S. Peter Magnusson

A mechanism for increased contractile strength of human pennate muscle in response to strength training: changes in muscle architecture

1 In human pennate muscle, changes in anatomical cross‐sectional area (CSA) or volume caused by training or inactivity may not necessarily reflect the change in physiological CSA, and thereby in maximal contractile force, since a simultaneous change in muscle fibre pennation angle could also occur. 2 Eleven male subjects undertook 14 weeks of heavy‐resistance strength training of the lower limb muscles. Before and after training anatomical CSA and volume of the human quadriceps femoris muscle were assessed by use of magnetic resonance imaging (MRI), muscle fibre pennation angle (θp) was measured in the vastus lateralis (VL) by use of ultrasonography, and muscle fibre CSA (CSAfibre) was obtained by needle biopsy sampling in VL. 3 Anatomical muscle CSA and volume increased with training from 77.5 ± 3.0 to 85.0 ± 2.7 cm2 and 1676 ± 63 to 1841 ± 57 cm3, respectively (±s.e.m.). Furthermore, VL pennation angle increased from 8.0 ± 0.4 to 10.7 ± 0.6 deg and CSAfibre increased from 3754 ± 271 to 4238 ± 202 μm2. Isometric quadriceps strength increased from 282.6 ± 11.7 to 327.0 ± 12.4 N m. 4 A positive relationship was observed between θp and quadriceps volume prior to training (r = 0.622). Multifactor regression analysis revealed a stronger relationship when θp and CSAfibre were combined (R= 0.728). Post‐training increases in CSAfibre were related to the increase in quadriceps volume (r = 0.749). 5 Myosin heavy chain (MHC) isoform distribution (type I and II) remained unaltered with training. 6 VL muscle fibre pennation angle was observed to increase in response to resistance training. This allowed single muscle fibre CSA and maximal contractile strength to increase more (+16 %) than anatomical muscle CSA and volume (+10 %). 7 Collectively, the present data suggest that the morphology, architecture and contractile capacity of human pennate muscle are interrelated, in vivo. This interaction seems to include the specific adaptation responses evoked by intensive resistance training.

Load-displacement properties of the human triceps surae aponeurosis in vivo

1 The present investigation measured the load‐displacement and stress‐strain characteristics of the proximal and distal human triceps surae aponeurosis and tendon in vivo during graded voluntary 10 s isometric plantarflexion efforts in five subjects. 2 During the contractions synchronous real‐time ultrasonography of aponeurosis displacement, electromyography of the gastrocnemius, soleus and dorsiflexor muscles, and joint angular rotation were obtained. Tendon cross‐sectional area and moment arm were obtained from magnetic resonance (MR) images. Force and electromyography data from dorsiflexion efforts were used to examine the effect of coactivation. 3 Tendon force was calculated from the joint moments and tendon moment arm, and stress was obtained by dividing force by cross‐sectional area. Aponeurosis and tendon strain were obtained from the displacements normalised to tendon length. 4 Tendon force was 3171 ± 201 N, which corresponded to 2.6 % less than the estimated force when coactivation was accounted for (3255 ± 206 N). Aponeurosis displacement (13.9‐ 12.9 mm) decreased 30 % (9.6‐10.7 mm) when accounting for joint angular rotation (3.6 deg). Coactivation and angular rotation‐corrected stiffness yielded a quadratic relationship, R2= 0.98± 0.01, which was similar for the proximal (467 N mm−1) and distal (494 N mm−1) aponeurosis and tendon. Maximal strain and stress were 4.4‐5.6 % and 41.6 ± 3.9 MPa, respectively, which resulted in a Youngs modulus of 1048‐1474 MPa. 5 The mechanical properties of the human triceps surae aponeurosis and tendon in vivo were for the first time examined. The stiffness and Youngs modulus exceeded those previously reported for the tibialis anterior tendon in vivo, but were similar to those obtained for various isolated mammalian and human tendons.

