The effects of distance and height on maximal isometric push and pull strength with reference to manual transmission truck drivers

This study investigates how different heights and distance of the hand from the body affect the maximal static force that can be either pushed or pulled while seated. Twenty female subjects performed maximum voluntary contractions (MVC) pulls and pushes while seated, at all nine combinations of the three height positions and three distance positions tested. The distances were measured from the seat reference point of the chair and were 18 cm (close), 36.1 cm (medium) and 45 cm (high). The three heights were measured from the ground and were: 41.1 cm (low), 64.3 cm (middle) and 82 cm (high). The subject's sitting and shoulder posture was controlled to prevent torso flexion, rotation or swaying back and forth. Surface EMG sensors were placed over the flexor digitorum profundus, tricep, bicep, anterior deltoid and posterior deltoid to monitor the muscle activity over each of these positions. The average force exerted at each position was analyzed using an s-beam load cell connected to a commercial stick shift handle. The optimal location was selected based on the ‘best position criteria’ which was defined as the location with the greatest force output and lowest muscle activity. The furthest distance tested and the middle height best fit these criteria in this experiment. This study also found that seated pushing tasks are more affected by the distance of the hand from the body than the starting height of the hand. This means that the increase in muscle strength is greater when the distance is increased versus when the height is increased. Finally, this study found that muscle strength during a pulling task is affected equally by the distance and height of the hand.Relevance to industryAlthough this task is applicable to manual transmission truck drivers, it can also be used to develop standards for other industries where seated pushing and pulling tasks are prevalent. This can include, but is not limited to, airline pilots, helicopter pilots, tractor drivers and bus drivers.