Poul Dyhre-Poulsen

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.

Load-displacement properties of the human triceps surae aponeurosis and tendon in runners and non-runners

The load‐displacement and stress–strain characteristics of the human triceps surae tendon and aponeurosis, in vivo, was examined during graded maximal voluntary plantarflexion efforts in runners who trained 80 km/ week or more and age‐matched non‐runners. Synchronous real‐time ultrasonography of triceps surae tendon and aponeurosis displacement, electromyography of the gastrocnemius, soleus and dorsiflexor muscles, and joint angular rotation were obtained. Tendon cross‐sectional area and ankle joint moment arm were obtained from magnetic resonance imaging. Tensile tendon force was calculated from the joint moments and tendon moment arm and stress was obtained by dividing force by cross‐sectional area. Strain was obtained from the displacements normalized to tendon length. Antagonist coactivation and small amounts of ankle joint rotation significantly affected tensile tendon force and aponeurosis and tendon displacement, respectively (P < 0.01). Plantarflexion moment was similar in runners (138 ± 27 Nm, mean ± SEM) and non‐runners (142 ± 17 Nm). Tendon moment arm was alike in non‐runner (58.3 ± 0.2 mm) and runners (55.1 ± 0.1 mm). Similarly, there was no difference in tendon tensile force between runners (2633 ± 465 N) and non‐runners (2556 ± 401 N). The cross‐sectional area of the Achilles tendon was larger in runners (95 ± 3 mm2) than non‐runners (73 ± 3 mm2) (P < 0.01). The load‐deformation data yielded similar stiffness (runners 306 ± 61 N/mm, non‐runners 319 ± 42 N/mm). The maximal strain and stress was 4.9 ± 0.8% and 38.2 ± 9.8 MPa in non‐runners and 4.1 ± 0.8% and 26.3 ± 5.1 MPa in runners. The larger tendon cross‐sectional area in trained runners suggests that chronic exposure to repetitive loading has resulted in a tissue adaptation.

Dynamic control of muscle stiffness and H reflex modulation during hopping and jumping in man.

1. The objective of the study was to evaluate the functional effects of reflexes on muscle mechanics during natural voluntary movements. The excitability of the H (Hoffmann) reflex was used as a measure of the excitability of the central component of the stretch reflex. 2. We recorded EMG, ground reaction forces and the H reflex in the soleus muscle in humans while landing from a downward jump, during drop jumping and during hopping. The movements were also recorded by high‐speed cinematography. 3. The EMG pattern was adapted to the motor task. When landing the EMG in the soleus muscle and in the anterior tibial muscle showed preinnervation and alternating activity after touch down. When hopping there was little preinnervation in the soleus muscle, and the activity was initiated about 45 ms after touch down by a peak and continued unbroken until lift off. In the drop jumps the EMG pattern depended on the jumping style used by the subject. 4. The H reflex in the soleus muscle was strongly modulated in a manner appropriate to the requirements of the motor task. During landing from a downward jump the H reflex was low at touch down whereas while hopping it was high at touch down. During drop jumping it was variable and influenced by the jumping technique. 5. Muscle stiffness in the ankle joint was negative after touch down when landing, but always positive when hopping. 6. It is suggested that during landing the alternating EMG pattern after touch down was programmed and little influenced by reflexes. During hopping reflexes could contribute to the initial peak and the EMG during lift off. 7. The programmed EMG activity and the suppression of the H reflex while landing probably contribute to the development of the negative stiffness and change the muscles from a spring to a damping unit.

Antagonist muscle coactivation during isokinetic knee extension

The aim of the present study was to quantify the amount of antagonist coactivation and the resultant moment of force generated by the hamstring muscles during maximal quadriceps contraction in slow isokinetic knee extension. The net joint moment at the knee joint and electromyographic (EMG) signals of the vastus medialis, vastus lateralis, rectus femoris muscles (quadriceps) and the biceps femoris caput longum and semitendinosus muscles (hamstrings) were obtained in 16 male subjects during maximal isokinetic knee joint extension (KinCom, ROM 90–10°, 30° · s−1). Two types of extension were performed: [1] maximal concentric quadriceps contractions and [2] maximal eccentric hamstring contractions. Hamstring antagonist EMG in [1] were converted into antagonist moment based on the EMG‐moment relationships determined in [2] and vice versa. Since antagonist muscle coactivation was present in both [1] and [2] a set of related equations was constructed to yield the moment/EMG relationships for the hamstring and quadriceps muscles, respectively. The equations were solved separately for every 0.05° knee joint angle in the 90–10° range of excursion (0°=full extension) ensuring that the specificity of muscle length and internal muscle lever arms were incorporated into the moment/EMG relationships established. Substantial hamstring coactivation was observed during quadriceps agonist contraction. This resulted in a constant level of antagonist hamstring moment of about 30 Nm throughout the range of motion. In the range of 30–10° from full knee extension this antagonist hamstring moment corresponded to 30–75% of the measured knee extensor moment. The level of antagonist coactivation was 3‐fold higher for the lateral (Bfcl) compared to medial (ST) hamstring muscles. The amount of EMG crosstalk between agonist–antagonist muscle pairs was negligible (RXY2<0.02–0.06). The present data show that substantial antagonist coactivation of the hamstring muscles may be present during slow isokinetic knee extension. In consequence substantial antagonist flexor moments are generated. The antagonist hamstring moments potentially counteract the anterior tibial shear and excessive internal tibial rotation induced by the contractile forces of the quadriceps near full knee extension. In doing so the hamstring coactivation is suggested to assist the mechanical and neurosensory functions of the anterior cruciate ligament (ACL).

