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21 - 26 MD SCREEN 19/7/06 12:10 pm Page 21 MUSCULOSKELETAL SCREENING

SCREENING TO IDENTIFY INJURY AND PERFORMANCE RISK:

MOVEMENT CONTROL TESTING – THE MISSING PIECE OF THE PUZZLE

By Mark J Comerford, MCSP

BACKGROUND To date, attempts to screen have been centred on testing either a strength para- meter (eg. force, power) joint range, or a mobility parameter (eg. joint range, muscle extensibility). Assessing these parameters invariably tends to isolate the individual joints or muscles in non- functional 'standard' situations. Some attempts have focused on developing functional tests based on work-specific tasks and sport-specific skills. When func- tionally orientated tests are used they tend to be highly specific to one task or sport-specific skill. All have been relative- ly unsuccessful at predicting risk of injury and have resulted in training programmes being biased towards high load muscle (strength) training and high force joint and muscle stretching (which in some cases may be a contributing factor to the cause of some overuse injuries).

The process of assessing the control of 'real' function, that is, the influence of the multiple muscle interactions acting on multiple joints in functionally orientated tasks has not be promoted because it has universally been put in the 'too hard bas- ket'. This is the missing piece of the screening puzzle.

MUSCLE RECRUITMENT To understand how multiple muscles act together around a joint it is necessary to be aware of the influence of recruitment threshold. Most muscles are composed predominantly of two different types of motor units. There are slow, low threshold (tonic) motor units and fast, high thresh- old (phasic) motor units. Research has identified other types of motor units, but this basic classification is useful for reha- bilitation purposes (1). The major differ- ences between low and high threshold recruitment are summarised in Table 1.

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Pre-participation screening of athletes during pre-season training is now almost mandatory in elite and professional sport and is commonplace in competitive sport even at junior levels. This screening is promoted as part of either an injury risk management strategy or as a performance optimisation strategy. The devel- opment of realistic and workable risk assessment and screening processes and subsequent training packages for sport and occupational health has been some- what of an unattainable dream among exercise, sporting and occupational health professionals for many years. This article outlines one possible approach that could be applied to patients and athletes alike.

FUNCTION

Contraction speed Contraction force

SLOW MOTOR UNITS FAST MOTOR UNITS (tonic recruitment)

slow low

Recruitment dominance primarily recruited at high increasingly recruited at

low % of MVC (< 25%) higher % of MVC (40+ %) or if plan to perform a fast movement

Recruitment threshold

Fatiguability ROLE

low (sensitive)- easily activated

fatigue resistant control of normal

Table 1. Main characteristics of low and high threshold muscles

The key points are that slow motor units have a low threshold of activation, slow speed of contraction, a low contraction force and are fatigue resistant. Fast motor units have a high threshold of activation, fast speed of contraction, a high contrac- tion force and fatigue quickly. Research has demonstrated that slow motor units are predominately recruited during normal daily activities (2).

Normal postural control and functional movement of the unloaded limbs and trunk should ideally demonstrate efficient recruitment of deeper, segmentally attaching muscles that provide a stability role. This is primarily a function of slow motor unit (tonic) recruitment. High load or high speed activities normally demon-

high (insensitive)

- requires higher stimulus fast fatiguing

rapid or accelerated

functional postures and movement and high load unloaded movements

activity Reproduced with permission of Kinetic Control

strate a dominance of recruitment of more superficial, multi-joint muscles that are biomechanically advantaged for high load, large range and high speed. High load activity or strength training (endurance or power overload training) is a function of both slow (tonic) and fast (phasic) motor unit recruitment. Functionally, efficient recruitment of slow motor units will optimise postural holding/anti-gravity and stability function, while efficient recruitment of fast motor units will optimise rapid/accelerated movement and the production of high force or power.

Recruitment and hypertrophy are very dif- ferent processes. Recruitment is modulat- ed by the higher central nervous system and is powerfully influenced by the affer-

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(phasic recruitment) fast

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