Injury Analysis & Prevention Lab
Relevant Papers
Risk Factors Of Muscle Injury And Falls Related To Load Handling
Technique
Author
Lars I.E. Oddsson
Abstract
The main goal of the postural control system is to maintain stability
in all situations and during all tasks that are performed. An external
perturbation of posture triggers specific automatic responses which act to
restore equilibrium. Few studies have investigated the interaction between
automatic postural responses and voluntary movements.
It has been hypothesized that certain combinations of voluntary
movement and postural perturbation, such as a slip or a trip, may cause a
conflict between postural and voluntary motor commands simultaneously
requiring different functions of the same muscle/muscle group. It is
further suggested that this conflict increases the risk of direct tissue
injury and/or a fall.
The occurrence of conflicts between motor commands is probably common
during different occupational activities such as walking, lifting or
carrying loads as well as during reaching for or pushing and pulling
objects. An unexpected slip or trip that coincides with such an ongoing
voluntary activity would have the potential to cause a motor command
conflict. However, most of these situations will not lead to an injury.
Nevertheless, the occurrence of a motor command conflict may still
represent an injury prone situation and the margin of safety, before an
injury occurs, is likely decreased. The current presentation will report
findings that suggest the presence of a motor command conflict in the
lumbar back muscles of subjects performing a lifting motion while being
perturbed on a balance platform.
A series of experiments were conducted where subjects were perturbed
randomly at the feet in an anterior-posterior direction during vertical
lifting of a 20 kg load. Subjects stood on a specially designed moving
platform (BALDER, BALance DisturbER) programmed to accelerate (9.81 m/s2)
and then decelerate (9.81 m/s2) over a distance of 0.11 m.
Kinematic information was obtained from a video based motion analysis
system (ARIEL, APAS). Muscle activity was recorded from soleus, tibialis
anterior, anterior deltoideus and erector spinae (ES).
Posterior platform perturbation triggered a hip strategy by the subject
which resulted in a sudden cessation of activity in the ES muscle. In this
phase of the movement, the task of lifting the load and supporting the
trunk would require the ES muscle to be continuously active. However,
instead there was an interruption of the lifting motion followed by a
brief sudden trunk flexion.
The trunk flexion coincided with in an extended period of rapid changes
in activation level of the ES muscle suggesting that the muscle was
activated in a lengthening contraction to brake the trunk flexion
movement. This effect was not seen during anterior platform perturbations.
The results support the hypothesis that certain combinations of voluntary
movement and postural perturbation elicit a conflict between motor
commands simultaneously trying to access the same muscle group.
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