The role of the length-tension curve in the control of movement

Effects of Kinesiology Taping on Shoulder Posture and Peak Torque in Junior Baseball Players with Rounded Shoulder Posture: A Pilot Study

Rounded shoulder posture (RSP) causes an imbalance of the adjacent joints due to the malalignment of the shoulder joint, and thus affects the strength of the muscles surrounding the shoulder. This study aimed to investigate the effect of rounded shoulder taping (RST) on shoulder posture and muscle strength in junior baseball players. Nineteen junior baseball players participated in the study, which used a crossover design at an interval of 1 week. The participants were randomized to receive rounded shoulder taping (RST) and sham kinesiology taping (SKT) using kinesiology tape. RSP was measured using two 12-inch combination squares, and shoulder peak torques were measured by isokinetic equipment. The results showed that RST led to significant changes in RSP (p < 0.05), but no significant changes were observed with SKT (p < 0.05). RST led to significant changes in the peak torques of external rotation and internal rotation of the shoulder (p < 0.05), but no significant changes were observed with SKT (p < 0.05). These results suggest that RST could help to correct RSP and improve peak torque of external and internal rotation of the shoulders of junior baseball players with RSP.

The Interaction of Compliance and Activation on the Force-Length Operating Range and Force Generating Capacity of Skeletal Muscle: A Computational Study using a Guinea Fowl Musculoskeletal Model

A muscle’s performance is influenced by where it operates on its force–length (F–L) curve. Here we explore how activation and tendon compliance interact to influence muscle operating lengths and force-generating capacity. To study this, we built a musculoskeletal model of the lower limb of the guinea fowl and simulated the F–L operating range during fixed-end fixed-posture contractions for 39 actuators under thousands of combinations of activation and posture using three different muscle models: Muscles with non-compliant tendons, muscles with compliant tendons but no activation-dependent shift in optimal fiber length (L0), and muscles with both compliant tendons and activation-dependent shifts in L0. We found that activation-dependent effects altered muscle fiber lengths up to 40% and increased or decreased force capacity by up to 50% during fixed-end contractions. Typically, activation-compliance effects reduce muscle force and are dominated by the effects of tendon compliance at high activations. At low activation, however, activation-dependent shifts in L0 are equally important and can result in relative force changes for low compliance muscles of up to 60%. There are regions of the F–L curve in which muscles are most sensitive to compliance and there are troughs of influence where these factors have little effect. These regions are hard to predict, though, because the magnitude and location of these areas of high and low sensitivity shift with compliance level. In this study we provide a map for when these effects will meaningfully influence force capacity and an example of their contributions to force production during a static task, namely standing.

Mechanical Evaluation of the Pronator Teres Rerouting Tendon Transfer

We simulated pronator teres rerouting using a three-dimensional biomechanical model of the arm. Simulations comprised the evaluation of changes in muscle length and the moment arm of pronator teres with changes in forearm axial rotation and elbow flexion. The rerouting of Pronator Teres was simulated by defining a path for it through the interosseous membrane with re-attachment to its original insertion. However the effect of moving the insertion to new positions, 2 cm below and above, the original position was also assessed. The effect on total internal rotation and external rotation capacity was determined by calculating the potential moments for pronator teres, supinator, pronator quadratus, biceps brachii and brachioradialis. Pronator teres was found to be a weak internal rotator in extreme pronation, but a strong internal rotator in neutral rotation and in supination. After rerouting pronator teres was only a strong external rotator in full pronation and not at other forearm positions, where the effect of rerouting was comparable to a release procedure.

Latissimus dorsi transfer to restore external rotation with reverse shoulder arthroplasty: a biomechanical study

In patients with pseudoparesis of the shoulder resulting from irreparable rotator cuff tears, reverse shoulder arthroplasty (RSA) can restore active elevation, but external rotation remains less predictable. Latissimus dorsi transfer (LDT) has been shown to be effective in restoring external rotation in patients with posterosuperior tears of the rotator cuff. The aim of this study is to determine the capacity of the LDT to restore external rotation in combination with RSA and to investigate the mechanical advantage produced by 3 different insertion sites. A biomechanical model was created using a reverse total shoulder prosthesis with 3 different transfer insertions. Moment arms were measured for 2 static positions and 1 motion of the humerus. The moment arm analysis showed that LDT can improve active external rotation in the setting of a reverse prosthesis. An insertion site on the posterior side of the greater tuberosity (adjacent to the teres minor insertion) produced a greater external rotation moment arm.

Exhaustive stretch-shortening cycle exercise: no contralateral effects on muscle activity in maximal motor performances

Minor cross-over effects of unilateral muscle fatigue have been reported after isometric exercises. The present study re-examined this possibility after an exhaustive stretch-shortening cycle (SSC)-type exercise. Twenty-five subjects performed on a sledge apparatus a unilateral exhaustive rebound exercise involving mostly the triceps surae muscle group. Ipsilateral vs contralateral fatigue effects were compared in uni- and bilateral tests that included a maximal isometric voluntary contraction (MVC) and a series of 10 maximal drop jumps (DJ). These tests were carried out just before and after (POST) the exhaustive SSC exercise, and were repeated 2 days later (D2), at the expected time of major inflammation and pain. The exercised (fatigued) leg analysis revealed significant declines in MVC and DJ performances at POST and D2, the latter ones being associated with significant decreases in voluntary muscle activity. In contrast, no significant change was found for the non-fatigued leg. These results do not support the existence of cross-over effects after exhaustive SSC exercise, at least when tested in maximal static and dynamic unilateral motor tasks.