The role of the neuromuscular control mechanism in motor output : do individuals share muscle activation features?

Although walking is a constrained movement, there is variation in the muscle recruitment
patterns. The processed wavelet-based electromyogram (EMG) [von Tscharner (2000)] signals
indicated that the task structure contained in walking doesn’t prescribe one single muscle
activation strategy, and that multiple configurations of muscle activation can result in
functionally equivalent postural control. By applying a principal component analysis approach,
the intra-muscular analysis of EMG signals of healthy females while walking has shown that any
activation pattern is a combination of (i) the group activation pattern used by all individuals, (ii)
two common strategies – either a pre impact muscle activation or a post heel strike reaction –
showing the subject-specific deviations and (iii) an unresolved, more random activation pattern.
Suggestions that individuals have a constrained flexibility in how they balance their pre and post
heel strike muscle activation.
Especially at heel strike, where the human locomotor system is affected by irregular impact
forces, controlled muscle activation strategies are essential for counteracting the destabilising
forces and thus for keeping the joint stable [Hurley (1999)]. The precisely timed co-activation of
Mm. quadriceps femoris and M. semitendinosus is important for a mobile, but stable knee joint.
A balanced activation of Mm. vastus medialis and vastus lateralis is furthermore required to
control the translation of the patella [Mellor and Hodges (2005)], and to maintain the dynamic
stability of the patellofemoral joint. In addition to a high intra-muscular coordination found
while walking, a significant inter-muscular interplay between muscles of the same muscle group
and between muscles either located on the medial or on the lateral side of the knee has been
found. Such an interaction showed that, in addition, to the within-muscle and within-muscle
group controlling, there is a mechanism regulating the knee rotation and the fine tuning of
muscles in order to control subtle changes in the interplay of structures surrounding the knee
joint due to destabilising forces to gain a stable knee joint.
The mechanism underlying neural control is complex, wherein typical temporal characteristics,
such as rhythm at about 40 ms within the EMG signal or triggering to heel strike of peak
activations while walking were structures occurring independent of the subject, muscle, task
and condition. In view of the fact that individuals share these features, there seems to be good
reasons that temporal features play an integral role within the neuromuscular control mechanism.
Furthermore, the comparison between the EMG frequency spectra of sprint- and endurance
trained athletes indicate that the EMG frequency spectra change systematically with training;
thus the EMG signals mirror the functional state of a muscle as a component of the state of the
individual and allow monitoring of training-related changes in muscles.
The results of this thesis may be suggest that rhythm, synchronicity and neuromuscular
activation strategies are brain-related features, which are quantifiable by combining waveletbased
EMG signals with pattern recognition approaches. This approach provides a deeper
insight into the muscular behaviour and the role of the neuromuscular control mechanisms in
motor output, in general and between individuals. Further, it has increased the knowledge about
the functional state of muscles that stabilise the knee joint. The various combinations in
interactions of activation patterns of the thigh muscles surrounding the knee joint may be key
functions to securing a stable knee joint before the occurrence of stressful events and may be a
neuromuscular component of dynamic joint stability adapted to the subject-specific circumstances.