Differential changes in muscle architecture and neuromuscular fatigability induced by isometric resistance training at short and long muscle-tendon unit lengths.

Brief Summary


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Clinicians in the performance and medical fields are often chasing changes in muscle-tendon architecture and performance that may improve athletic performance and tissue resilience or robustness. Muscle or tendon cross-sectional area, muscle fascicle pennation angle, fascicle length, muscle strength, power and endurance, or tendon stiffness may be targeted in the prescription of any training/resistance program. 

In the cited article by Akagi et al 2020, the authors explore changes in muscle architecture and fatiguability in response to 8 weeks of isometric training at short and long muscle lengths. In doing so, primary and secondary findings with important clinical implications are highlighted.

Materials and Methods: 7 males and 6 females were recruited. Dominant and non-dominant legs were randomly assigned to undertake isometric training of the tibialis anterior muscle over an 8-week period. Training dose and frequency was 3x8x5s MVIC, 3x per week. One leg was trained at a short (0°) and the other at a long (40°) muscle-tendon unit length.

Neuromuscular Measures: Electrically evoked peak dorsiflexion torque during peak doublet twitches, Maximum Voluntary Isometric Contraction (MVIC) peak ankle dorsiflexion torque, voluntary activation (VA), and the torque angle relationship across 9 ankle joint angles, and isotonic peak power @ 20% MVIC resistance.

Real-Time Ultrasound Measures: Muscle thickness, pennation Angle (PA), and Fascicle Length (FL) of the tibialis Anterior.

Electromyography (EMG) Measures: Surface EMG of the tibialis Anterior (TA) and Soleus (SOL) muscle.

The Fatiguing task involved subjects performing maximal effort 20% MVC Isotonic dorsiflexion repetitions from 40° - 0° plantarflexion until peak power dropped below 60% of the 2 previous contractions. 



Results / Findings

Isometric dorsiflexion strength (MVIC), peak power and dynamic peak torque were increased in both groups following the prescribed Isometric training. 

The plateau region of the torque angle curve was broader at 8 weeks (5°-40°) than at 0 weeks (10°-30°) in the group training at an increased muscle-tendon unit length. So, while the angle for optimal torque generation had not changed, an extended plateau or ability to generate high force over a greater total range, was evident in those subjects trained at a longer muscle-tendon length.

Muscle thickness and PA were increased in both groups regardless of the length at which training occurred. An increase in FL was only demonstrated in the group training at a long muscle-tendon length. 

Consistent with current literature, this data confirms that Isometric training increases muscle size, isometric and isotonic muscle strength and peak power. Changes in muscle architecture (pennation angle, fascicle length and thickness) are also associated with Isometric training. However, it is worth noting that the associated changes in fascicle length are dependent on the muscle-tendon unit length at which this isometric training occurs.

While this study illustrates the positive effects of isometric training at an augmented muscle-tendon unit length on dorsiflexion strength, consideration should look to the broader applicability of this notion to other force vectors. Further inquiry is essential to deepen our comprehension of the relationship between deficiencies in isometric knee flexor strength and the susceptibility to hamstring injuries. Notably, Nara (2022) identified that post-hamstring injury, deficits in isometric peak muscle force were observed at the lengthened muscle position rather than the unstretched position. Consequently, it would make sense to integrate assessments of strength, endurance, and control of hamstring muscle groups at outer ranges into profiling and monitoring strategies in locomotor sports.

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Moreover, clinicians should consider training or correcting any discernible deficits evident in this risk factor evident, through analysis of asymmetries and or comparisons with normalised data from reference populations, as part of a multifactorial approach aimed at mitigating hamstring injury risk.

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