View Full Version : quantifying co-contraction during gait

unknown user
11-26-2007, 03:09 AM
Dear colleagues,

I am conducting a study in which I want to quantify agonist-antagonist
muscle co-contraction during gait. In this study, subjects walked at
several different slopes (including level walking). For each trial I
collected VO2 and EMG from eight muscles of one leg (Tibialis Anterior,
Medial Gastrocnemius, Lateral Gastrocnemius., Soleus, Vastus Medialis.,
Vastus Lateralis, Biceps Femoris, Gluteus Maximus) using a Noraxon Telemyo
system (sampling rate = 1000 Hz). I also collected foot switch data (both
feet). Prior to the walking trials, I collected EMG during quiet standing.
I am interested in determining if the reduced delta mechanical efficiency
observed in several of the subjects is related to an increase in
antagonist muscle co-contraction.

I would like to know what the best method for analyzing the EMG data is.

Here are some proposed steps for the initial EMG processing prior to
co-contraction analysis:
1) Filter EMG data (band pass: 10- 500 Hz; 2nd or 4th order Butterworth)

2) Rectify EMG data

3) Filter to get linear envelope (low pass: 6 Hz)

4) Normalize amplitude to mean integrated EMG of same muscle during level
walking trial (do not have MVC data)

5) Normalize time to step time using footswitch data (0% = heels strike,
100% = ipsilateral heel strike)

6) Determine when each muscle is “OFF”
• Visually inspect and subjectively determine?
• Or,…Use prior research that gives the time (percentage of gait cycle)
that the muscle should not be activated?
• Or, …should I use some other method?

7) Determine when each muscle is “ON”
• ON = When muscle activation is 3 standard deviations above the mean of
when muscle is “OFF”
• Or,…. Some other methods?

Steps to determine co-contraction (based on Falconer & Winter, 1985)
1) Determine what muscle is acting as the antagonist
• Should I do this for each portion of step (weight acceptance, single
support, push off, swing) or for the whole step cycle?
• The agonist will be determined as the muscle that has the smaller
integrated EMG envelope
• Or,…The agonist will be determined as the muscle that has the smaller
peak EMG.
• Or is there some other method of determining which is the antagonist

2) Get integral of linear envelope for both the agonist and antagonist
• Should we only compare one agonist muscle to one antagonist muscle at a
time (i.e. TA compared to Soleus)?
• Or,… should we compare all the agonist muscles about a joint (ankle) to
all antagonist muscle at the joint (i.e. TA compared to Soleus + Med. Gas
+ Lat. Gas)?

3) Get Total Integral of EMG (Itot)
• Sum the two integrals from step six: Itot = Iant + Iagon

4) Calculate the co-contraction Index
• CCI (%) = (2 x Iant)/Itot x 100

I worry this step could give me some misleading results. For example, lets
say I get a integral for the antagonist (Iant) equal to 1, and an integral
for the agonist (Iagon) equal to 2. Then the total integral of the EMG
(Itot) would be equal to 2 + 1 = 3. This would then give me a
co-contraction index of (2 x 1)/3 x 100 = 66%.

Then , lets say on the second trial I get a integral for the antagonist
(Iant) equal to 2, and a integral for the agonist (Iagon) equal to 4. Then
the total integral of the EMG (Itot) would be equal to 2 + 4 = 6 Vsec.
This would then give me a co-contraction index of (2 x 2)/6 x 100 = 66%.
Although the absolute magnitude of muscle activation increased for both
muscles but the co-contraction index stays the same. Does this seem

Other studies have quantified co-contraction using the duration of agonist
and antagonist muscle activation (Lamontagne et al, 2000; Dierick, et al,
2002). Is this a better method? What are its pros and cons?

Has anyone used the Noraxon Myoresearch Master Ed to analyze
co-contraction during gait? If so, is there an analysis protocol for this
software that is most appropriate.

Thank you for any help you may have to offer.


Justus Ortega, Ph.D.
Biomechanics Lab
Department of Kinesiology
Humboldt State University
1 Harpst Street, Arcata CA 95521
(707) 826-4274