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.

Viscoelastic stress relaxation during static stretch in human skeletal muscle in the absence of EMG activity

The present study sought to investigate the role of EMG activity during passive static stretch. EMG and passive resistance were measured during static stretching of human skeletal muscle in eight neurologically intact control subjects and six spinal cord‐injured (SCI) subjects with complete motor loss. Resistance to stretch offered by the hamstring muscles during passive knee extension was defined as passive torque (Nm). The knee was passively extended at 5o/s to a predetermined final position, where it remained stationary for 90 s (static phase) while force and integrated EMG of the hamstring muscle were recorded. EMG was sampled for frequency domain analysis in a second stretch maneuver in five control and three SCI subjects. There was a decline in passive torque in the 90‐s static phase for both control and SCI subjects, P<0.05. Although peak passive torque was greater in control subjects, P<0.05, there was no difference in time‐dependent passive torque response between control (33%) and SCI (38%) subjects. Initial and final 5‐s IEMG ranged from 1.8 to 3.4 μ V.s and did not change during a stretch or differ between control and SCI subjects. Frequency domain analysis yielded similar results in both groups, with an equal energy distribution in all harmonics, indicative of ‘white noise’. The present data demonstrate that no measurable EMG activity was detected in either group during the static stretch maneuver. Therefore, the decline in resistance to static stretch was a viscoelastic stress relaxation response.

Amplitude of the human soleus H reflex during walking and running

1 The objective of the study was to investigate the amplitude and modulation of the human soleus Hoffmann (H) reflex during walking and during running at different speeds. 2 EMGs were recorded with surface electrodes from the soleus, the medial and lateral head of the gastrocnemius, the vastus lateralis and the anterior tibial muscles. The EMGs and the soleus H reflex were recorded while walking on a treadmill at 4.5 km h−1 and during running at 8, 12 and 15 km h−1. 3 The amplitudes of the M wave and the H reflex were normalized to the amplitude of a maximal M wave elicited by a supramaximal stimulus just after the H reflex to compensate for movements of the recording and stimulus electrodes relative to the nerve and muscle fibres. The stimulus intensity was set to produce M waves that had an amplitude near to 25% of the maximal M wave measured during the movements. As an alternative, the method of averaging of sweeps in sixteen intervals of the gait cycle was applied to the data. In this case the amplitude of the H reflex was expressed relative to the maximal M wave measured whilst in the standing position. 4 The amplitude of the H reflex was modulated during the gait cycle at all speeds. During the stance phase the reflex was facilitated and during the swing and flight phases it was suppressed. The size of the maximal M wave varied during the gait cycle and this variation was consistent for each subject although different among subjects. 5 The peak amplitude of the H reflex increased significantly (P= 0.04) from walking at 4.5 km h−1 to running at 12 and 15 km h−1 when using the method of correcting for variations of the maximal M wave during the gait cycle. The sweep averaging method showed a small but non‐significant decrease (P= 0.3) from walking to running at 8 km h−1 and a small decrease with running speed (P= 0.3). The amplitude of the EMG increased from walking to running and with running speed. 6 The relatively large H reflex recorded during the stance phase in running indicates that the stretch reflex may influence the muscle mechanics during the stance phase by contributing to the motor output and enhancing muscle stiffness.

Mechanical and muscular factors influencing the performance in maximal vertical jumping after different prestretch loads.

The objective of the present work was to study the interaction between the tendon elasticity, the muscle activation-loading dynamics, specific actions of the biarticular muscles, preloading and jumping performance during maximal vertical jumping. Six male expert jumpers participated in the study. They performed maximal vertical jumps with five different preloads. The kinematics and dynamics of the jumping movements were analysed from force plate and high speed film recordings. The amount of elastic energy stored in the tendons of the leg extensor muscles was calculated by a generalised tendon model, and the muscle coordination was analysed by surface EMG. The best jumping performances were achieved in the jumps with low preloads (counter movement jumps and drop jumps from 0.3 m). A considerable amount of the energy imposed on the legs by prestretch loading was stored in the tendons (26 +/- 3%), but the increased performance could not be explained by a contribution of elastic energy to the positive work performed during the push off. During the preloading, the involved muscles were activated at the onset of the loading. Slow prestretches at the onset of muscle activation under relatively low average stretch loads, as observed during counter movement jumps and drop jumps from 0.3 m, prevented excessive stretching of the muscle fibres in relation to the tendon length changes. This consequently conserved the potential of the muscle fibres to produce positive work during the following muscle-tendon shortening in concert with the release of the tendon strain energy. A significant increase in the activity of m. rectus femoris between jumps with and without prestretch indicated a pronounced action of m. rectus femoris in a transport of mechanical energy produced by the proximal monoarticular m. gluteus maximus at the hip to the knee and thereby enhanced the transformation of rotational joint work to translational work on the mass centre of the body. The changes in muscle activity were reflected in the net muscle powers. Vertical jumping is like most movements constrained by the intended direction of the movement. The movements of the body segments during the prestretches induced a forward rotation and during the take off, a backward rotation of the body. A reciprocal shift in the activities of the biarticular m. rectus femoris and m. semitendinosus indicated that these rotations were counteracted by changes in the direction of the resultant ground reaction vector controlled by these muscles.(ABSTRACT TRUNCATED AT 400 WORDS)