Mechanical, morphological and biochemical adaptations of bone and muscle to hindlimb suspension and exercise

The influences of weightbearing forces on the structural remodeling, matrix biochemistry, and mechanical characteristics of the rat tibia and femur and surrounding musculature were examined by means of a hindlimb suspension protocol and highly intensive treadmill running. Female, young adult, Sprague-Dawley rats were designated as either normal control, sedentary suspended, or exercise suspended rats. For 4 weeks, sedentary suspended rats were deprived of hindlimb-to-ground contact forces, while the exercise suspended rats experienced hindlimb ground reaction forces only during daily intensive treadmill training sessions. The suspension produced generalized atrophy of hindlimb skeletal muscles, with greater atrophy occurring in predominantly slow-twitch extensors and adductors, as compared with the mixed fiber-type extensors and flexors. Region-specific cortical thinning and endosteal resorption in tibial and femoral diaphyses occurred in conjunction with decrements in bone mechanical properties. Tibial and femoral regional remodeling was related to both the absence of cyclic bending strains due to normal weightbearing forces and the decrease in forces applied to bone by antigravity muscles. To a moderate extent, the superimposed strenuous running counteracted muscular atrophy during the suspension, particularly in the predominantly slow-twitch extensor and adductor muscles. The exercise did not, however, mitigate changes in bone mechanical properties and cross-sectional morphologies, and in some cases exacerbated the changes. Suspension with or without exercise did not alter the normal concentrations of collagen, phosphorus, and calcium in either tibia or femur.

Structural and mechanical adaptation of immature bone to strenuous exercise

To investigate the adaptive responses of immature bone to increased loads, young (3-wk-old) White Leghorn roosters were subjected to moderately intense treadmill running for 5 or 9 wk. The training program induced significant increases in maximal O2 consumption and muscle fumarase activity in the 12-wk-old birds, demonstrating that growing chickens have the ability to enhance their aerobic capacity. The structural and mechanical properties of the runners' tarsometatarsus bones were compared with sedentary age-matched controls at 8 and 12 wk of age. Suppression of circumferential growth occurred with exercise at both ages, whereas exercise enhanced middiaphysial cortical thickening, especially on the bones' concave surfaces. Although cross-sectional area moments of inertia did not change with exercise, significant decreases in bending stiffness, energy to yield, and energy to fracture were observed. It was concluded that strenuous exercise may retard long-bone maturation, resulting in more compliant bones.

Adaptation of rat knee meniscus to prolonged exercise

The morphological and biochemical adaptations of knee meniscus to prolonged exercise were studied. Female Sprague-Dawley rats maintained under controlled environmental conditions were randomly assigned to either an endurance-trained or a sedentary group. Training consisted of a progressive exercise protocol on a motor-driven treadmill, 5 days/wk for 12 wk. Knee lateral menisci were obtained from anesthetized rats and used for morphological and biochemical analyses. Gastrocnemius succinate dehydrogenase increased 65% in the endurance-trained group, as evidence for a training effect. In the trained group, collagen, proteoglycan, and calcium concentrations increased significantly in the posterior region of the lateral meniscus. In contrast, no significant changes were found in the anterior region of the lateral meniscus. The region-specific changes in meniscal concentrations of calcium and matrix macromolecules in response to prolonged exercise are consistent with the distinctly different mechanical properties and functional roles of the anterior and posterior regions of the rat knee meniscus.

