Anthony V. Seaber

Viscoelastic properties of muscle-tendon units The biomechanical effects of stretching

Most muscle stretching studies have focused on defin ing the biomechanical properties of isolated elements of the muscle-tendon unit or on comparing different stretching techniques. We developed an experimental model that was designed to evaluate clinically relevant biomechanical stretching properties in an entire muscle- tendon unit. Our objectives were to characterize the viscoelastic behavior of the muscle-tendon unit and to consider the clinical applications of these viscoelastic properties. Rabbit extensor digitorum longus and tibialis anterior muscle-tendon units were evaluated using methods designed to simulate widely used stretching tech niques. Additionally, the effects of varying stretch rates and of reflex influences were evaluated. We found that muscle-tendon units respond viscoelastically to tensile loads. Reflex activity did not influence the biomechani cal characteristics of the muscle-tendon unit in this model. Experimental techniques simulating cyclic stretching and static stretching resulted in sustained muscle-ten don unit elongations, suggesting that greater flexibility can result if these techniques are used in the clinical setting. With repetitive stretching, we found that after four stretches there was little alteration of the muscle- tendon unit, implying that a minimum number of stretches will lead to most of the elongation in repetitive stretching. Also, greater peak tensions and greater energy absorptions occurred at faster stretch rates, suggesting that the risk of injury in a stretching regimen may be related to the stretch rate, and not to the actual technique. All of these clinically important considera tions can be related to the viscoelastic characteristics of the muscle-tendon unit.

The role of fatigue in susceptibility to acute muscle strain injury.

We investigated the role of fatigue in muscle strain injuries using the extensor digitorum longus muscles of 48 rabbits. The muscles of the rabbits were fatigued by 25% or 50% then stretched to failure and compared with the contralateral controls. Three rates of stretch were used. The force to muscle failure was reduced in the fatigued leg in all groups (range, 93% to 97.4% compared with the controls). The change in muscle length in the fatigue groups was not different from the controls. The amount of energy absorbed in the fa tigued muscle was 69.7% to 92% that of the energy absorbed in the control muscle. The lowest energy absorption occurred in muscles that were more fa tigued. In eight additional rabbits, fatigued extensor digitorum longus muscles were compared with sub- maximally stimulated muscles with the equivalent con tractile properties, and no difference was seen. Mus cles subjected to strains are frequently injured under high-intensity eccentric loading conditions. Under these conditions, muscles absorb energy and provide control and regulation of limb movement. Our data showed that muscles are injured at the same length, regardless of the effects of fatigue. However, fatigued muscles are able to absorb less energy before reach ing the degree of stretch that causes injuries.

Warm-up and muscular injury prevention: An update

SummaryMusculotendinous injuries are responsible for a significant proportion of injuries incurred by athletes. Many of these injuries are preventable. Importantly, musculotendinous injuries have a high incidence of recurrence. Thus, muscle injury prevention is advocated by coaches and trainers. Yet, most of the recommendations for muscle injury prevention are attempted by athletes and taught by coaches without supporting scientific evidence.This paper reviews the mechanics of muscular injury, associated and predisposing factors, and methods of prevention with a review of the supporting research and rationale for these methods with an emphasis on warm-up, stretching and strengthening.Muscles that are capable of producing a greater force, a faster contraction speed and subjected to a greater stretch are more likely to become injured. Many factors have been associated with muscular injury. From current research, some conclusions and recommendations for muscle injury prevention can be made. Overall and muscular conditioning and nutrition are important. Proper training and balanced strengthening are key factors in prevention of musculotendinous injuries as well. Warm-up and stretching are essential to preventing muscle injuries by increasing the elasticity of muscles and smoothing muscular contractions. Improper or excessive stretching and warming up can, however, predispose to muscle injury. Much research is still needed in this important aspect of sports medicine.

