Speed-specific optimal contractile conditions of the human soleus muscle from slow to maximum running speed

Biarticular gastrocnemii muscles increase their joint energy transfer potential at high running speeds

The involvement of energy transfer mechanisms between the ankle and the knee joint by the biarticular gastrocnemii muscles at high running speeds is currently unknown. During running at seven speeds (3.0-8.5 m s-1), the ankle and knee joint kinematics as well as the electromyographic activity of the gastrocnemius medialis and lateralis were captured. By means of the ankle-knee joint coupling angles, we determined the energy transfer potential between the two joints as the fraction of contact time where the joint angles are in-phase. At speeds above 6.0 m s-1, the ankle-to-knee joint energy transfer potential during the first part of stance and the knee-to-ankle energy transfer potential during the second part of stance were increased by 37% and 12%, respectively. This was accompanied by a 2.8-fold and 2.0-fold increase of the gastrocnemii muscle activation. The findings demonstrate a speed-dependent modification of the ankle-knee joint coordination towards an in-phase pattern in combination with an increase in muscle activation, which enhances the possibility of energy transfer between the two joints by the biarticular gastrocnemii muscles. An increased energy transfer from the knee to the ankle joint is probably necessary to increase the power output at the ankle joint, required for the highest running speeds.

Optimizing Resistance Training for Sprint and Endurance Athletes: Balancing Positive and Negative Adaptations

Resistance training (RT) triggers diverse morphological and physiological adaptations that are broadly considered beneficial for performance enhancement as well as injury risk reduction. Some athletes and coaches therefore engage in, or prescribe, substantial amounts of RT under the assumption that continued increments in maximal strength capacity and/or muscle mass will lead to improved sports performance. In contrast, others employ minimal or no RT under the assumption that RT may impair endurance or sprint performances. However, the morphological and physiological adaptations by which RT might impair physical performance, the likelihood of these being evoked, and the training program specifications that might promote such impairments, remain largely undefined. Here, we discuss how selected adaptations to RT may enhance or impair speed and endurance performances while also addressing the RT program variables under which these adaptations are likely to occur. Specifically, we argue that while some myofibrillar (muscle) hypertrophy can be beneficial for increasing maximum strength, substantial hypertrophy can lead to macro- and microscopic adaptations such as increases in body (or limb) mass and internal moment arms that might, under some conditions, impair both sprint and endurance performances. Further, we discuss how changes in muscle architecture, fiber typology, microscopic muscle structure, and intra- and intermuscular coordination with RT may maximize speed at the expense of endurance, or maximize strength at the expense of speed. The beneficial effect of RT for sprint and endurance sports can be further improved by considering the adaptive trade-offs and practical implications discussed in this review.

Acute effects of the short-foot exercise in runners with medial tibial stress syndrome: A quasi-experimental study

OBJECTIVES

Analyze whether there are immediate changes in peak soleus activation and peak hindfoot eversion after short-foot exercise (SFE) in runners with medial tibial stress syndrome (MTSS). Secondarily, establish differences in peak soleus activation and peak hindfoot eversion between asymptomatic individuals and those presenting MTSS.

DESIGN

Quasi-experimental study.

SETTING

University Laboratory.

PARTICIPANTS

Thirty-two runners participated: 16 with MTSS and 16 in the no-pain group (NPG).

MAIN OUTCOME MEASURES

Soleus activation was measured using electromyography, and hindfoot eversion via 3D kinematic analysis. Participants performed SFE, and running data were collected at 9,12 and 15 km/h pre- and post-intervention.

RESULTS

SFE reduced peak soleus activation at 9 (p = 0.017) and 15 km/h (p = 0.019) for the MTSS group and at 15 km/h (p < 0.001) for the NPG, suggesting improved neuromuscular efficiency and potentially reduced tibial stress. SFE did not significantly affect peak hindfoot eversion. Significant correlations were found between ankle dorsiflexion range of motion and muscle activation (r = 0.585 to 0.849, p < 0.05). These findings suggest SFE could improve neuromuscular efficiency and reduce tibial stress, and highlights ankle flexibility's role in muscle activation.

CONCLUSIONS

SFE significantly reduces soleus activation, potentially improving neuromuscular efficiency and decreasing tibial stress.

Spatiotemporal modulation of a common set of muscle synergies during unpredictable and predictable gait perturbations in older adults

Muscle synergies as functional low-dimensional building blocks of the neuromotor system regulate the activation patterns of muscle groups in a modular structure during locomotion. The purpose of the current study was to explore how older adults organize locomotor muscle synergies to counteract unpredictable and predictable gait perturbations during the perturbed steps and the recovery steps. Sixty-three healthy older adults (71.2±5.2 years) participated in the study. Mediolateral and anteroposterior unpredictable and predictable perturbations during walking were introduced using a treadmill. Muscle synergies were extracted from the electromyographic activity of 13 lower limb muscles using Gaussian non-negative matrix factorization. The four basic synergies responsible for unperturbed walking (weight acceptance, propulsion, early swing and late swing) were preserved in all applied gait perturbations, yet their temporal recruitment and muscle contribution in each synergy were modified (P<0.05). These modifications were observed for up to four recovery steps and were more pronounced (P<0.05) following unpredictable perturbations. The recruitment of the four basic walking synergies in the perturbed and recovery gait cycles indicates a robust neuromotor control of locomotion by using activation patterns of a few and well-known muscle synergies with specific adjustments within the synergies. The selection of pre-existing muscle synergies while adjusting the time of their recruitment during challenging locomotor conditions may improve the effectiveness to deal with perturbations and promote the transfer of adaptation between different kinds of perturbations.

Assessment and modelling of the activation-dependent shift in optimal length of the human soleus muscle in vivo

Previous in vitro and in situ studies have reported a shift in optimal muscle fibre length for force generation (L0) towards longer length at decreasing activation levels (also referred to as length-dependent activation), yet the relevance for in vivo human muscle contractions with a variable activation pattern remains largely unclear. By a combination of dynamometry, ultrasound and electromyography (EMG), we experimentally obtained muscle force-fascicle length curves of the human soleus at 100%, 60% and 30% EMGmax levels from 15 participants aiming to investigate activation-dependent shifts in L0 in vivo. The results showed a significant increase in L0 of 6.5 ± 6.0% from 100% to 60% EMGmax and of 9.1 ± 7.2% from 100% to 30% EMGmax (both P < 0.001), respectively, providing evidence of a moderate in vivo activation dependence of the soleus force-length relationship. Based on the experimental results, an approximation model of an activation-dependent force-length relationship was defined for each individual separately and for the collective data of all participants, both with sufficiently high accuracy (R2 of 0.899 ± 0.056 and R2 = 0.858). This individual approximation approach and the general approximation model outcome are freely accessible and may be used to integrate activation-dependent shifts in L0 in experimental and musculoskeletal modelling studies to improve muscle force predictions. KEY POINTS: The phenomenon of the activation-dependent shift in optimal muscle fibre length for force generation (length-dependent activation) is poorly understood for human muscle in vivo dynamic contractions. We experimentally observed a moderate shift in optimal fascicle length towards longer length at decreasing electromyographic activity levels for the human soleus muscle in vivo. Based on the experimental results, we developed a freely accessible approximation model that allows the consideration of activation-dependent shifts in optimal length in future experimental and musculoskeletal modelling studies to improve muscle force predictions.