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This study aimed to identify muscle synergies of the lower limb during treadmill running on level and inclined ground. Eight subjects ran on a treadmill at three speeds (2.5, 3.3, and 4.1 m/s) and two grades (level and 10% grade). Surface electromyographic (EMG) signals were recorded from 10 muscles of the lower limb, including deeper muscles such as vastus intermedius, adductor magnus, and adductor longus. Muscle synergies were extracted applying a non-negative matrix factorization algorithm, and relative co-activations across muscles and the temporal recruitment pattern were identified by muscle synergy vector and synergy activation coefficient, respectively. The scalar product between pairs of synergy vectors and synergy activation coefficients during level and uphill running conditions were analyzed as a similarity index, with values above 0.8 recognized as similar. Approximately 4 muscle synergies controlled the majority of variability in 10 EMGs during running, and were common between level and uphill conditions. At each running speed, inter-condition similarity was observed in synergy vector (r > 0.83) and synergy activation coefficients (r > 0.84) at each type of synergy. These results suggest that types of synergy are consistent between level and uphill running.

BackgroundOlder individuals have been shown to present muscle atrophy in conjunction with increased fat fraction in some muscles. The proportion of fat and connective tissue within the skeletal muscle can be estimated from axial B-mode ultrasound images using echo intensity (EI). EI was used to calculate the index of muscle quality. Walking, home-based weight-bearing resistance training, and its combinations are considered simple, easy, and practical exercise interventions for older adults. The purpose of this study was to quantify the effects of walking and walking with home-based resistance training on muscle quality of older individuals.MethodsThirty-one participants performed walking training only (W-group; 72 ± 5 years) and 33 participants performed walking and home-based resistance training (WR-group; 73 ± 6 years). This study was a non-randomized controlled trial with no control group. All participants were instructed to walk 2 or 3 sets per week for 10 weeks (one set: 30-min continuous walking). In addition, the WR-group performed home-based weight-bearing resistance training. EI was measured as a muscle quality index using axial B-mode ultrasound images of the rectus femoris and vastus lateralis of the mid-thigh. We further averaged these parameters to obtain the EI of the quadriceps femoris (QF). Participants further performed five functional tests: sit-ups, supine up, sit-to-stand, 5-m maximal walk, and 6-min walk.ResultsQF EI was significantly decreased in both groups after training (W-group 69.9 ± 7.4 a.u. to 61.7 ± 7.0 a.u., WR-group 64.0 ± 9.5 a.u. to 51.1 ± 10.0 a.u.; P < 0.05), suggesting improved muscle quality. QF EI was further decreased in the WR-group compared with the W-group. The sit-up test in both groups and the sit-to-stand and 5-m maximal walk tests in the W-group were significantly improved after training.ConclusionThese results suggest that training-induced stimulation is associated with a decrease in EI in some thigh regions. Furthermore, the addition of home-based resistance training to walking would be effective for a greater reduction of EI.

Assessments of both neural and muscular adaptations during interventions would provide valuable information for developing countermeasures to age-related muscle dysfunctions. We investigated the effect of fish protein ingestion on training-induced neural and muscular adaptations in older adults. Twenty older adults participated 8 weeks of isometric knee extension training intervention. The participants were divided into two groups who took fish protein (n = 10, Alaska pollack protein, APP) or casein (n = 10, CAS). Maximal muscle strength during knee extension, lower extremity muscle mass (body impedance method), and motor unit firing pattern of knee extensor muscle (high-density surface electromyography) were measured before, during, and after the intervention. Muscle strength were significantly increased in both CAS (124.7 ± 5.8%) and APP (117.1 ± 4.4%) after intervention (p &lt; .05), but no significant differences between the groups were observed (p &gt; .05). Significant increases in lower extremity muscle mass from 0 to 8 weeks were demonstrated only for APP (102.0 ± 3.2, p &lt; .05). Greater changes in motor unit firing pattern following intervention were represented in CAS more than in APP. These results suggest that nutritional supplementations could modulate neural and muscular adaptations following resistance training and fish protein ingestion preferentially induces muscular adaptation without the detectable neural adaptation in older adults.

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