YIA Winner presentation at ECSS MetropolisRuhr 2017

Muscle-tendon length and force affect human tibialis anterior aponeurosis stiffness in vivo
The University of Queensland

MUSCLE-TENDON LENGTH AND FORCE AFFECT HUMAN TIBIALIS ANTERIOR CENTRAL APONEUROSIS
STIFFNESS IN VIVO
Raiteri, B., Cresswell, A.G., Lichtwark, G.A.

Introduction
In pennate muscle, the elastic sheet-like aponeurosis is important for transmitting force from its muscle fibres to the free tendon.
However, unlike the free tendon, the aponeurosis undergoes a complex three-dimensional loading regime during contraction, which can alter its stiffness in passive versus active muscle (Azizi et al., 2009). If additional factors can also drive changes in aponeurosis stiffness during a contraction (which remains to be tested), then this would have implications for how we model muscle and tendon interactions; and may also serve to optimise muscle efficiency during everyday tasks such as walking. Therefore, the aim of this study was to determine if factors other than muscle activation could modulate the apparent in vivo longitudinal stiffness of the human tibialis anterior (TA) central aponeurosis.

Methods
We employed two ultrasound techniques to examine in vivo fascicle behaviour and central aponeurosis stiffness of the human TA during force-matched, voluntary isometric dorsiflexion contractions at three muscle-tendon unit (MTU) lengths (n=11). Using elastography and the measured muscle shear modulus, we determined an index of muscle force (Hug et al., 2015), which could be used to validate if our TA muscle force estimates across MTU lengths were similar.

Results
Mean TA muscle shear modulus values were very similar for the three MTU lengths at the low (mean range; 55 kPa) and moderate
(mean range: 97-99 kPa) muscle forces. TA MTU length increased both the length and apparent longitudinal stiffness of its central
aponeurosis at the low (short: 30.8±16.2 N/mm; medium: 47.2±26.5 N/mm; long: 69.0±28.8 N/mm) and moderate (short: 44.1±15.5 N/mm; medium: 66.3±28.0 N/mm; long: 104.7±41.8 N/mm) muscle forces (P less than 0.01). The above were associated with reduced magnitudes of TA fascicle shortening at both forces as MTU length increased (P less than 0.01).

Discussion
This study provides the first in vivo evidence that MTU length and muscle force are able to modulate the apparent in vivo longitudinal
stiffness of the human TA. We found that the central aponeurosis was stretched less from a longer starting length for the low and
moderate force-matched conditions. The rightward shift and increase in slope of the aponeurosis force-length curves with increasing MTU length was likely due to a combination of parallel lengthening of the aponeurosis under passive conditions and altered transverse aponeurosis strains. Further research on muscles that produce power during walking and running is warranted to examine the influence of variable aponeurosis stiffness on muscle efficiency.

References
Azizi E., Roberts, TJ. (2009). J Physiol, 587(17), 4309-4318.
Hug, F., et al. (2015). Exerc Sport Sci Rev, 43(3), 125-133.

Contact
brent.raiteri@rub.d

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