The role of intersegmental dynamics during rapid limb oscillations

The interactive dynamic effects of muscular, inertial and gravitational moments on rapid, multi-segmented limb oscillations were studied. Using three-segment, rigid-body equations of motion, hip, knee and ankle intersegmental dynamics were calculated for the steady-state cycles of the paw-shake response in adult spinal cats. Hindlimb trajectories were filmed to obtain segmental kinematics, and myopotentials of flexors and extensors at each of the three joints were recorded synchronously with the ciné film. The segmental oscillations that emerged during the paw-shake response were a consequence of an interplay between active and passive musculotendinous forces, inertial forces, and gravity. During steady-state oscillations, the amplitudes of joint excursions, peak angular velocities, and peak angular accelerations increased monotonically and significantly in magnitude from the proximal joint (hip) to the most distal joint (ankle). In contrast to these kinematic relationships, the maximal values of net moments at the hip and knee were equal in magnitude, but of significantly lower magnitude than the large net moment at the ankle joint. At both the ankle and the knee, the flexor and extensor muscle moments were equal, but at the hip the magnitude of the peak flexor muscle moment was significantly greater than the extensor muscle moment. Muscle moments at the hip not only acted to counterbalance accelerations of the more distal segments, but also acted to maintain the postural orientation of the hindlimb. Large muscle moments at the knee functioned to counterbalance the large inertial moments generated by the large angular accelerations of the paw. At the ankle, the muscle moments dominated the generation of the paw accelerations. At the ankle and the knee, muscle moments controlled limb dynamics by slowing and reversing joint motions, and the active muscle forces contributing to ankle and knee moments were derived from lengthening of active musculotendinous units. In contrast to the more distal joints, the active muscles crossing the hip predominantly shortened as a result of the interplay among inertial forces and gravitational moments. The muscle function and kinetic data explain key features of the complex interactions that occur between central control mechanisms and multi-segmented, oscillating limb segments during the paw-shake response.

Heterogeneous mechanical response of rat knee menisci to thermomechanical stress

Thermodilatometric, dynamic thermomechanical, and light-microscopic analyses were done on the anterior and posterior regions of the rat knee menisci to correlate regional differences in morphology and extracellular matrix composition with regional mechanical behavior. Following the administration of a general anesthetic, menisci were excised from 12 young female Sprague-Dawley rats. During thermodilatometric and thermomechanical testing, tissue temperature was increased at a constant rate of 3.5 degrees C/min from 30 to 100 degrees C. Light microscopy revealed regional differences in cell density and proteoglycan content. The anterior horn was significantly heavier (greater than 87%) and thicker (greater than 60%) than the posterior region. During thermal analyses, both the anterior and posterior horns decreased in tissue thickness as the temperature increased from 30 to 73 degrees C. After 73 degrees C, however, the posterior horn expanded significantly, whereas the anterior remained in a comparatively contracted state. The rates of linear contraction and expansion of the posterior horn were seven times those of the anterior horn, and the stiffness of the anterior horn was significantly greater than the posterior horn.

Intralimb coordination of the paw-shake response: a novel mixed synergy

Intralimb coordination of the paw-shake response (PSR) was studied in five normal and eleven spinal adult cats. Representative extensor and flexor muscles that function at the hip, knee, and ankle joints were recorded, and in six spinal cats the kinematics of these joints were determined from high-speed cinefilm. The PSR was characterized uniquely by mixed (flexor-extensor) synergies. Knee extensor (VL) and ankle flexor (TA) coactivity constituted one synergy, while the second synergy included hip extensors (GM, BF), knee flexors (BF, LG), and ankle extensor (LG). Joint displacements reflected the mixed synergy. Motions at the knee and ankle were out of phase, while motions at the hip were in phase with movements of the knee. Electromyographic burst durations and onset latencies were similar for normal and spinal cats, and in all cycles of a given PSR, the recruitment pattern was consistent for all muscles, except VL. High variability and missing bursts marked the activity of VL in some spinal cats. In PSRs with missing VL bursts, oscillations at the knee joint were not coordinated with cyclic actions at the hip and ankle. From the kinematic records three distinct phases of the PSR were identified: start-up consisted of the initial four to six cycles during which hip, knee, and ankle actions progressively became organized; steady-state included the middle three to five cycles that were characterized by consistent displacement at all three joints; and slow-down comprised the last three to four cycles during which the rate of oscillations slowed, and joint excursions decreased. During steady-state cycles, muscle contractions acted to reverse joint motions at the knee and ankle joints. Thus, knee and ankle extensor recruitment coincided with joint flexion, while joint flexors were recruited during joint extension. Muscle activity at the hip, however, was in phase with displacement. While neural input to muscle is consistent throughout the three phases of the PSR, segment motions can become progressively organized during start-up to achieve stable oscillations. Whether the PSR attains steady-state or not may hinge on the sensitive interplay that occurs between muscle activities and intersegmental mechanical interactions. That kinetic interplay is detailed in the following paper.

