Biomedical subjects

S Peter Magnusson

Publications and source records attributed to S Peter Magnusson.

10 recordsLinked to original sources

PubMed2005

Muscle performance during maximal isometric and dynamic contractions is influenced by the stiffness of the tendinous structures.

Contractile force is transmitted to the skeleton through tendons and aponeuroses, and, although it is appreciated that the mechanocharacteristics of these tissues play an important role for movement performance with respect to energy storage, the association between tendon mechanical properties and the contractile muscle output during high-force movement tasks remains elusive. The purpose of the study was to investigate the relation between the mechanical properties of the connective tissue and muscle performance in maximal isometric and dynamic muscle actions. Sixteen trained men participated in the study. The mechanical properties of the vastus lateralis tendon-aponeurosis complex were assessed by ultrasonography. Maximal isometric knee extensor force and rate of torque development (RTD) were determined. Dynamic performance was assessed by maximal squat jumps and countermovement jumps on a force plate. From the vertical ground reaction force, maximal jump height, jump power, and force-/velocity-related determinants of jump performance were obtained. RTD was positively related to the stiffness of the tendinous structures (r = 0.55, P < 0.05), indicating that tendon mechanical properties may account for up to 30% of the variance in RTD. A correlation was observed between stiffness and maximal jump height in squat jumps and countermovement jumps (r = 0.64, P < 0.05 and r = 0.55, P < 0.05). Power, force, and velocity parameters obtained during the jumps were significantly correlated to tendon stiffness. These data indicate that muscle output in high-force isometric and dynamic muscle actions is positively related to the stiffness of the tendinous structures, possibly by means of a more effective force transmission from the contractile elements to the bone.

Adult↗

PubMed2005

Changes in the human muscle force-velocity relationship in response to resistance training and subsequent detraining.

Previous studies show that cessation of resistance training, commonly known as "detraining," is associated with strength loss, decreased neural drive, and muscular atrophy. Detraining may also increase the expression of fast muscle myosin heavy chain (MHC) isoforms. The present study examined the effect of detraining subsequent to resistance training on contractile performance during slow-to-medium velocity isokinetic muscle contraction vs. performance of maximal velocity "unloaded" limb movement (i.e., no external loading of the limb). Maximal knee extensor strength was measured in an isokinetic dynamometer at 30 and 240 degrees/s, and performance of maximal velocity limb movement was measured with a goniometer during maximal unloaded knee extension. Muscle cross-sectional area was determined with MRI. Electromyographic signals were measured in the quadriceps and hamstring muscles. Twitch contractions were evoked in the passive vastus lateralis muscle. MHC isoform composition was determined with SDS-PAGE. Isokinetic muscle strength increased 18% (P < 0.01) and 10% (P < 0.05) at slow and medium velocities, respectively, along with gains in muscle cross-sectional area and increased electromyogram in response to 3 mo of resistance training. After 3 mo of detraining these gains were lost, whereas in contrast maximal unloaded knee extension velocity and power increased 14% (P < 0.05) and 44% (P < 0.05), respectively. Additionally, faster muscle twitch contractile properties along with an increased and decreased amount of MHC type II and MHC type I isoforms, respectively, were observed. In conclusion, detraining subsequent to resistance training increases maximal unloaded movement speed and power in previously untrained subjects. A phenotypic shift toward faster muscle MHC isoforms (I --> IIA --> IIX) and faster electrically evoked muscle contractile properties in response to detraining may explain the present results.

Adaptation, Physiological↗

PubMed2005

Neuromuscular adaptations to detraining following resistance training in previously untrained subjects.

Resistance training has been shown to considerably increase strength and neural drive during maximal eccentric muscle contraction; however, less is known about the adaptive change induced by subsequent detraining. The purpose of the study was to examine the effect of dynamic resistance training followed by detraining on changes in maximal eccentric and concentric isokinetic muscle strength, as well as to examine the corresponding adaptations in muscle cross-sectional area (CSA) and EMG activity. Maximal concentric and eccentric isokinetic knee extensor moment of force was measured in 13 young sedentary males (age 23.5+/-3.2 years), before and after 3 months of heavy resistance training and again after 3 months of detraining. Following training, moment of force increased during slow eccentric (50%, P<0.001), fast eccentric (25%, P<0.01), slow concentric (19%, P<0.001) and fast concentric contraction (11%, P<0.05). Corresponding increases in EMG were observed during eccentric and slow concentric contraction. Significant correlations were observed between the training-induced changes in moment of force and EMG (R(2)=0.33-0.77). Muscle CSA (measured by MRI) increased by 10% (P<0.001). After 3 months of detraining maximal muscle strength and EMG remained preserved during eccentric contraction but not concentric contraction. The present findings suggest that heavy resistance training induces long-lasting strength gains and neural adaptations during maximal eccentric muscle contraction in previously untrained subjects.

Adaptation, Physiological↗

PubMed2005

Exercise-induced rib stress fractures: influence of reduced bone mineral density.

