Can fast-twitch muscle fibres be selectively recruited during lengthening contractions? Review and applications to sport movements

Literature examining the recruitment order of motor units during lengthening (eccentric) contractions was reviewed to determine if fast-twitch motor units can be active while lower threshold slow-twitch motor units are not active. Studies utilizing surface electromyogram (EMG) amplitude, single motor unit activity, spike amplitude–frequency analyses, EMG power spectrum, mechanomyographic, and phosphocreatine-to-creatine ratio (PCr/Cr) techniques were reviewed. Only single motor unit and PCr/Cr data were found to be suitable to address the goals of this review. Nine of ten single motor unit studies, examining joint movement velocities up to 225°/s and forces up to 53% of a maximum voluntary contraction, found that the size principle of motor unit recruitment applied during lengthening contractions. Deviation from the size principle was demonstrated by one study examining movements within a small range of low velocities and modest forces, although other studies examining similar low forces and lengthening velocities reported size-ordered recruitment. The PCr/Cr data demonstrated the activation of all fibre types in lengthening maximal contractions. Most evidence indicates that for lengthening contractions of a wide range of efforts and speeds, fast-twitch muscle fibres cannot be selectively recruited without activity of the slow-twitch fibres of the same muscle.     

Is there a low-back cost to hip-centric exercise? Quantifying the lumbar spine joint compression and shear forces during movements used to overload the hips

The aim of this study was to quantify joint compression and shear forces at L4/L5 during exercises used to overload the hips. Nine men performed 36 "walking" trials using two modalities: (1) sled towing and (2) exercise bands placed around the ankles. Participants completed forward, backward, and lateral trials with bent and straight legs at three separate loads. Surface electromyography (EMG) was recorded bilaterally from eight torso and thigh sites, upper body and lumbar spine motion were quantified, and hand forces were measured. An EMG-driven musculoskeletal model was used to estimate the muscular contribution to joint compression and shear. Peak reaction, muscle and joint compression and shear forces, and peak gluteus medius and maximus activity were calculated. Significant differences were noted in each dependent measure; however, they were dependent on direction of travel, leg position, and load. The highest joint compression and shear forces for the sled and band conditions were 4378 N and 626 N, and 3306 N and 713 N, respectively. In general, increasing the band tension had little effect on all dependent measures, although a load-response was found during the sled conditions. Before using any exercise to improve hip function, the potential benefits should be weighed against "costs" to neighbouring joints.     

How many times per week should a muscle be trained to maximize muscle hypertrophy? A systematic review and meta-analysis of studies examining the effects of resistance training frequency

Training frequency is considered an important variable in the hypertrophic response to regimented resistance exercise. The purpose of this paper was to conduct a systematic review and meta-analysis of experimental studies designed to investigate the effects of weekly training frequency on hypertrophic adaptations. Following a systematic search of PubMed/MEDLINE, Scoups, and SPORTDiscus databases, a total of 25 studies were deemed to meet inclusion criteria. Results showed no significant difference between higher and lower frequency on a volume-equated basis. Moreover, no significant differences were seen between frequencies of training across all categories when taking into account direct measures of growth, in those considered resistance-trained, and when segmenting into training for the upper body and lower body. Meta-regression analysis of non-volume-equated studies showed a significant effect favoring higher frequencies, although the overall difference in magnitude of effect between frequencies of 1 and 3+ days per week was modest. In conclusion, there is strong evidence that resistance training frequency does not significantly or meaningfully impact muscle hypertrophy when volume is equated. Thus, for a given training volume, individuals can choose a weekly frequency per muscle groups based on personal preference.