Ground reaction forces and center of pressure patterns in the gait of children with amputation: preliminary report

Ground reaction forces and center of pressure (CP) were studied during gait in children with unilateral lower extremity amputations. Five children, three with knee disarticulations and two with above-knee amputations, walked at slow, normal, and fast speeds, while wearing a conventional SACH prosthetic foot and again, while wearing an experimental CAPP prosthetic foot. Fore-aft (F-A) and vertical force (VF) components and CP patterns were examined for a total of 90 trials. Walking speed had a significant effect on both F-A and VF amplitudes. No differences, however, were found between the force amplitudes of the SACH foot and CAPP foot. Significant asymmetries were found in the force and amplitudes of a child's natural limb versus the prosthetic limb; the retarding and propelling F-A forces in the prosthetic limb were significantly less than the corresponding forces in a child's natural limb. The CP patterns during stance phase were markedly different for a child's natural limb, prosthetic limb with the SACH foot, and prosthetic limb with the CAPP foot. With the CAPP foot, the CP remained in the forefoot region during stance. In contrast with the CAPP foot, the SACH foot had a potential for producing a flexor moment at the knee joint at the initiation of the foot-ground contact. Stability in the prosthetic knee was enhanced when the children wore a CAPP prosthetic foot.

Contrasting roles of inertial and muscle moments at knee and ankle during paw-shake response

Intralimb kinetics of the paw-shake response (PSR) were studied in four spinal, adult cats. Using rigid body equations of motion to determine the dynamic interactions between limb segments, knee and ankle joint kinetics were calculated for the steady-state cycles as defined in the preceding paper. Hindlimb motion was filmed (200 frames/s) to obtain knee and ankle kinematics. Responses of flexors and extensors at both joints were recorded synchronously with cinefilm. Ankle and knee joint kinematics were determined from 51 steady-state cycles of 16 PSRs. Average maximum displacements, velocities, and accelerations were substantially greater for the ankle than for the knee joint. Knee and ankle motions were out of phase in the first part of the cycle; knee extension occurred simultaneously with ankle flexion. In the second part of the cycle, motions at the two joints were sequential; rapid knee flexion, accompanied by negligible ankle displacement, preceded rapid ankle extension with minimal knee displacement. At the ankle joint, peak net moments tending to cause flexion and extension were similar in magnitude and determined primarily by muscle moments. Moments due to leg angular acceleration contributed significantly to an extensor peak in the net moment near the end of the cycle. Other inertial and gravitational moments were small. At the knee joint, net moments tending to cause flexion and extension were also similar, but smaller than those at the ankle. The knee muscle moments, however, were large and counteracted large inertial moments due to paw angular acceleration. Also, moments due to leg angular acceleration and knee linear acceleration were substantial and opposite in effect. Other inertial and the gravitational moments were negligible. Muscle moments slowed and reversed joint motions, and active muscle force components of muscle moments were derived from lengthening of active musculotendinous units. Segmental interactions, in which proximal segment motion augmented distal segment velocity, increased the effectiveness of PSR steady-state cycles by facilitating the generation of extremely large paw linear accelerations. Limb oscillations during PSR steady-state result from interactions between muscle synergies and motion-dependent limb dynamics. At the ankle, muscle activity functioned to control paw acceleration, whereas at the knee, muscle activity functioned to control leg and paw inertial interactions.(ABSTRACT TRUNCATED AT 400 WORDS)