Exercise-induced rib stress fractures have been reported frequently in elite rowers during the past decade. The etiology of rib stress fractures is unclear, but low bone mineral density (BMD) has been suggested to be a potential risk factor for stress fractures in weight-bearing bones. The present study investigated BMD in seven Danish national team rowers with previous rib stress fracture (RSF) and 7 controls (C) matched for gender, age, height, weight and training experience. Total body scan and specific scans of the lumbar spine (L2-L4), femoral neck and distal radius were performed using a DEXA scanner. The RSF subjects showed significantly lower L2-L4 BMD: RSF: 1.22+/-0.05 g cm(-2) (mean+/-SEM) (median 1.19 g cm(-2), range 1.02-1.37 g cm(-2)) compared to C: 140+/-0.04 g cm(-2) (median 1.41 g cm(-2), range 1.27-1.57 g cm(-2)) (P=0.028). The present results suggest that low bone mineral density may be a potential risk factor for the development of exercise-induced rib stress fractures in elite rowers.

Absorptiometry, Photon↗

PubMed2004

Training-induced changes in muscle CSA, muscle strength, EMG, and rate of force development in elderly subjects after long-term unilateral disuse.

The ability to develop muscle force rapidly may be a very important factor to prevent a fall and to perform other tasks of daily life. However, information is still lacking on the range of training-induced neuromuscular adaptations in elderly humans recovering from a period of disuse. Therefore, the present study examined the effect of three types of training regimes after unilateral prolonged disuse and subsequent hip-replacement surgery on maximal muscle strength, rapid muscle force [rate of force development (RFD)], muscle activation, and muscle size. Thirty-six subjects (60-86 yr) were randomized to a 12-wk rehabilitation program consisting of either 1) strength training (3 times/wk for 12 wk), 2) electrical muscle stimulation (1 h/day for 12 wk), or 3) standard rehabilitation (1 h/day for 12 wk). The nonoperated side did not receive any intervention and thereby served as a within-subject control. Thirty subjects completed the trial. In the strength-training group, significant increases were observed in maximal isometric muscle strength (24%, P < 0.01), contractile RFD (26-45%, P < 0.05), and contractile impulse (27-32%, P < 0.05). No significant changes were seen in the two other training groups or in the nontrained legs of all three groups. Mean electromyogram signal amplitude of vastus lateralis was larger in the strength-training than in the standard-rehabilitation group at 5 and 12 wk (P < 0.05). In contrast to traditional physiotherapy and electrical stimulation, strength training increased muscle mass, maximal isometric strength, RFD, and muscle activation in elderly men and women recovering from long-term muscle disuse and subsequent hip surgery. The improvement in both muscle mass and neural function is likely to have important functional implications for elderly individuals.

Aged↗

PubMed2004

Differential displacement of the human soleus and medial gastrocnemius aponeuroses during isometric plantar flexor contractions in vivo.

The human triceps surae muscle-tendon complex is a unique structure with three separate muscle compartments that merge via their aponeuroses into the Achilles tendon. The mechanical function and properties of these structures during muscular contraction are not well understood. The purpose of the study was to investigate the extent to which differential displacement occurs between the aponeuroses of the medial gastrocnemius (MG) and soleus (Sol) muscles during plantar flexion. Eight subjects (mean +/- SD; age 30 +/- 7 yr, body mass 76.8 +/- 5.5 kg, height 1.83 +/- 0.06 m) performed maximal isometric ramp contractions with the plantar flexor muscles. The experiment was performed in two positions: position 1, in which the knee joint was maximally extended, and position 2, in which the knee joint was maximally flexed (125 degrees ). Plantarflexion moment was assessed with a strain gauge load cell, and the corresponding displacement of the MG and Sol aponeuroses was measured by ultrasonography. Differential shear displacement of the aponeurosis was quantified by subtracting displacement of Sol from that of MG. Maximal plantar flexion moment was 36% greater in position 1 than in position 2 (132 +/- 20 vs. 97 +/- 11 N.m). In position 1, the displacement of the MG aponeurosis at maximal force exceeded that of the Sol (12.6 +/- 1.7 vs. 8.9 +/- 1.5 mm), whereas in position 2 displacement of the Sol was greater than displacement of the MG (9.6 +/- 1.0 vs. 7.9 +/- 1.2 mm). The amount and "direction" of shear between the aponeuroses differed significantly between the two positions across the entire range of contraction, indicating that the Achilles tendon may be exposed to intratendinous shear and stress gradients during human locomotion.