A New Concept For Isokinetic Hamstring: Quadriceps Muscle Strength Ratio

Conventionally, the hamstring:quadriceps strength ratio is calculated by dividing the maximal knee flexor (hamstring) moment by the maximal knee extensor (quadriceps) moment measured at identical angular velocity and contraction mode. The agonist-antagonist strength relationship for knee extension and flexion may, however, be better described by the more functional ratios of eccentric hamstring to concentric quadriceps moments (extension), and concentric hamstring to eccentric quadriceps moments (flexion). We compared functional and conventional isokinetic hamstring: quadriceps strength ratios and examined their relation to knee joint angle and joint angular velocity. Peak and angle-specific (50°, 40°, and 30° of knee flexion) moments were determined during maximal concentric and eccentric muscle contractions (10° to 90° of motion; 30 and 240 deg/sec). Across movement speeds and contraction modes the functional ratios for different moments varied between 0.3 and 1.0 (peak and 50°), 0.4 and 1.1 (40°), and 0.4 and 1.4 (30°). In contrast, conventional hamstring:quadriceps ratios were 0.5 to 0.6 based on peak and 50° moments, 0.6 to 0.7 based on 40° moment, and 0.6 to 0.8 based on 30° moment. The functional hamstring:quadriceps ratio for fast knee extension yielded a 1:1 relationship, which increased with extended knee joint position, indicating a significant capacity of the hamstring muscles to provide dynamic knee joint stability in these conditions. The evaluation of knee joint function by use of isokinetic dynamometry should comprise data on functional and conventional hamstring:quadriceps ratios as well as data on absolute muscle strength.

Biomechanical responses to repeated stretches in human hamstring muscle in vivo

To examine stiffness, energy, and passive torque in the dynamic and static phases of a stretch maneuver in the human hamstring muscle in vivo we used a test- retest protocol and a repeated stretches protocol. Re sistance to stretch was defined as passive torque (in newton-meters) offered by the hamstring muscle group during passive knee extension as measured using an isokinetic dynamometer with a modified thigh pad. In 13 uninjured subjects, the knee was passively ex tended to a predetermined final position (0.0875 rad/ sec, dynamic phase) where it remained stationary for 90 seconds (static phase). The test-retest protocol in cluded two tests administered 1 hour apart. On a sep arate occasion, five consecutive static stretches were administered separated by 30 seconds and followed by a sixth stretch 1 hour later. For the test-retest phase, stiffness and energy in the dynamic phase and passive torque in the static phase did not differ and yielded correlations of r = 0.91 to 0.99. During the static phase, passive torque declined in both tests (P < 0.0001). For the repeated stretches, decreases were observed for energy (P < 0.01) and stiffness (P < 0.05) in the dynamic phase and for passive torque (P < 0.0001) in the static phase. However, the decline in the variables returned to baseline within 1 hour. The data show that the method employed is a useful tool for measuring biomechanical variables during a stretch maneuver. This may provide a more detailed method to examine skeletal muscle flexibility.

Human tendon behaviour and adaptation, in vivo

Tendon properties contribute to the complex interaction of the central nervous system, muscle–tendon unit and bony structures to produce joint movement. Until recently limited information on human tendon behaviour in vivo was available; however, novel methodological advancements have enabled new insights to be gained in this area. The present review summarizes the progress made with respect to human tendon and aponeurosis function in vivo, and how tendons adapt to ageing, loading and unloading conditions. During low tensile loading or with passive lengthening not only the muscle is elongated, but also the tendon undergoes significant length changes, which may have implications for reflex responses. During active loading, the length change of the tendon far exceeds that of the aponeurosis, indicating that the aponeurosis may more effectively transfer force onto the tendon, which lengthens and stores elastic energy subsequently released during unloading, in a spring‐like manner. In fact, data recently obtained in vivo confirm that, during walking, the human Achilles tendon provides elastic strain energy that can decrease the energy cost of locomotion. Also, new experimental evidence shows that, contrary to earlier beliefs, the metabolic activity in human tendon is remarkably high and this affords the tendon the ability to adapt to changing demands. With ageing and disuse there is a reduction in tendon stiffness, which can be mitigated with resistance exercises. Such adaptations seem advantageous for maintaining movement rapidity, reducing tendon stress and risk of injury, and possibly, for enabling muscles to operate closer to the optimum region of the length–tension relationship.

Mechanical and physiological responses to stretching with and without preisometric contraction in human skeletal muscle

Abstract Objective: To examine electromyography (EMG) activity, passive torque, and stretch perception during static stretch and contract-relax stretch. Design: Two separate randomized crossover protocols: (1) a constant angle protocol on the right side, and (2) a variable angle protocol on the left side. Subjects: 10 male volunteers. Intervention: Stretch-induced mechanical response in the hamstring muscles during passive knee extension was measured as knee flexion torque (Nm) while hamstring surface EMG was measured. Final position was determined by extending the knee to an angle that provoked a sensation similar to a stretch maneuver. Constant angle stretch: The knee was extended to 10/dg below final position, held 10sec, then extended to the final position and held for 80sec. Variable angle stretch: The knee was extended from the starting position to 10/dg below the final position, held 10sec, then extended to the onset of pain. Subjects produced a 6-sec isometric contraction with the hamstring muscles 10/dg below the final position in the contract-relax stretch, but not in the static stretch. Main Outcome Measures: Passive torque, joint range of motion, velocity, and hamstring EMG were continuously recorded. Results: Constant angle contract-relax and static stretch did not differ in passive torque or EMG response. In the final position, passive torque declined 18% to 21% in both contract-relax and static stretch ( p p Conclusion: At a constant angle the viscoelastic and EMG response was unaffected by the isometric contraction. The variable angle protocol demonstrated that PNF stretching altered stretch perception.