Regional biochemical and morphological characteristics of rat knee meniscus

The biochemical and morphological characteristics of the anterior and posterior regions of the rat knee meniscus were studied. The anterior meniscal horn was thicker and contained a lower concentration of DNA, hydroxyproline, and uronic acid as compared to the posterior region. The calcium concentration in the anterior region, however, was significantly greater than the calcium concentration in the posterior horn. Presence of a significant concentration of calcium in the normal rat knee meniscus is unique to rats and uncommon in other mammalian species.

Modulation of limb dynamics in the swing phase of locomotion

A method was presented for quantifying cat (Felis catus) hind limb dynamics during swing phase of locomotion using a two-link rigid body model of leg and paw, which highlighted the dynamic interactions between segments. Comprehensive determination was made of cat segment parameters necessary for dynamic analysis, and regression equations were formulated to predict the inertial parameters of any comparable cat. Modulations in muscle and non-muscle components of knee and ankle joint moments were examined at two treadmill speeds using three gaits: (a) pace-like walk and trot-like walk, at 1.0 ms-1, and (b) gallop, at 2.1 ms-1. Results showed that muscle and segment interactive moments significantly effected limb trajectories during swing. Some moment components were greater in galloping than in walking, but net joint maxima were not significantly different between speeds. Moment magnitudes typically were greater for pace-like walking than for trot-like walking at the same speed. Generally, across gaits, the net and muscle moments were in phase with the direction of distal joint motion, and these same moments were out of phase with proximal joint motion. Intersegmental dynamics were not modulated exclusively by speed of locomotion, but interactive moments were also influenced significantly by gait mode.

Strain topography of human tendon and fascia

Discretized surface strains for human tendon and fascia were photogrammetrically determined with high-speed cinematography and were displayed topographically using three-dimensional computer graphics. Substantial differences were found between estimates of tissue strain measured from grip motion versus discretized strain estimates from high-speed films. The computer-generated contour maps also provide a useful technique for analyzing the nonhomogeneity of tendon and fascial strains during high rate tests.

Biomechanical analysis of human ligament grafts used in knee-ligament repairs and reconstructions

Virtually all types of collagenous tissues have been transferred in and around the knee joint for intra-articular and extra-articular ligament reconstructions. However, the mechanical properties (in particular, strength) of such grafts have not been determined in tissues from young adult donors, where age and disuse-related effects have been excluded. To provide this information, we subjected ligament graft tissues to high-strain-rate failure tests to determine their strength and elongation properties. The results were compared with the mechanical properties of anterior cruciate ligaments from a similar young-adult donor population. The study indicated that some graft tissues used in ligament reconstructions are markedly weak and therefore are at risk for elongation and failure at low forces. Grafts utilizing prepatellar retinacular tissues (as in certain anterior-cruciate reconstructions) and others in which a somewhat narrow width of fascia lata or distal iliotibial tract is utilized are included in this at-risk group. Wider grafts from the iliotibial tract or fascia lata would of course proportionally increase ultimate strength. The semitendinosus and gracilis tendons are stronger, having 70 and 49 per cent, respectively, of the initial strength of anterior cruciate ligaments. The bone-patellar tendon-bone graft (fourteen to fifteen millimeters wide, medial or central portion) was the strongest, with a mean strength of 159 to 168 per cent of that of anterior cruciate ligaments. Patellar tendon-bone units, based on grip-to-grip motions, were found to be three to four times stiffer than similarly gripped anterior cruciate ligaments, while gracilis and semitendinosus tendon preparations had values that were nearly identical to those of anterior cruciate ligaments.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of structure and strain measurement technique on the material properties of young human tendons and fascia

