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New Findings: What is the central question of this study? Which structures of the medial gastrocnemius muscle-tendon unit contribute to its lengthening during joint rotation and thus receive the stretching stimulus? What is the main finding and its importance? We show, for the first time, that muscle and tendon lengthen in a different manner in children with cerebral palsy compared with typically developing children during a similar amount of muscle-tendon unit lengthening or joint rotation. This indicates possible differences in mechanical muscle and tendon properties attributable to cerebral palsy, which are not evident by assessment of muscle function at the level of a joint., Abstract: Children with cerebral palsy (CP) commonly present with reduced ankle range of motion (ROM) attributable, in part, to changes in mechanical properties of the muscle-tendon unit (MTU). Detailed information about how muscle and tendon interact to contribute to joint rotation is currently lacking but might provide essential information to explain the limited effectiveness of stretching interventions in children with CP. The purpose of this study was to quantify which structures contribute to MTU lengthening and thus receive the stretch during passive ankle joint rotation. Fifteen children with CP (age, in mean ± SD, 11.4 ± 3 years) and 16 typically developing (TD) children (age, in mean ± SD, 10.2 ± 3 years) participated. Ultrasound was combined with motion tracking, joint torque and EMG to record fascicle, muscle and tendon lengthening of the medial gastrocnemius during passive ankle joint rotations over the full ROM and a common ROM. In children with CP, relative to MTU lengthening, muscle and fascicles lengthened less (CP, 50.4% of MTU lengthening; TD, 63% of MTU lengthening; P < 0.04) and tendon lengthened more (CP, 49.6% of MTU lengthening; TD, 37% of MTU lengthening; P < 0.01) regardless of the ROM studied. Differences between groups in the amount of lengthening of the underlying structures during a similar amount of joint rotation and MTU displacement indicate possible differences in tissue mechanical properties attributable to CP, which are not evident by assessment at the level of a joint. These factors should be considered when assessing and treating muscle function in children with CP, for example during stretching exercises, because the muscle might not receive much of the applied lengthening stimulus., (© 2018 The Authors. Experimental Physiology © 2018 The Physiological Society.)

New Findings: What is the central question of this study? Can the increased range of motion seen acutely after stretching in children with cerebral palsy be explained by changes in the stiffness of the medial gastrocnemius fascicles? What is the main finding and its importance? We show, for the first time, that passive muscle and tendon properties are not changed acutely after a single bout of stretching in children with cerebral palsy and, therefore, do not contribute to the increase in range of motion. This contradicts common belief and what happens in healthy adults., Abstract: Stretching is often used to increase or maintain the joint range of motion (ROM) in children with cerebral palsy (CP), but the effectiveness of these interventions is limited. Therefore, our aim was to determine the acute changes in muscle-tendon lengthening properties that contribute to increased ROM after a bout of stretching in children with CP. Eleven children with spastic CP [age 12.1 (3 SD) years, 5/6 hemiplegia/diplegia, 7/4 gross motor function classification system level I/II] participated. Each child received three sets of five × 20 s passive, manual static dorsiflexion stretches separated by 30 s rest, with 60 s rest between sets. Before and immediately after stretching, ultrasound was used to measure medial gastrocnemius fascicle lengthening continuously over the full ROM and an individual common ROM pre- to post-stretching. Simultaneously, three-dimensional motion of two marker clusters on the shank and the foot was captured to calculate ankle angle, and ankle joint torque was calculated from manually applied torques and forces on a six degrees-of-freedom load cell. After stretching, the ROM was increased [by 9.9 (12.0) deg, P = 0.005]. Over a ROM common to both pre- and post-measurements, there were no changes in fascicle lengthening or torque. The maximal ankle joint torque tolerated by the participants increased [by 2.9 (2.4) N m, P = 0.003], and at this highest passive torque the maximal fascicle length was 2.8 (2.4) mm greater (P = 0.009) when compared with before stretching. These results indicate that the stiffness of the muscle fascicles in children with CP remains unaltered by an acute bout of stretching., (© 2017 The Authors. Experimental Physiology © 2017 The Physiological Society.)

To better understand the effects of pubertal maturation on the contractile properties of skeletal muscle in vivo, the present study investigated whether there are any differences in the specific tension of the quadriceps muscle in 20 adults and 20 prepubertal children of both sexes. Specific tension was calculated as the ratio between the quadriceps tendon force and the sum of the physiological cross-sectional area (PCSA) multiplied by the cosine of the angle of pennation of each head within the quadriceps muscle. The maximal quadriceps tendon force was calculated from the knee extension maximal voluntary contraction (MVC) by accounting for EMG-based estimates of antagonist co-activation, incomplete quadriceps activation using the interpolation twitch technique and magnetic resonance imaging (MRI)-based measurements of the patellar tendon moment arm. The PCSA was calculated as the muscle volume, measured from MRI scans, divided by optimal fascicle length, measured from ultrasound images during MVC at the estimated angle of peak quadriceps muscle force. It was found that the quadriceps tendon force and PCSA of men (11.4 kN, 214 cm(2)) were significantly greater than those of the women (8.7 kN, 152 cm(2); P < 0.01). Both adult groups had greater values than the children (P < 0.01) but there were no differences between boys (5.2 kN, 99 cm(2)) and girls (6.1 kN, 102 cm(2)). Agonist activation was greater in men and women than in girls (P < 0.05), and antagonist activation was greater in men than in boys (P < 0.05). Moment arm length was greater in men than in boys or girls and greater in women than in boys (P < 0.05). The angle of pennation did not differ between the groups in any of the quadriceps heads. The specific tension was similar (P > 0.05) between groups: men, 55 +/- 11 N cm(-2); women, 57.3 +/- 13 N cm(-2); boys, 54 +/- 14 N cm(-2); and girls, 59.8 +/- 15 N cm(-2). These findings indicate that the increased muscle strength with maturation is not due to an increase in the specific tension of muscle; instead, it can be attributed to increases in muscle size, moment arm length and voluntary activation level.