Biomechanical comparison of stimulated and nonstimulated skeletal muscle pulled to failure

We compared the biomechanical properties of passive and stimulated muscle rapidly lengthened to failure in an experimental animal model. The mechanical param eters compared were force to tear, change in length to tear, site of failure, and energy absorbed by the muscle- tendon unit before failure. Paired comparisons were made between 1) muscles stimulated at 64 Hz (tetanic stimulation) and passive (no stimulation) muscles, 2) muscles stimulated at 16 Hz (wave-summated stimu lation) and passive muscles, and 3) muscles stimulated at 64 Hz and at 16 Hz. Both tetanically stimulated and wave-summation con tracted muscles required a greater force to tear (at 64 Hz, 12.86 N more, P < 0.0004; and at 16 Hz, 17.79 N more, P < 0.003) than their nonstimulated controls, while there was no statistical difference in failure force between muscles stimulated at 16 Hz and 64 Hz. The energy absorbed was statistically greater for the stim ulated muscles than for the passive muscles in Groups 1 and 2 (at 64 Hz, 100% more, P < 0.0003; and 16 Hz, 88% more, P < 0.0002). In Group 3, the tetanically contracted muscle-tendon units absorbed 18% more energy than the wave-summated stimulated muscles (P < 0.01). All muscles tore at the distal musculotendi nous junction, and there was no difference in the length increase at tear between muscles in each group. These findings may lead to enhanced understanding of the mechanism and physiology of muscle strain injuries.

The role of warmup in muscular injury prevention

This study is an attempt to provide biomechanical sup port for the athletic practice of warming up prior to an exercise task to reduce the incidence of injury. Tears in isometrically preconditioned (stimulated before stretching) muscle were compared to tears in control (nonstimulated) muscle by examining four parameters: 1) force and 2) change of length required to tear the muscle, 3) site of failure, and 4) length-tension defor mation. The tibialis anterior (TA), the extensor digitorum longus (EDL), and flexor digitorum longus (EDL) mus cles from both hindlimbs of rabbits comprised our ex perimental model. Isometrically preconditioned TA (P < 0.001), EDL (P < 0.005), and FDL (P < 0.01) muscles required more force to fail than their contralateral controls. Precondi tioned TA (P < 0.05), EDL (P < 0.001), and FDL (P < 0.01) muscles also stretched to a greater length from rest before failing than their nonpreconditioned con trols. The site of failure in all of the muscles was the musculotendinous junction; thus, the site of failure was not altered by condition. The length-tension deforma tion curves for all three muscle types showed that in every case the preconditioned muscles attained a lesser force at each given increase in length before failure, showing a relative increase in elasticity, although only the EDL showed a statistically significant differ ence. From our data, it may be inferred that physiologic warming (isometric preconditioning) is of benefit in pre venting muscular injury by increasing the and length to failure and elasticity of the muscle-tendon unit.

Biomechanical and histological evaluation of muscle after controlled strain injury

This study correlates force generation and healing in muscle after controlled strain injury. Right tibialis ante rior (TA) muscles from 30 rabbits were strained to approximately 80% of failure while the left TA muscles served as control. Both injured and control muscles were then tested for ability to generate force. Seven animals were sacrificed immediately after testing and the muscles were examined grossly and histologically. Remaining animals were retested at 24 hours (N = 7), 48 hours (N = 8), and 7 days (N = 8). Contractile ability following injury was 70.5% of control immediately, 51.1% at 24 hours, 74.5% at 48 hours, and 92.5% at 7 days. Immediate histology showed limited distal fiber rupture and hemorrhage. By 24 hours, histology showed fiber necrosis, infiltration of inflammatory cells, edema, and hemorrhage. At 48 hours there was com plete fiber breakdown and intense inflammatory cell proliferation. At 7 days inflammation was reduced and collagen fibrosis more advanced. Our findings demon strate that injured muscle begins functional recovery by 48 hours despite inflammation and active healing. This suggests that decreasing muscle function seen clinically between 24 and 48 hours following strain injury may result from pain due to inflammation. Scarring and fibrosis seen at 7 days may explain the frequent recur rence of injury to strained muscles.