It is generally recognized that the organization of collagen bundles in soft tissues strongly influences their material properties. To study this, sixty failure tests were conducted on double-layered fascia lata, 'isolated' parallel-bundled tendons (gracilis and semitendinosus) and parallel-bundled bone-patellar tendon-bone units taken from about the knees of eighteen young human donors (mean age of 26 yr). Surprisingly, most material parameters for the two-layered fascia lata did not differ significantly from corresponding values for the isolated tendons and tendon-bone preparations, suggesting their longitudinal fibers predominated during loading. Differences were present however between the gracilis tendon and all other tissues for both modulus and maximum stress. The large variations in reported maximum and failure strains for tendons, fascia and other collagenous tissues prompted the other phase of the study. During 15 of the 60 failure tests, surface markers were simultaneously filmed to determine; differences between local surface strains and grip to grip values; the amount of tissue slippage and/or failure occurring in the grips; and the effect of strain measurement technique on tissue moduli and failure energy densities. Maximum local strains were found to be 25-30% of grip strains for all tissues tested. Some slippage and/or failure could be seen in all isolated tissues which were gripped directly although their maximum grip strains were similar to values for tendon-bone units. For all tissues, two to three fold differences were also found in moduli and failure energy densities between grip and midregion measurements.

Correlation of Movement Patterns via Pattern Recognition

A structural pattern recognition method for the quantitative determination of equivalence or similarity between movement patterns was examined. A chain encoding technique was implemented for the analysis of lower limb trajectories during walking and running. Conjoint angular displacement or angular velocity patterns provided kinematic data which were cross-correlated to determine geometric congruity of within and between subject motor patterns. The correlations of the movement patterns during different speeds of locomotion revealed numerical coefficients which consistently and quantitatively discriminated the similarity or dissimilarity of limb movement patterns.

Stride kinematics and knee joint kinetics of child amputee gait

As a few quantitative data exist characterizing the gait of child lower extremity amputee (CLEA), this study provides such data by examining selected kinematic and kinetic variables during locomotion. Five unilateral CLEA were analyzed in an experimental design which comprised 3 different speeds of walking, 2 types of prosthetic feet (SACH-foot and an experimental CAPP-foot), and normal versus prosthetic limbs for a total of 104 trials. Cinematographic data and footprint information where used to quantify walking speed and uniplanar lower extremity angles. A rigid-body, linked model was used to evaluate the extent and range to thigh and leg motion and moments of force at the knee joint. The kinematic and kinetic characteristics of CLEA gait were affected significantly by variations in walking speed. Stride length, step length, and walking velocity decreased and stride width increased when the children used the experimental foot component; as speed of walking increased, stride length and step length increased with both prosthetic feet. Foot angles increased with walking velocity except for fast walking with the experimental foot component. Joint angles were significantly different between the normal and prosthetic limb. The only significant limb angle differences between the 2 prosthetic feet were in maximum hip flexion and the total range of thigh movement; the experimental foot elicited less hip flexion and and smaller range of thigh movement.

Biomechanical evaluation of bilateral tibial spiral fractures during skiing--a case study

The utility of a biomechanical evaluation of an injury is shown for an incident involving bilateral tibial spiral fractures during skiing. With incorporation of directly measured binding release forces into a biomechanical model, estimates of torques transmitted to the skier's leg were determined. The analysis revealed a high potential for torsional injury to the leg with the particular release mechanism and settings used by this skier; for nearly all applications of lateral loads to the ski, this skier's bindings would not have released prior to exceeding the torsional elastic threshold of the tibia.

