Coordination of a task such as plantar flexion might seem simple, but is in reality achieved
through the activation of several muscles—muscles that play an important role in locomotion
and posture control. These can be divided in to two groups, superficial and deep plantar flexors.
It has been shown that although people usually produce certain well learned movements very
similarly, some variability in the coordination might occur. This is also the case with the ankle
plantar flexors as shown by Finni et al. (2006) and Masood et al (2014). Purpose of the study
was to examine whether some individuals use their deeper plantar flexors to a greater extent
than others and to see whether the relative contribution of the deeper muscles is affected by the
type of movement or torque level. The two methods of measuring muscle activity, EMG and
VE-PC MRI, were also compared.
Fifteen subjects took part in two measurement sessions where the activity of the plantar flexor
muscles was meas
ured. The first session took place in a biomechanics laboratory where EMG
data from gastrocnemius medialis, soleus and flexor hallucis longus was recorded. The other
session was done in the MRI scanner and produced muscle tissue velocity data for the same
three muscles. Subjects did both isometric and concentric plantar flexion tasks at torque levels
of 10% MVC and 30% MVC.
The relative contribution of the deeper muscles was assessed by calculating velocity ratios between
GM and FHL as well as Sol and FHL. Clear differences were found between individuals.
The lowest ratios, indicating greater activity of deep muscles were 1.02 for GM/FHL and 1.09
for Sol/FHL, whereas, the highest ratios were 2.08 and 2.44 respectively. At 10% torque level
the two different movement types differed slightly. For GM/FHL velocity ratio was 0.41
(p<0.05), and for Sol/FHL 0.38 (p=0.68) smaller in isometric task compared to concentric.
Torque level had no effect on the relative contribution of the deeper muscles. For GM (r=0.52,
p<0.001) and FHL (r=0.59, p<0.001) there was a correlation between the EMG and peak mean
negative velocity. The correlation between peak mean negative velocity and torque, and EMG
and torque were also similar to each other.
Differences in velocity ratios show that individuals use their deeper plantar flexors to different
extent. Some of the subjects showed little to no movement in the deeper muscles whereas others
had equal peak mean velocities in the deeper muscles. With the conflicting results no clear
conclusion can be drawn on whether torque or type of movement affect the relative contribution
of the deeper muscles. The two measurement methods yielded similar results and there was also
a correlation between peak mean velocity and EMG for GM and FHL.
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