Plantarflexor fiber length and tendon slack length are the strongest determinates of simulated single-leg heel raise function

Abstract

Achilles tendon ruptures lead to reduced ankle function and often limits recreational activity. Single-leg heel raises are often used clinically to characterize patient function. However, it is unclear how the structure of the Achilles tendon and plantarflexor muscles affects single-leg heel raise function. Therefore, the purpose of this study was to develop a musculoskeletal model in order to simulate the effects of muscle-tendon unit (MTU) parameters on peak plantarflexion during this clinically-relevant task. The ankle joint was plantarflexed by two MTUs that represented the soleus and gastrocnemius muscles. The optimal fiber length, maximal muscle force, muscle pennation, tendon stiffness, and resting ankle anglea surrogate measure of tendon slack length were iteratively adjusted to test the combined effects of each of these MTU parameters. Singleleg heel raises were simulated by maximally exciting the two plantarflexor MTUs for each model configuration (N = 161,051 simulations). Optimal muscle fiber and tendon slack lengths had the greatest effect on peak plantarflexion during simulated single-leg heel raises. Simulations that were unable to produce at least 30 degrees of plantarflexion had muscle fibers that were shorter than healthy muscle and longer tendon slack lengths. These findings highlight the importance of preserving muscle fascicle and tendon length following Achilles tendon injuries.

Gastrocnemius fascicles are shorter and more pennate throughout the first month following acute Achilles tendon rupture

“…Long-term functional deficits have been linked to tendon elongation (30), which occurs within the first month following the initial injury (16). Preliminary reports suggest that plantarflexor muscle structure is the primary determinant of function following Achilles tendon ruptures (1,2). Sudden changes in muscletendon tension, whether imposed by immobilization (32) or tendon repairs (18), elicit rapid remodeling of skeletal muscle.…”

“…Once cleared for activity, we will test plantarflexor strength and power in these patients using dynamometry and single-leg heel raises (2,3). Prior research also suggests that these structural changes in muscle are permanent(2) and the magnitude of these changes may be predictive of long-term functional deficits (1,30).…”

“…Decreased resting tension in the muscle-tendon unit stimulates sarcomere subtraction (32,33), effectively restoring the resting tension in the muscle. Both increased tendon length and decreased muscle fascicle length have been shown to negatively affect single-leg heel raise performance in computational simulations (1).…”

Peer Review #1 of "Gastrocnemius fascicles are shorter and more pennate throughout the first month following acute Achilles tendon rupture (v0.2)"

“…Once cleared for activity, we will test plantarflexor strength and power in these patients using dynamometry and single-leg heel raises (Baxter & Piazza, 2014;Baxter, Hullfish & Chao, 2018). Prior research also suggests that these structural changes in muscle are permanent (Baxter, Hullfish & Chao, 2018) and the magnitude of these changes may be predictive of long-term functional deficits (Silbernagel, Steele & Manal, 2012;Baxter, Farber & Hast, 2018).…”

“…Decreased resting tension in the muscle-tendon unit stimulates sarcomere subtraction in small animals (Williams & Goldspink, 1978;Williams, 1990), effectively restoring the resting tension in the muscle. Both increased tendon length and decreased muscle fascicle length have been shown to negatively affect single-leg heel raise performance in computational simulations (Baxter, Farber & Hast, 2018).…”