Anterior cruciate ligament allograft transplantation: Long-term function, histology, revascularization, and operative technique

In recent years much effort has been devoted to finding a satisfactory replacement for the injured ACL. None of the reconstruction techniques used in the past can be considered ideal because of their inability to dupli cate the complex geometry, structure, and function of the ligament. Current advances in allograft transplan tation and cryopreservation have led us to design and implement an experimental model for testing the feasi bility of cryopreserved ACL allotransplantation. Groups of dogs were used to evaluate the effect of cryopreservation on ligament strength and to compare the relative performance of both autograft and allograft ACL transplants up to 18 months after implantation. The ligaments were examined mechanically, histologi cally, and microangiographically. The cryopreservation process and duration of stor age had no effect on the biomechanical or structural properties of the ligament. The mechanical integrity of the allografts was similar to that of the autografts, with both achieving nearly 90% of control ligament strength by 36 weeks. Revascularization approached normal by 24 weeks in both autograft and allograft. No evidence of structural degradation or immunological reaction was seen. Based on these results, we believe that a cryopre served ACL allograft can provide the ideal material for ACL reconstruction. We have outlined a surgical tech nique for harvesting and implanting this graft clinically.

A biomechanical comparison of the methods for treating Kienböck's disease

Reduction of lunate compression is thought to promote revascularization of the lunate in patients with Kienböcks disease. The decompressing abilities of ulnar lengthening, radial shortening, capitate-hamate fusion, and scaphoid-trapezium-trapezoid (STT) fusion were examined in axially loaded, whole arm specimens. Lunate strain was monitored by electronic strain gauges and found to be proportional to the axial load borne by the bone. The STT fusions and the procedures to alter relative radial and ulnar length were successful in relieving lunate loading throughout a functional range of wrist motion and forearm rotation, but the capitate-hamate fusion was ineffective. Only the STT fusion resulted in a significant decrease in wrist range of motion. Incremental ulnar lengthening and radial shortening revealed that approximately 2 mm of length change maximizes lunate decompression without greatly increasing the risk of disorders of the distal radioulnar joint and ulnocarpal impingement.

Experimental muscle strain injury Early functional and structural deficits and the increased risk for reinjury

The structural and functional strength of a muscle immediately after an experimentally created strain injury was examined to provide clinically relevant information for the early treatment of muscle strain injuries. The extensor digitorum longus muscles of 12 adult male rabbits were studied. Contractile force and shortening, and peak load were determined for control muscles. A nondisruptive strain injury was created by stretching the experimental muscles just short of complete rup ture. Contractile force generation and shortening, and peak load were determined after the experimental strain injury. Peak load was 63% and elongation to rupture was 79% for the experimental muscles relative to the controls. Statistically significant lower values for con tractile force generation and shortening were also seen in the experimental muscles. Histologic and gross ex aminations revealed that incomplete disruptions oc curred near the distal muscle-tendon junction. These experimental data suggest clinical implications, such as 1) a muscle-tendon unit is significantly more susceptible to injury following a strain injury than normal muscle, 2) early return to the uncontrolled environment of athletic competition may place the injured muscle at risk for further injury, and 3) therapeutic regimens designed to achieve an early return to competition may further increase the risk for additional injury by eliminating protective pain mechanisms. Although the decrements in peak load and elongation to failure are less than normal muscle, the values seem high enough to allow mobilization of the injured extremity and functional re habilitation.

The effect of muscle architecture on the biomechanical failure properties of skeletal muscle under passive extension

This study investigates the biomechanical failure prop erties of five architecturally different skeletal muscles and examines the role muscle structure plays in the passive extension characteristics of musculotendinous units. The muscles used in this study fall into four morphologic categories: fusiform, unipennate, bipen nate, and multipennate. Each muscle was pulled to failure at three different rates of strain (1, 10, and 100 cm/min). Specimens of fusiform, unipennate, and bipennate muscles were pulled from their proximal as well as distal attachments. The relationship of elongation to failure of the entire musculotendinous unit to resting muscle fiber length was examined to determine the effect of angle of pennation and fiber length on the failure properties. Our results demonstrate that all four muscle types tested show injury and rupture at the musculotendinous junction whether pulled from proximal or distal attach ment, regardless of muscle structure and rate of strain. There was a statistically significant difference (P < 0.005) in the degree of elongation to failure relative to resting muscle fiber length, with a tendency to greater elongation relative to fiber length for muscles with more pennate structure (tibialis anterior, 72.7% ± 1.0%; ex tensor digitorum longus, 113.1% ± 3.5%; rectus fem oris, 225.5% ± 3.7% elongation in percent resting fiber length).