Negligible epimuscular myofascial force transmission between the human rectus femoris and vastus lateralis muscles in passive conditions

PURPOSE

There have been contradictory reports of the effects of epimuscular myofascial force transmission in humans. This study investigated the transmission of myofascial force to the human vastus lateralis muscle by determining whether vastus lateralis slack angle changed with hip angle. Since the distance between the origin and insertion of the vastus lateralis muscle does not change when hip angle changes, any change in vastus lateralis slack angle with hip position can be attributed to epimuscular myofascial force transmission.

METHODS

Nineteen young adults were tested in hip flexed ([Formula: see text]) and neutral ([Formula: see text]) positions. Ultrasound images of the vastus lateralis muscle were obtained as the knee was passively flexed at [Formula: see text]/s. The knee angle at which vastus lateralis muscle fascicles began to lengthen was used to identify muscle slack angle.

RESULTS

Overall, there was a negligible effect of hip position on vastus lateralis slack angle ([Formula: see text] [[Formula: see text] to 1.9]; mean [95% confidence interval]). However, a small and variable effect was noted in 3/19 participants.

CONCLUSION

This result indicates that, over the range of joint angles tested here, there is little or no epimuscular myofascial force transmission between the vastus lateralis muscle and neighbouring bi-articular structures under passive conditions. More broadly, this result provides additional evidence that epimuscular myofascial force transmission tends to be small and variable under passive conditions in healthy human muscle.

Brief report: Passive mechanical properties of gastrocnemius in multiple sclerosis and ankle contracture

BACKGROUND

Ankle contracture is common in people with multiple sclerosis (MS) but the mechanisms of contracture are not clear. This study aimed to identify the mechanisms of contracture in MS by comparing passive muscle length and stiffness at known tension, separated into contributions by muscle fascicles and tendons, between people with MS who had contracture and healthy people.

METHODS

Passive length-tension curves of the gastrocnemius muscle-tendon unit were derived from passive ankle torque and angle using a published biomechanical method. Ultrasound images of medial gastrocnemius muscle fascicles were used to partition length-tension curves into fascicle and tendon components. Lengths and stiffness of the muscle-tendon unit, muscle fascicles and tendons were compared between groups with linear regression.

FINDINGS

Data were obtained from 15 participants with MS who had contracture [age 53 (12) years, mean (SD)] and 25 healthy participants [48 (20) years]. Participants with MS had clinically significant ankle contracture, and had shorter fascicles at slack length (between-groups mean difference -0.8 cm, 95% CI -1.2 to -0.4 cm, p < 0.001) and at 100 N (-0.7 cm, 95% CI -1.3 to -0.1 cm, p = 0.02) compared to healthy participants. There were no differences between groups in all other outcomes.

INTERPRETATION

Tension-referenced comparisons of passive muscle length and stiffness show that people with MS who had contracture had shorter fascicles at low and high tension compared to healthy people, but there were no changes to the muscle-tendon unit or tendon. Further studies are needed to identify the causes and mechanisms of contracture in neurological conditions.

Plantarflexor metabolics are sensitive to resting ankle angle and optimal fiber length in computational simulations of gait

BACKGROUND

Plantarflexor structure is an important predictor of function in healthy, athletic, and some patient populations. Computational simulations are powerful tools capable of testing the isolated effects of muscle-tendon structure on gait function.

RESEARCH QUESTION

The purpose of this study was to characterize the sensitivity of plantarflexor muscle function based on muscle-tendon unit (MTU) parameters. We hypothesized that plantarflexor metabolics and shortening dynamics would be sensitive to MTU parameters.

METHODS

Stance phase of gait was simulated using a musculoskeletal model and computed muscle control algorithm. Optimal muscle fiber length, resting ankle angle, and tendon stiffness parameters were systematically changed to test these effects on plantarflexor metabolics, activation, and power. Dorsiflexor metabolics were also measured to determine the impact of the action of the antagonist muscle group.

RESULTS AND SIGNIFICANCE

Plantarflexor metabolic demands were 1.5 and 2.7 times more sensitive to optimal fiber length and resting ankle angle, respectively, compared to the effect of tendon stiffness. Increased resting ankle plantarflexion induced a large passive plantarflexion moment during early stance, which required non-physiologic dorsiflexor contractions. Conversely, longer optimal fiber and more neutral resting ankle angles increased the shortening demands of the plantarflexors. These findings highlight the importance of carefully selecting MTU parameters when modeling gait with musculoskeletal models, especially in pathologic or high-performance athlete populations.