We hypothesized that training with eccentric contractions only (therefore using higher loads) would yield greater muscle structural and strength gains compared with conventional resistance training. Nine older adults (mean +/- s.d. age, 74 +/- 3 years) were assigned to a conventional (CONV) resistance training group performing both concentric and eccentric contractions and 10 (age, 67 +/- 2 years) to an eccentric-only (ECC) resistance training group. Both groups trained three times per week for 14 weeks at 80% of the five-repetition maximum, specific to each training mode. Maximal knee extensor torque was assessed during isometric, concentric and eccentric contractions across a range of angular velocities (0-3.49 rad s(-1)). Vastus lateralis muscle architecture (fascicle length, pennation angle and muscle thickness) was assessed in vivo at rest using ultrasonography. Training increased fascicle length in both groups, but the increase was significantly greater in the ECC (20% increase) than the CONV group (8% increase). Conversely, pennation angle significantly increased in the CONV (35% increase) but not in the ECC group (5% increase). Muscle thickness increased to a similar extent in both groups (approximately 12% increase). In the ECC group, eccentric knee extensor torque increased by 9-17% across velocities, but concentric torque was unchanged. Conversely, in the CONV group, concentric torque increased by 22-37% across velocities, but eccentric torque was unchanged. Instead, isometric torque increased to a similar extent in both groups (approximately 8% increase). Thus, the two training regimens resulted in differential adaptations in muscle architecture and strength. These results suggest that the stimulus for adding sarcomeres in-series and in-parallel may be different, which implies that different myogenic responses were induced by the two different training methods.

The present study investigated whether differences between adults and children in mechanical power during single-joint knee extension tasks and the complex multijoint task of jumping could be explained by differences in the quadriceps femoris muscle volume. Peak power was calculated during squat jumps, from the integral of the vertical force measured by a force plate, and during concentric knee extensions at 30, 90, 120, 180 and 240 deg s(-1), and muscle volume was measured from magnetic resonance images for 10 men, 10 women, 10 prepubertal boys and 10 prepubertal girls. Peak power during jumping and isokinetic knee extension was significantly higher in men than in women, and in both adult groups compared with children (P < 0.01), although there were no differences between boys and girls. When power was normalized to muscle volume, the intergroup differences ceased to exist for both tasks. Peak power correlated significantly with quadriceps volume (P < 0.01), with r(2) values of 0.8, 0.86, 0.81, 0.78 and 0.81 from isokinetic knee extension at angular velocities of 30, 90, 120, 180 and 240 deg s(-1), respectively, and with an r(2) value of 0.9 from squat jumps. These results indicate that the quadriceps femoris muscle volume accounts largely for the increase in power that occurs with maturation in the two genders not only in kinematically constrained knee extensions but also in multijoint tasks. Future studies should examine the role of other factors relating to the generation and transmission of contractile power, such as muscle architecture, tendon stiffness and external mechanical leverage.

The age-related loss of muscle mass known as senile sarcopenia is one of the main determinants of frailty in old age. Molecular, cellular, nutritional and hormonal mechanisms are at the basis of sarcopenia and are responsible for a progressive deterioration in skeletal muscle size and function. Both at single-fibre and at whole-muscle level, the loss of force exceeds that predicted by the decrease in muscle size. For single fibres, the loss of intrinsic force is mostly due to a loss in myofibrillar protein content. For whole muscle, in addition to changes in neural drive, alterations in muscle architecture and in tendon mechanical properties, exemplified by a reduction in tendon stiffness, have recently been shown to contribute to this phenomenon. Resistance training can, however, cause substantial gains in muscle mass and strength and provides a protective effect against several of the cellular and molecular changes associated with muscle wasting and weakness. In old age, not only muscles but also tendons are highly responsive to training, since an increase in tendon stiffness has been observed after a period of increased loading. Many of the myotendinous factors characterizing ageing can be at least partly reversed by resistance training.

This study investigated changes in elderly muscle joint angle-torque relation induced by resistance training. Older adults were assigned to either training (n = 9, age 74.3 +/- 3.5 years; mean +/-s.d.) or to control groups (n = 9, age 67.1 +/- 2 years). Leg-extension and leg-press exercises were performed three times per week for 14 weeks. Maximal isometric knee extension torque was measured across the knee joint angle range of movement. Vastus lateralis muscle architecture was examined in vivo using ultrasonography. The vastus lateralis muscle fascicle force was estimated from the measured joint torque, enabling construction of the fascicle length-force relation. Electromyographic (EMG) activity was measured from representative agonist and antagonist muscles. Training altered the angle-torque relation: (a) displacing it by 9-31% towards higher torque values (P < 0.05); and (b) shifting the optimal angle from 70 deg (corresponding torque: 121.4 +/- 61 N m) before to 60 deg (134.2 +/- 57.2 N m; P < 0.05) after training. Training also altered the fascicle length-force relation: (a) displacing it by 11-35% towards higher force values; and (b) shifting the optimal fascicle length from 83.7 +/- 8 mm (corresponding force: 847.9 +/- 365.3 N) before to 93.2 +/- 12.5 mm (939.3 +/- 347.8 N; P < 0.01) after training. The upward displacement of the angle-torque relation was mainly due to a training-induced increase in agonist activation, whilst the shift in the optimal angle was associated with changes in muscle-tendon properties.