Shoulder Strength and Range of Motion in Symptomatic and Pain-Free Elite Swimmers

To evaluate differences in shoulder strength and range of motion between painful and pain-free shoulders we examined two matched groups of athletes. Fifteen competitive swimmers were allocated to two groups. Group 1 consisted of seven swimmers with unilateral shoulder pain related to swimming (Neer and Welsh phase I to II). The control group (Group 2) consisted of eight swimmers with no present or previous history of shoulder pain. Concentric and eccentric internal rota tional torques were reduced in painful shoulders in between-group comparisons as well as in side-to-side comparisons. The decrease in internal rotational torque resulted in significantly greater concentric and eccentric external-to-internal rotational strength ratios of the painful shoulder in Group 1 swimmers compared with the controls. Furthermore, the functional ratio (ec centric external rotation:concentric internal rotation) was significantly greater in the painful shoulder in both between-group and side-to-side comparisons. Both groups of swimmers exhibited increased external range of motion and reduced internal range of motion compared with normalized data, but no between-group or side-to-side differences were detected. Our findings suggest that prevention or rehabilitation of swimmers shoulder might not solely involve strengthening of the external rotators of the shoulder joint. Attention might also be drawn toward correction of a possible deficit in internal rotational strength. Changes in shoulder range of motion seem unrelated to the occurrence of shoul der pain.

Resistance Training in the Early Postoperative Phase Reduces Hospitalization and Leads to Muscle Hypertrophy in Elderly Hip Surgery Patients—A Controlled, Randomized Study

Objectives: To better understand how immobilization and surgery affect muscle size and function in the elderly and to identify effective training regimes.

Viscoelastic stress relaxation in human skeletal muscle

Viscoelastic stress relaxation refers to the decrease in tensile stress over time that occurs when a body under tensile stress is held at a fixed length. The purpose of this study was to demonstrate viscoelastic stress relaxation in human skeletal muscle. Resistance to stretch (tensile force), hip flexion range of motion (ROM), and reflex contractile activity (IEMG) of the hamstring muscle group were measured during a passive straight leg raise. The testing protocol involved a first stretch to the maximum tolerated ROM with the lower extremity held at that point for 45 s (test 1). All 15 subjects tested (9 men, 6 women) had a stretch induced EMG response. The onset of a sustained EMG response occurred at a specific hip flexion angle in 10 subjects. These 10 subjects (6 men, 4 women) underwent a second straight leg raise stretch (test 2) to a ROM 5 degrees below the ROM at which the onset of EMG activity occurred in test 1. The stretch was held at this hip flexion angle for 45 s. There was a significant decrease in force at final ROM during the 45 s in test 1 (11.35 +/- 1.75 N, P < 0.0001) and in test 2 (4.2 +/- 1.55 N, P < 0.05). The percent decrease from the force at the respective final ROM was not significantly different between the tests (14.4 +/- 2.2% in test 1 and 13 +/- 2.3% in test 2). In test 1 there was a significant decrease over time in IEMG of 59.71 +/- 16.01 microV.s (P < 0.01) which was not significantly correlated to the decrease in force.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of neuromuscular training on knee joint motor control during sidecutting in female elite soccer and handball players.

Objective:The project aimed to implement neuromuscular training during a full soccer and handball league season and to experimentally analyze the neuromuscular adaptation mechanisms elicited by this training during a standardized sidecutting maneuver known to be associated with non-contact anterior cruciate ligament (ACL) injury. Design:The players were tested before and after 1 season without implementation of the prophylactic training and subsequently before and after a full season with the implementation of prophylactic training. Participants:A total of 12 female elite soccer players and 8 female elite team handball players aged 26 ± 3 years at the start of the study. Intervention:The subjects participated in a specific neuromuscular training program previously shown to reduce non-contact ACL injury. Methods:Neuromuscular activity at the knee joint, joint angles at the hip and knee, and ground reaction forces were recorded during a sidecutting maneuver. Neuromuscular activity in the prelanding phase was obtained 10 and 50 ms before foot strike on a force plate and at 10 and 50 ms after foot strike on a force plate. Results:Neuromuscular training markedly increased before activity and landing activity electromyography (EMG) of the semitendinosus (P < 0.05), while quadriceps EMG activity remained unchanged. Conclusions:Neuromuscular training increased EMG activity for the medial hamstring muscles, thereby decreasing the risk of dynamic valgus. This observed neuromuscular adaptation during sidecutting could potentially reduce the risk for non-contact ACL injury.