Tendon and ligament adaptation to exercise, immobilization, and remobilization

Tendon and ligament adaptation to exercise, immobilization,
and remobilization
Tishya A . L. Wren, PhD ; Gary S . Beaupre, PhD; Dennis R . Carter, PhD
Rehabilitation Research and Development Center ; Veterans Affairs Health Care System, Palo Alto, CA;
Biomechanical Engineering Division, Mechanical Engineering Department, Stanford University, Stanford, CA.
Abstract—This study provides atheoretical and computational
basis for understanding and predicting how tendons and ligaments
adapt to exercise, immobilization, and remobilization.
In a previous study, we introduced a model that described the
growth and development of tendons and ligaments . In this
study, we use the same model to predict changes in the crosssectional
area, modulus, and strength of tendons and ligaments
dueto increased or decreased loading . The model predictions
are consistent with the results of experimental exercise and
obilization studies performed by other investigators . These
results suggest that the same fundamental principles guide both
development and adaptation . A basic understanding of these
principles can contribute both to prevention of tendon and ligament
injuries and to moreeffective rehabilitation when injury
does occur.
Keywords : adaptation, exercise, immobilization, ligament,
mechanobiology, tendon.
This material is based upon work supported by the Veterans Administration
Rehabilitation Research and Development Program, Washington, DC 20420.
Address all correspondence and requests for reprints to : Tishya Wren,
Rehabilitation Research and DevelopmentCenter, Palo Alto Veterans Affairs
Health Care System (153), 3801 Miranda Avenue, Palo Alto, CA 94304;
email : wren@stanford .edu
INTRODUCTION
Hard and soft skeletal connective tissues adapt in
response to mechanical loading . This adaptation allows
the tissues to withstand the mechanical loads imposed on
them during normal activities of daily living. While
numerous computational andtheoretical studies have
examined the functional adaptation of bone (1-5), few
analytical studies have considered the adaptation of soft
tissues such as tendons and ligaments . In a previous study
(6), we introduced a computational model relating
changes in the geometric and material properties of tendons
and ligaments to biological and mechanobiological
influences . In this paper, we use thesame model to predict
the response of tendons and ligaments to increased or
decreased loading experienced during exercise, immobilization,
and remobilization.
A number of experimental studies have examined the
effects of exercise, immobilization, and remobilization on
tendons and ligaments. Experimental exercise studies have
compared the biochemical composition and mechanical
properties oftendons and ligaments from exercised animals
with those from sedentary controls . The exercised
animals undergo a prescribed regimen of running exercises,
while the control animals engage in nolnlal cage activ-
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ity. In some cases, exercise of mature animals leads to
increases in tendon weight, cross-sectionalarea, collagen
content, modulus, and strength (7,8) . In other cases, exercise
has no effect on these properties (9,10) . Similarly,
exercise before maturity may lead to an increase in mature
tendon weight (7), or it may not affect mature tendon
weight (11-43) . These inconsistencies may stem from differences
in the magnitude of loading applied to various
structures during general exerciseprograms (14).
Immobilization studies have compared the biochemical
composition and mechanical properties of tendons
and ligaments subject to reduced loading with those from
controls experiencing normal loading . Various procedures
have been used to reduce the loading, including
ankle disarticulation (15), cast immobilization (16,17),
and internal fixation (18,19) . In mature animals,...