Abstract
The effect of stretch-shortening cycles (SSCs) is often studied in laboratory settings, yet it remains unclear whether highly active muscle SSCs actually occur during in vivo movement. Nine highly trained jumping athletes performed single-leg pre-hop forward jumps at maximal effort. We hypothesized that these jumps would induce a SSC at the level of the muscle in the medial gastrocnemius. Kinematic and kinetic data were collected together with electromyography signals (EMG) and muscle fascicle length and pennation angle changes of the medial gastrocnemius of both legs and combined with a musculoskeletal model to calculate the stretch-shortening behavior of the muscle (fascicles) and tendon (series-elastic element). The length changes of the fascicles, longitudinal muscle displacement, series-elastic element, and whole muscle-tendon unit further allowed for a detailed analysis of the architectural gearing ratio between different phases of the SSC within a single movement. We found a SSC at the level of the joint, muscle-tendon unit and tendon but not at the muscle. We further found that the average architectural gearing ratio was higher during the stretching of the series-elastic element as compared to when the series-elastic element was shortening, yet this was not statistically tested because of low sample size for this parameter. However, we found no correlation when plotting the architectural gearing ratio as a function of the fascicle velocities at each instance in time. Despite the athletes having a clear preferred leg for jumping, we found no differences in any kinematic or kinetic parameter between the preferred and non-preferred leg or any parameter from the muscle-tendon interaction analysis other than a reduced longitudinal muscle shortening in the non-preferred leg (p = 0.008). We conclude that, although common at the level of the joints, MTUs, and tendon (series-elastic element), highly active SSCs very rarely occur in the medial gastrocnemius, even in movements that induce high loading. This has important implications for the translation of ex vivo findings on SSC effects, such as residual force enhancement, in this muscle. We further conclude that there is no precise tuning of the architectural gearing ratio in the medial gastrocnemius throughout the whole movement.
Highlights
The stretch-shortening cycle (SSC) is a common phenomenon in many naturally occurring movements and has long been identified as a performance-enhancing mechanism (Bosco et al, 1982; Seiberl et al, 2015)
Despite the athletes having a clear preference for one leg over the other for jumping, we found no significant difference between the body mass normalized peak resultant GRF or impulse generated between the preferred and non-preferred leg (Table 1)
We found no differences between the preferred and non-preferred leg in any of the minimal or maximal joint angles, maximal angular velocities, maximal body mass normalized moments, maximal power absorption and generation, or total negative, positive or net work done at the ankle or knee joint (Table 1)
Summary
The stretch-shortening cycle (SSC) is a common phenomenon in many naturally occurring movements and has long been identified as a performance-enhancing mechanism (Bosco et al, 1982; Seiberl et al, 2015). We were able to overcome the lack of MTU stretching in a classic squat jump and counter-movement jump and induced a SSC at the level of the medial gastrocnemius muscle-tendon unit by adding a specific pre-hop to the squat jump, effectively changing the behavior of the muscle and tendon of the plantar flexors (Aeles et al, 2018). This resulted in a SSC at the level of the joint, muscle-tendon unit and tendon. Our understanding of the mechanical functioning of the muscle during highly active muscle SSCs in vivo remains poor
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