The mechanical output of the muscle-tendon unit (MTU) is in part governed by the viscoelastic nature of tendinous tissues and the tendon contributes to beneficial contractile conditions during various types of movement. Yet, knowledge about the modulation of muscle-tendon behaviour under different external constraints and with different tendon stiffness is scarce. Moreover, the role of human tendons has mainly been examined in the context of energy conservation or power amplification and has not been investigated during tasks requiring energy dissipation.
Place: Norges Idrettshøgskole
The role of Achilles tendon properties in the mechanical function of the human triceps surae
The aim of the present thesis was to examine muscle-tendon interaction: during a landing task; under external constraints (i.e. loading and speed) during landing and running; and after a training-induced increase in tendon stiffness during landing and running.
We conducted a cross-sectional study (A) and a controlled longitudinal study (B), to investigate muscle-tendon interaction during a landing task (n=39 and n=21, respectively) and during running on a treadmill (n=16 and n=21, respectively). In study A, ankle and knee joint mechanics and gastrocnemius and soleus MTU length were derived from kinematic and kinetic data during both tasks. In addition, we synchronised ultrasound with these data to measure Achilles tendon and muscle fascicle length. Loading (+20% of body mass) was added to the subjects using weighted vests after completing the tasks unloaded. Running was performed at preferred speed and increased speed (+20%). In study B, the training group (n = 11) underwent 10 weeks of resistance training consisting of single-leg isometric plantar flexion contractions. Similar measurements as in study A were performed during unloaded landing and unloaded running at preferred speed, additionally including the assessment of plantarflexion force, tendon stiffness and strain, measured during isometric contractions. Repeated-measures ANOVAs were used to test differences in the variables of interest between conditions of task execution (study A) and between baseline and post-training tests (study B).
The results showed that during landing, the rapid MTU stretch was buffered by the Achilles tendon to delimit muscle fascicle strains. Additional loading was taken up by the tendon and was met by increased soleus muscle activity, effectively preserving the contractile length and velocity of muscle fascicles. Similarly, fascicle behaviour was preserved during running with load and at increased speed. We observed different strategies to increase ankle joint work under the two constraints; higher elastic energy utilisation was favoured with added load, whereas increasing speed resulted in higher muscle activity. The training-induced increase in Achilles tendon stiffness also altered muscle-tendon behaviour during landing and running. During landing, the buffering action of the tendon was not limited despite similar forces and increased tendon stiffness. Mechanisms for the reduction in gastrocnemius lengthening and greater length at touch-down during landing remain to be determined. During running, gastrocnemius and soleus fascicle shortening patterns were preserved, despite a reduced tendon recoil.