Time course of stress relaxation and recovery in human ankles
Introduction
When soft tissues are stretched for a period of seconds or minutes they undergo progressive deformation, so that they can then be extended further with a constant force (“creep”) or exert less force when stretched to a constant length (“stress relaxation”) [1]. In everyday parlance, stretching is said to make the tissues more flexible. This property is exploited by clinicians and athletes when they stretch muscles and joints.
A number of studies have examined the time course of the effects of stretching [2], [3], [4], [5], because it has been thought that this might enable recommendations to be made concerning optimal stretch duration. In a frequently cited study, Madding and colleagues [5] found no significant difference between the effects of hip adductor stretches of 15, 45 s or 2 min duration. They suggested that it is reasonable to stretch muscles for only 15 s when immediate increases in the range of motion are desired. However, in most tissues the effects of stretching continue indefinitely, albeit at a diminishing rate [6], [7]. Madding et al.'s study may have detected further effects of stretching at 2 min if it utilised a larger sample or a design that provided greater statistical precision.
The Madding study investigated the time course of stretching by allocating subjects to groups that were stretched for different durations. A better way to examine the time course of the effects of stretching is to monitor the time course of stretch on each individual, either by stretching tissues to a constant torque and continuously measuring joint angle (i.e., monitor creep), or by stretching tissues to a constant-angle and continuously measuring torque (i.e., monitor stress relaxation). Toft and colleagues [8] showed that when the ankle was stretched to a constant dorsiflexion angle, torque initially declined rapidly and thereafter declined linearly with the logarithm of time. These authors monitored the stress relaxation for 5 min, at which time passive torque was still declining significantly.
The purpose of the present study is to extend these observations to the time course of stress relaxation in the human ankle. We sought to apply a dorsiflexion stretch to the ankle long enough to be able to confidently predict the effects of a hypothetical stretch of infinite duration. This would make it possible to describe the effects of short duration stretches in terms of a proportion of the greatest possible effect of stretching.
In vivo studies on isolated tissues and studies on human joints show that stress relaxation is a reversible phenomenon [9], [10], [11], [12]. That is, when the stretch is released, the degree of stress sustained at any length slowly returns to pre-stretch values. Few studies have examined the time course of recovery from sustained stretch in humans [13] and none, to our knowledge, have examined the time course of recovery of stress relaxation. Therefore, a second purpose of the present study was to make a preliminary investigation of the time course of recovery from stretch. To do this, we examined the degree of recovery that occurred when a 20-min stretch was released for 2 min. As the mechanical properties of resting skeletal muscle depend on the muscle's contraction history (“thixotropy”), [14], [15], we also investigated whether the degree of recovery from a stretch to the ankle is influenced by muscle contraction during the recovery period.
A second experiment examined the time course of recovery in more detail. The purpose was to determine how much recovery occurred at any time following removal of the stretch. In this experiment we manipulated the duration of the recovery period following a 20-min stretch.
Section snippets
Experiment 1
Subjects. Eight healthy university students volunteered to participate in the first experiment. There were seven females and one male with a mean age of 21.0 years (SD 0.82). None had a history of major musculoskeletal trauma to the ankle.
Procedures. All procedures were approved by the institutional ethics committee. Subjects were required to attend the laboratory on four separate days. On each day the ankle was stretched to a fixed dorsiflexion angle for 20 min. There was then a 2-min recovery
Experiment 1
The initial ankle angles attained when a “strong but not painful” stretch was applied varied considerably between-subjects (range 101–125°, mean 114°, SD 8°).
Time course of stress relaxation. A representative stress relaxation profile is shown in Fig. 2. The time course of stress relaxation was closely fitted by a double exponential function of the formwhere T is the torque (N m) and t is the stretch time (s). and τ2 are the constants obtained from the non-linear
Discussion
In experiment 1, subjects in the relax-long condition received an uninterrupted 42-min stretch to the ankle. Over this period, passive ankle torque declined to within 8% of the final value anticipated had the double exponential decline continued indefinitely. This made it possible to accurately characterise most of the time course of stress relaxation. The main findings were that torque declines as a double exponential function, and the time course of this decline is slow. For example, it takes
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