Muscle fatigue does not lead to increased instability of upper extremity repetitive movements

Abstract

Muscle fatigue alters neuromuscular responses. This may lead to increased sensitivity to perturbations and possibly to subsequent injury risk. We studied the effects of muscle fatigue on movement stability during a repetitive upper extremity task. Twenty healthy young subjects performed a repetitive work task, similar to sawing, synchronized with a metronome before and after performing each of two fatiguing tasks. The first fatigue task (LIFT) primarily fatigued the shoulder flexor muscles, while the second fatigue task (SAW) fatigued all of the muscles of the arm. Subjects performed each task in random order on two different days at least seven days apart. Instantaneous mean EMG frequencies (IMNF) decreased over both fatiguing tasks indicating that subjects did experience significant muscle fatigue. The slopes of the IMNF over time and the decreases in maximum force measurements demonstrated that the LIFT fatigue task successfully fatigued the shoulder flexors to a greater extent than any other muscle. On average, subjects exhibited more locally stable shoulder movements after the LIFT fatigue task (p=0.035). They also exhibited more orbitally stable shoulder (p=0.021) and elbow (p=0.013) movements after the SAW fatigue task. Subjects also had decreased cocontraction at the wrist post-fatigue for both tasks (p=0.001) and at the shoulder (p<0.001) for the LIFT fatigue task. Therefore, increased dynamic stability of these repeated movements cannot be explained by increased muscle cocontraction. Possible alternative mechanisms are discussed.

Figure 1

A) General protocol for the experiment. On both visits, all 20 subjects completed all activities. Only the fatigue task differed. B) During the sawing task subjects were seated in a high-back chair and restrained by a five-point harness (Corbeau, Sandy, UT) across their waist and shoulders. A handle with an adjustable weight stack was able to slide with low friction across a horizontal track. The device was adjusted so subjects sat with a knee angle of 90°. The height of the metal track was adjusted so the midpoint between the third and fourth finger was at the level of the xiphoid process. The front/back position of the chair was adjusted to be comfortable for the subject and allow for a full range of motion. This was defined as being at a maximum point when almost to full extension (no hyperextension) and at a minimum point at the level of the sternum. C) In the sawing (Saw) fatigue task subjects pushed 25% of their pushing/pulling MVC for four minutes. In the lifting (Lift) fatigue task, subjects lifted 10% of their shoulder flexion MVC from at their side to approximately 90 degrees in the sagittal plane for three minutes. In both tasks, subjects could stop at any point they felt they could no longer continue.

Figure 2

EMG linear envelopes for one trial of a representative subject are shown. Here EMG_ago was the anterior deltoid, EMG_ant was the posterior deltoid and EMG_min was whichever had the lower value at that percent of the movement cycle. Subjects pushed the weight forward during the first 50 % of the movement cycle and then brought it back toward them during the last 50 %.

Figure 3

The slope of the IMNF vs. cycle curves are shown for each condition. Errorbars represent 95% confidence intervals about the mean of the 20 subjects. Subjects showed significant fatigue in all muscles as a result of both fatigue protocols (95% confidence intervals do not include 0).

Figure 4

MVC measures were taken at four time points in the experiment (Fig. 1A). A) MVCs are shown as a percent of maximum for Top: shoulder flexion and extension, Middle: shoulder internal and external rotation, Bottom: elbow flexion and extension. B) Balance ratios of shoulder flexion to extension and shoulder internal to external rotation MVCs are shown for each time point. Significant differences from MVC1 are represented as ‘*’ for both fatigue protocols, ‘†’ for the lifting protocol only, and ‘§’ for the sawing protocol only. Significant differences between conditions at that time point are represented by ‘c’.

Figure 5

Mean values of λ_s^* are shown for the shoulder, elbow and wrist. Data are shown prior to fatigue ‘○’ and after ‘x’ either the sawing or lifting fatigue protocol. Errorbars are the 95% confidence intervals across subjects about the mean. ‘^*’ represent significant pre/post effects. n = 20.

Figure 6

A) The MaxFM across the movement cycle is shown after the LIFT and SAW tasks. 0% was at the start of the pushing phase. At 50%, the subject began to pull the weight back toward them. B) The peak value of the MaxFM across the movement cycle is shown before and after each fatigue condition. ‘○’ represents the pre-fatigue sawing trials while ‘x’ denotes the post-fatigue sawing trials. Significant differences from pre to post-fatigue for that condition are denoted with ‘^*’. n = 20.

Figure 7

Cocontraction indicies (CCI) are shown for the shoulder, elbow and wrist pre and post fatigue for each fatigue condition. ‘○’ represents the pre-fatigue sawing trials while ‘x’ denotes the post-fatigue sawing trials. Significant differences from pre to post-fatigue for that condition are denoted with ‘^*’.

Figure 8

The average movement pattern for the shoulder is shown pre and and post fatigue for each condition. From right to left, these angles are shoulder elevation, humeral plane angle, and humeral rotation angle. Dashed lines represent the standard deviation across subjects for that condition.