one of the many reasons why you can't look at EMG activity and reliably predict what the contractile force is within that muscle at that instant in time. The central nervous system may have started a burst of efferent electrical activity to the gastroc-soleus but, 15 msecs later, the gastrocnemius-soleus muscle is still in a relaxed, non-contractile state.

    In addition, the center of mass (CoM) displacement is normally relatively slow in balance activities so that the central nervous system (CNS) has plenty of time to send increased efferent signals to the gastroc-soleus complex to shift the CoP forward when anterior CoM movement is detected by the CNS. This is not the case in running or in faster walking where both CoM and CoP velocities are increased relative to the plantar foot so that the CNS must "plan ahead" for foot contact at the initiation of stance phase of gait by preactivating the muscles necessary to allow optimum foot contact kinematics. We see this preactivation of the gastroc-soleus especially occurring in the barefoot runners where they are changing their running kinematics to avoid heel contact.

    I don't know if I agree with your statement "the muscles are activated centrally in a pro-active manner, rather than a re-active manner". This is because, during balance, the CNS is reacting to changes in CoM movement by shifting the CoP either anteriorly, posteriorly, medially or laterally in order to "push" the CoM back toward its "safe position" where balance can best be maintained. David Winters has the best material on this and I have written about in one of my books from over a decade ago (Kirby KA: Foot and Lower Extremity Biomechanics II: Precision Intricast Newsletters, 1997-2002. Precision Intricast, Inc., Payson, AZ, 2002, pp. 133-136). If you said that the CNS reacts to CoM/CoP changes but also may be proactive in anticipating expected CoM/CoP changes, then I would agree.

    My guess is the reason that you did not see any correlation between anterior-posterior displacement and signal intensity is because CoP motions were too slow and of too low of a magnitude in standing balance activities. I would imagine that once you increase the velocity of motion and increase the magnitude of ground reaction force, you will see some correlation between anterior-posterior displacement and EMG signal intensity in the plantar intrinsics.

    Excellent discussion!:drinks
     
  20. pod29

    pod29 Active Member

    Definitely, the electromechanical delay can be quite significant (28-41 msec in elbow muscles) [http://www.ncbi.nlm.nih.gov/pubmed/506761] especially when you consider faster moving activities such as running or sprinting where the time spent on the ground for the foot is less than 200 msec. This is one of the many reasons why you can't look at EMG activity and reliably predict what the contractile force is within that muscle at that instant in time. The central nervous system may have started a burst of efferent electrical activity to the gastroc-soleus but, 15 msecs later, the gastrocnemius-soleus muscle is still in a relaxed, non-contractile state.

    Previous research looking at time difference in EMG peaks and COP excusrion peaks in postural tasks has reported anywhere between 50msec and 100msec, our results, from memory were somewhere in the middle of that. I think there is certainly an element of electromechanical delay, but there does remain a significant portion of time in advance that could potentially be explained by a "feedforward"control mechanism.

    If you said that the CNS reacts to CoM/CoP changes but also may be proactive in anticipating expected CoM/CoP changes, then I would agree.

    I think you're description probably sums situation with much more clarity than mine, thanks! If you are interested in this topic, I really suggest reading some of Ian Loram's papers in the Journal of physiology from the last couple of years. The references are quoted in our manuscript. He has really extended on from some of the earlier work of Winter.

    My guess is the reason that you did not see any correlation between anterior-posterior displacement and signal intensity is because CoP motions were too slow and of too low of a magnitude in standing balance activities. I would imagine that once you increase the velocity of motion and increase the magnitude of ground reaction force, you will see some correlation between anterior-posterior displacement and EMG signal intensity in the plantar intrinsics.

    Quite possibly, would be interesting to find that out

    :good: Thanks for the input!

    Cheers
     
  21. I am very interested. Please send the papers by Basmajian and Loram my way when convenient.

    kevinakirby@comcast.net
     
  22. The idea that greater magnitude of GRF increases EMG of the plantar intrinsics is supported by the previous research by Basmajian and Stecko in 1963. They saw an increase in EMG of abductor hallucis and flexor digitorum brevis with increasing loads from 100 to 400 pounds. Wouldn't you then expect that increasing the GRF on the plantar foot by fast walking or running would tend to increase EMG activity of the plantar intrinsics?

    I might add, that since you only looked at the medial longitudinal arch intrinsic muscles, then I would expect more activity in these muscles with medial CoP movement. However, it would also be interesting to see if lateral CoP movement increased the EMG activity of the abductor digiti minimi, for example.

    Just give me a couple more days, Luke......I'll fill up your research schedule for the next decade....;)
     
  23. pod29

    pod29 Active Member

    Agreed, agreed and agreed!

    P.S - will send the Loram papers across,

    Regards
     
  24. pod29

    pod29 Active Member

  25. Which comes back to the point I was making earlier and links with the delay issue. Is the centre of pressure driving the EMG activity or is the EMG driving the centre of pressure?

    If, for example we knew that the medial plantar intrinsics were working concentrically, then surely increased activity should tend to shift the centre of pressure laterally.
     
  26. EMG, by itself, can not determine whether a muscle contractile force is concentric, eccentric or isometric in nature, at least to my undestanding of the technology.
     
  27. That was my point, that we do not know the nature of the muscular activity; merely that there is activity. Take a muscle let it contract concentrically, take the same muscle let it contract isometrically, now let the muscle contract eccentrically. The EMG signal might be the same in all three conditions, yet the kinetic and kinematic response to these three contractions might be very different. Thus, in many ways, EMG is a somewhat blunt instrument when employed in isolation.
     
  28. NewsBot

    NewsBot The Admin that posts the news.

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  29. NewsBot

    NewsBot The Admin that posts the news.

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