Adult↗

PubMed2004

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. DESIGN: A prospective randomized, controlled study. SETTING: Bispebjerg University Hospital, Copenhagen, Denmark. PARTICIPANTS: Thirty-six patients (aged 60-86) scheduled for unilateral hip replacement due to primary hip osteoarthrosis. INTERVENTION: Patients were randomized to standard home-based rehabilitation (1 h/d x 12 weeks), unilateral neuromuscular electrical stimulation of the operated side (1 h/d x 12 weeks), or unilateral resistance training of the operated side (3/wk x 12 weeks). MEASUREMENTS: Hospital length of stay (LOS), quadriceps muscle cross-sectional area (CSA), isokinetic muscle strength, and functional performance. Patients were tested presurgery and 5 and 12 weeks postsurgery. RESULTS: Mean+/-standard error LOS was shorter for the resistance training group (10.0+/-2.4 days, P<.05) than for the standard rehabilitation group (16.0+/-7.2 days). Resistance training, but not electrical stimulation or standard rehabilitation, resulted in increased CSA (12%, P<.05) and muscle strength (22-28%, P<.05). Functional muscle performance increased after resistance training (30%, P<.001) and electrical stimulation (15%, P<.05) but not after standard rehabilitation. CONCLUSION: Postoperative resistance training effectively increased maximal muscle strength, muscle mass, and muscle function more than a standard rehabilitation regime. Furthermore, it markedly reduced LOS in elderly postoperative patients.

Activities of Daily Living↗

PubMed2003

Region-specific differences in Achilles tendon cross-sectional area in runners and non-runners.

The present study examined the cross-sectional area (CSA) of the Achilles tendon in subjects who repeatedly exposed their tendons to large loads (habitual runners) compared to control subjects (non-runners). Six male habitual runners [36 (7) years, 70.9 (4.4) kg and 1.84 (0.05) m, +/-SD] who had performed distance running (approximately 80 km per week) for the last 5 years were compared to six non-runners [34 (3) years, 81.2 (8.7) kg and 1.81 (0.02) m, +/-SD]. Tendon CSA was obtained from MR images obtained with the ankle in a neutral position (90 degrees ). The most proximal aspect of the tuberosity of calcaneus was used as a landmark to standardize the levels of images: the most distal image (1) was obtained 10 mm above the proximal tuberosity of calcaneus, and the most proximal image (7) was obtained 70 mm above the proximal tuberosity of calcaneus. There was a significant difference in CSA along the length of the tendon both in runners ( P<0.001) and non-runners ( P<0.01). In non-runners and runners the CSA of the most distal part was 51% and 85% greater than the most proximal part of the tendon, respectively. Furthermore, there was a difference in tendon CSA between the groups, such that runners had a greater CSA (36%) than non-runners at the most distal part of the tendon ( P<0.05). The greater CSA in the distal tendon may reflect differences in structural properties along the length of the human Achilles tendon, while the greater CSA in runners compared to non-runners may indicate a region-specific hypertrophy in response to the habitual loading of running.

Achilles Tendon↗

PubMed2003

Increased cross-sectional area and reduced tensile stress of the Achilles tendon in elderly compared with young women.

The Achilles tendon cross-sectional area (CSA), tensile force, and stress during an isometric contraction were examined in healthy young (n = 9, age = 29 +/- 1 years, mean +/- SEM) and elderly (n = 10, 79 +/- 2 years) women. CSA area was obtained with magnetic resonance imaging 3 cm proximal to the insertion, and tendon force was obtained from the isometric ankle moment. The moment of force about the ankle joint was greater in young women (95 +/- 17 N m) than in elderly women (51 +/- 5 N m; p <.05). The Achilles tendon CSA was significantly greater in elderly women (56.3 +/- 3.0 mm(2)) than in young women (46.0 +/- 1.9 mm(2); p <.01). These data show that young women can exert a greater force than elderly women on the Achilles tendon during voluntary contraction, although elderly women have an increased (22%) tendon CSA, and a lower tendon force than young women. The greater tendon size combines to lower the stress on the tendon markedly, which may reduce the risk of injury to the tendon.

Achilles Tendon↗

PubMed2001

Interindividual differences in H reflex modulation during normal walking.

Based on previous studies, at least two different types of soleus Hoffmann (H) reflex modulation were likely to be found during normal human walking. Accordingly, the aim of the present study was to identify different patterns of modulation of the soleus H reflex and to examine whether or not subjects with different H reflex modulation would exhibit different walking mechanics and different EMG activity. Fifteen subjects walked across two force platforms at 4.5 km/h (+/-10%) while the movements were recorded on video. The soleus H reflex and EMG activity were recorded separately during treadmill walking at 4.5 km/h. Using a two-dimensional analysis joint angles, angular velocities, accelerations, linear velocities and accelerations were calculated, and net joint moments about the ankle, knee and hip joint were computed by inverse dynamics from the video and force plate data. Six subjects (group S) showed a suppressed H reflex during the swing phase, and 9 subjects (group LS) showed increasing reflex excitability during the swing phase. The plantar flexor dominated moment about the ankle joint was greater for group LS. In contrast, the extensor dominated moment about the knee joint was greater for the S group. The hip joint moment was similar for the groups. The EMG activity in the vastus lateralis and anterior tibial muscles was greater prior to heel strike for the S group. These data indicate that human walking exhibits at least two different motor patterns as evaluated by gating of afferent input to the spinal cord, by EMG activity and by walking mechanics. Increasing H reflex excitability during the swing phase appears to protect the subject against unexpected perturbations around heel strike by a facilitated stretch reflex in the triceps surae muscle. Alternatively, in subjects with a suppressed H reflex in the swing phase the knee joint extensors seem to form the primary protection around heel strike.

Adult↗