Muscle volume is known to correlate with maximal joint torque in humans, but the role of muscle moment arm in determining maximal torque is less clear. Moderate correlations have been reported between maximal isometric knee extensor torque and knee extensor moment arm, but no such observations have been made for the ankle joint. It has been suggested that smaller muscle moment arms may enhance force generation at high rates of joint rotation, but this has not yet been observed for ankle muscles in vivo. The purpose of the present study was to correlate plantar flexor moment arm and plantar flexor muscle volume with maximal plantar flexor torque measured at different rates of plantar flexion. Magnetic resonance imaging was used to quantify the plantar flexor moment arm and muscle volume of the posterior compartment in 20 healthy young men. Maximal plantar flexor torque was measured isometrically and at three plantar flexion speeds using an isokinetic dynamometer. Plantar flexor torque was significantly correlated with muscle volume (0.222 < R²< 0.322) and with muscle moment arm at each speed (0.323 < R²< 0.494). While muscle volume was strongly correlated with body mass and stature, moment arm was not. The slope of the torque-moment arm regression line decreased as the rate of joint rotation increased, indicating that subjects with small moment arms experienced smaller reductions in torque at high speeds. The findings of this study suggest that plantar flexor moment arm is a determinant of joint strength that is at least as important as muscle size.

Purpose: The purposes of our study were to evaluate Achilles tendon loading profiles of various exercises and to develop guidelines to incrementally increase the rate and magnitude of Achilles tendon loading during rehabilitation. Methods: Eight healthy young adults completed a battery of rehabilitation exercises. During each exercise, we collected three-dimensional motion capture and ground reaction force data to estimate Achilles tendon loading biomechanics. Using these loading estimates, we developed an exercise progression that incrementally increases Achilles tendon loading based on the magnitude, duration, and rate of tendon loading. Results: We found that Achilles tendon loading could be incrementally increased using a set of either isolated ankle movements or multijoint movements. Peak Achilles tendon loads varied more than 12-fold, from 0.5 bodyweights during a seated heel raise to 7.3 bodyweights during a forward single-leg hop. Asymmetric stepping movements like lunges, step ups, and step downs provide additional flexibility for prescribing tendon loading on a side-specific manner. Conclusion: By establishing progressions for Achilles tendon loading, rehabilitative care can be tailored to address the specific needs of each patient. Our comprehensive data set also provides clinicians and researchers guidelines on how to alter magnitude, duration, and rate of loading to design new exercises and exercise progressions based on the clinical need.

Recent studies of sprinters and distance runners have suggested that variations in human foot proportions and plantarflexor muscle moment arm correspond to the level of sprint performance or running economy. Less clear, however, is whether differences in muscle moment arm are mediated by altered tendon paths or by variation in the centre of ankle joint rotation. Previous measurements of these differences have relied upon assumed joint centres and measurements of bone geometry made externally, such that they would be affected by the thickness of the overlying soft tissue. Using magnetic resonance imaging, we found that trained sprinters have shorter plantarflexor moment arms (p= 0.011) and longer forefoot bones (p= 0.019) than non-sprinters. The shorter moment arms of sprinters are attributable to differences in the location of the centre of rotation (p< 0.001) rather than to differences in the path of the Achilles tendon. A simple computer model suggests that increasing the ratio of forefoot to rearfoot length permits more plantarflexor muscle work during plantarflexion that occurs at rates expected during the acceleration phase following the sprint start.

Several limited-open Achilles tendon repair techniques that use locking or nonlocking sutures have been developed, but direct comparisons of in vitro mechanical properties have not yet been reported in the literature. It was our hypothesis that loads applied to the repaired Achilles tendon would be better resisted by limited-open techniques that use locking stitches compared with limited-open repairs that use nonlocking stitches. The Achilles tendons of 31 fresh-frozen cadaver lower limbs were incised 4 cm proximal to the calcaneal insertion. Tendons were then repaired using 1 of 2 limited-open Achilles tendon repair tools, one using 3 nonlocking sutures and the other using a combination of locking and nonlocking sutures. Repaired specimens were cycled to 1000 cycles from 20 to 100 N and from 20 to 190 N followed by a single load to failure test. Nonparametric analyses were performed to compare the number of cycles to gapping and total load to failure between the 2 repair techniques. During cyclic loading, more cycles occurred prior to detection of 2-mm and 9.5-mm gaps in the locking suture construct compared with the nonlocking suture construct (P = .012 and P = .005, respectively). There was no difference in the number of cycles to a gap of 5 mm (P = .053). The locking suture construct also resisted a significantly greater load to failure compared with the nonlocking suture construct (P < .001; median 385.0 and 299.6 N, respectively). Limited-open repair techniques using locking sutures provided greater construct strength under both cyclic and ultimate loads compared with a repair technique that used only nonlocking sutures. Limited-open Achilles tendon repairs using locking sutures are better able to resist forces simulating early accelerated rehabilitation than repairs using nonlocking sutures.

Background: Suture-button constructs are an alternative to screw fixation for syndesmotic injuries, and proponents advocate that suture-button constructs may allow physiological motion of the syndesmosis. Recent biomechanical data suggest that fibular instability with syndesmotic injuries is greatest in the sagittal plane, but the design of a suture-button construct, being a rope and 2 retention washers, is most effective along the axis of the rope (in the coronal plane). Some studies report that suture-button constructs are able to constrain fibular motion in the coronal plane, but the ability of a tightrope to constrain sagittal fibular motion is unknown. The purpose of this study was to assess fibular motion in response to an external rotation stress test in a syndesmotic injury model after fixation with a screw or suture-button constructs. Methods: Eleven fresh-frozen cadaver whole legs with intact tibia-fibula articulations were secured to a custom fixture. Fibular motion (coronal, sagittal, and rotational planes) in response to a 6.5-Nm external rotation moment applied to the foot was recorded with fluoroscopy and a high-resolution motion capture system. Measures were taken for the following syndesmotic conditions: intact, complete lateral injury, complete lateral and deltoid injury, repair with a tetracortical 4.0-mm screw, and repair with a suture button construct (Tightrope; Arthrex, Naples, FL) aimed from the lateral fibula to the anterior medial malleolus. Results: The suture-button construct allowed significantly more sagittal plane motion than the syndesmotic screw. Measurements acquired with mortise imaging did not detect differences between the intact, lateral injury, and 2 repair conditions. External rotation of the fibula was significantly increased in both injury conditions and was not restored to intact levels with the screw or the suture-button construct. Conclusion: A single suture-button placed from the lateral fibula to the anterior medial malleolus was unable to replicate the motion observed in the intact specimen when subjected to an external rotation stress test and allowed significantly more posterior motion of the fibula than when fixed with a screw in simulated highly unstable injuries. Clinical Relevance: Fixation of a syndesmotic injury with a single suture-button construct did not restore physiological fibular motion, which may have implications for postoperative care and clinical outcomes.