The effect of manual therapy on gastrocnemius muscle stiffness in healthy individuals

Highlights

• MyotonPRO is a reliable tool to assess gastrocnemius stiffness in weightbearing.

• A single measure of the MyotonPRO is reliable.

• Individuals receiving manual therapy demonstrated changes in muscle stiffness

Abstract

Study

Design

Randomized clinical trial.

Background

Muscle stiffness is a potential complication after injury and has been shown to be a risk factor for injury in healthy individuals.

Objectives

The primary purpose of this study was to assess the short-term effects of manual therapy (MT) on muscle stiffness of the gastrocnemius in both a relaxed and contracted state. The secondary purpose was to assess the reliability of a novel clinical tool (MyotonPRO) to measure muscle stiffness in the gastrocnemius in both a passive and contracted state.

Methods

Eighty-four consecutive healthy individuals were randomized to receive Manual Therapy (MT group) directed at the right-side ankle and foot or no treatment (CONTROL group). Muscle stiffness of the gastrocnemius was assessed bilaterally in all participants at baseline and then immediately after intervention in a relaxed and contracted state. Group (MT vs. CONTROL) by side (ipsilateral vs. contralateral) by time (pre vs. post) effects were compared through a 3-way interaction utilizing mixed model ANOVA. Reliability of the MyotonPRO was assessed with two-way mixed model intraclass correlation coefficients.

Results

There was a significant 3-way interaction for muscle stiffness of the gastrocnemius in a relaxed state (p < 0.01), but not contracted state (p = 0.54). All conditions had increased resting muscle stiffness from pre to post measures except for the ipsilateral limb of the MT group. There was not a significant interaction for muscle stiffness in a contracted state. Reliability estimates (ICC) for muscle stiffness measures ranged between 0.898 and 0.986.

Conclusion

The change in muscle stiffness of the gastrocnemius in a relaxed state depended upon whether individuals received MT. Muscle stiffness measures were highly reliable based on single measurements.

Level of evidence

Therapy, level 2.

Introduction

Muscle stiffness is most commonly quantified as the slope of a strain-stress curve of a material in the elastic deformation region of interest or Young’s modulus, and is an intrinsic biomechanical muscle property [1]. In healthy individuals, muscle stiffness is primarily a function of both active and passive muscle tension or force and is measurable with techniques such as ultrasound elastography and myotonometry [2].

After injury, aberrant muscle stiffness may develop that can potentially impede the return of normal function [3]. A recent study in patients with low back pain found that increased stiffness of the lumbar multidifus was the strongest independent predictor of back pain status (odds ratio = 4.13) of the seven included demographic, anthropometric, and medical history variables [4]. Several studies also suggest that increased overall stiffness of the lower extremities maybe associated with increased risk of Achilles tendon injury in runners, perhaps due to the resultant increase in braking forces [5], [6]. These findings and others have led to the suggestion that reducing muscle stiffness can possibly lead to a decrease in muscle injury [7].

While one recent study reported increased calf muscle stiffness could be beneficial to athletic performance in football players, other studies have reported that increased muscle stiffness may be a predisposing factor to AT injury [5], [6], [8]. Thus the assessment of muscle stiffness may be an important measure to prevent injury as well as enhance athletic performance. In either case, it would appear important to assess muscle stiffness in a relaxed state as well as in a contracted state while weight bearing since the majority of athletic activities occur in a closed kinetic chain system. While recent research has evaluated the ability of the MyotonPRO to assess muscle stiffness in a relaxed state while in a prone position, no studies to our knowledge have assessed it in a weight bearing contracted state, which may be important in an athletic setting [9]. It would also appear, based on current evidence, that quantifying muscle stiffness could be beneficial from an athletic injury prevention perspective.

Manual therapy is defined as “skilled passive movements of joints and soft tissue” [10] and has been shown to impart changes in range of motion, pain thresholds, and load distributions of the foot [11], [12], [13], [14]. Manual therapy has also been shown to increase monaminergic output centrally (reducing pain levels) [15] that can lead to alterations in monoamine levels. These changes that occur as a result of manual therapy treatments can lead to decreased motoneuron excitability [16], These mechanical and functional changes could conceivably reduce muscle stiffness and have been demonstrated in both healthy and injured patient populations [11], [17], [18].

The MyotonPRO is a relatively new handheld device that provides a simple and noninvasive way to characterize mechanical stiffness of skeletal muscle [19], [20]. It operates by applying a mechanical impulse to the skin, which is then transmitted to the underlying soft tissue and muscle (.58 N for 15 ms). The exterior mechanical impulse causes the muscle to respond by a damped natural oscillation which is recorded by an accelerometer in the form of an acceleration signal. The oscillation of the muscles is recorded by the probe to calculate mechanical stiffness (N/m) of the muscle [21]. A built-in gravity compensation system enables measurements to be taken at any angle in relation to the gravity vector and is not affected by changes in altitude. The MyotonPRO has been shown to be both a valid and reliable tool to measure muscle stiffness [8], [9]. In addition, the MyotonPRO has good reliability to measure stiffness of the gastrocnemius in both a resting position and a contracted position in prone utilizing a handheld dynamometer to determine MVIC [9]; however, the authors are not aware of any studies to date assessing gastrocnemius muscle stiffness in a weightbearing position.

To the authors knowledge, no studies have assessed the effects of manual therapy on muscle stiffness of the gastrocnemius muscle as assessed by the MyotonPRO. In addition, the intra-operator reliability of the MyotonPRO has not been determined when assessing muscle stiffness in a weight-bearing position. The primary purpose of this study was to assess the short-term effect of manual therapy (MT), directed at the foot and ankle, on muscle stiffness of the gastrocnemius in both a relaxed and contracted state in a healthy asymptomatic population.

In the present study, it is hypothesized that resting stiffness would decrease, and contracted stiffness would increase from pre to post measures in the treated limbs of the manual therapy group only. The secondary purpose of this study was to assess the within-day and between-day reliability of a single rater using the MyotonPRO for the assessment of gastrocnemius muscle stiffness in weight-bearing.