Muscle architecture and force-velocity characteristics of cat soleus and medial gastrocnemius: implications for motor control

1. Isometric and isotonic contractile parameters of the soleus (SOL) and medial gastrocnemius (MG) muscles of seven adult cats were studied. In addition, architectural characteristics of six contralateral pairs of these ankle extensors were determined. 2. The in situ peak isometric tetanic tension developed by the MG at the Achilles tendon is nearly 5 times (9,846 vs 2,125 g) that of the SOL muscle. However, when differences between the MG and SOL in fiber length (2.01 vs 3.66 cm), muscle mass (9.80 vs. 3.31 g), and angle of pinnation (21.4 vs. 6.4 degrees) are considered, the specific tensions of these muscles are similar (approximately 2.3 kg x cm-2). 3. When the effects of muscle architecture are eliminated, the nearly threefold greater maximum isotonic shortening velocity (Vmax) of sarcomeres of the MG (38.2 micron/s) relative to the SOL (13.4 micron/s) is presumably due to intrinsic differences in the biochemical properties of these muscle. However, the Vmax developed by the MG at the Achilles tendon (258.6 mm/s) during a shortening contraction is only 1.5 times that of the SOL (176.3 mm/s) due to the influence of these muscles' specific architectures. 4. Variations in geometrical characteristics of the SOL and MG are consonant with the relative amounts of participation of these muscles during posture, locomotion, and jumping. Posture requires the development of low forces for prolonged periods for which the SOL seems best suited both architecturally and physiologically. The MG, relatively inactive during quiet standing, becomes responsible for a greater percentage of tension and shortening speed during plantar flexion (E3) as gait speeds increase, which is consistent with this muscle's greater tension- and velocity-generating capacity. 5. At high speeds of locomotion (3.0 m/s) and jumping, the shortening velocities developed at the end of E3 (approximately 20-40 ms before paw off) exceed Vmax of the SOL. Consequently, the SOL, although electrically active, cannot contribute to the tensions required to generate the shortening velocities dictated by these movements. 6. These data demonstrate the influence of the differing geometries of the SOL and MG on the roles of these muscles in generating forces at varying velocities, as demanded by the dynamics of the movement.

Rapid ankle extension during paw shakes: selective recruitment of fast ankle extensors

1. Electromyographic (EMG) signals from slow (soleus) and fast (lateral gastrocnemius) ankle extensors of six cats were recorded during rapid and alternate flexion-extension of the hindlimb elicited by placing the paw in water or by sticking tape to the plantar pads. High-speed 16-mm film, taken at 100 or 200 frames/s, was analyzed to determine the knee and ankle joint kinematics. 2. During 77 typical records, which averaged eight paw shakes each, a single extension-flexion cycle measured by the paw shake interval (PSI) of the electromyogram record, averaged 88 ms and ranged from 55 to 110 ms. LG EMG bursts of 10 ms in duration were synchronized with the peak displacement of ankle flexion. The SOL was inactive throughout these typical records. 3. During four atypical records from one cat, the average OSI was 141 ms, and both lateral gastrocnemius (LG and soleus (SOL) were active simultaneously. At a range of 6--8 cycles/s, these slower shakes are comparable to rhythmic actions of scratching )12) and locomotion (27); cyclic movements that typically include the recruitment of soleus. 4. It is suggested that paw shaking is an automatic movement triggered primarily by large, low-threshold afferents innervating the central plantar pads, which may selectively recruit the fast extensors while inhibiting the slow extensor. This is the only movement of the hindlimb recorded to date in our laboratory in which the tlg was active without the SOL. This unique dissociation of recruitment of slow and fast ankle extensors may be dictated by the time constraints imposed by the rapid cyclic movements of paw shaking.

Human patellar-tendon rupture

The first biomechanical analysis of a human patellar-tendon rupture during actual sports competition is reported. Cinematographic data for analysis were collected at a national weight-lifting championship. Dynamic equations to mathematically model the lifter were developed to compute time course and magnitudes of hip, knee and ankle-joint moments of force and of tensile loading of the patellar tendon before and during tendon trauma. Results provided evidence that the range of maximum tensile stress of the tendon may be considerably greater during rapid dynamic loading conditions, as in many sports situations, than maximum tensile stress obtained during static test conditions.