A recent United States study involving power grip strength and endurance of 148 health workers from various occupations led the investigators to create a graphic analysis method called the Power Grip Graphic Analyzer. The method provides a visual comparison of an individual’s strength-endurance relative to a gender norm. It also produces a vector which reflects fatigue magnitude (through length and direction) and allows the calculation of a numeric representation of an individual’s gender specific strength-endurance.
Study data revealed that males had greater mean initial and mean final strength over a repetitive, 60 second duration evaluation. However, there was little difference in strength endurance between genders (as measured by (initial strength-final strength)/(initial strength x 100)).
Specifically, power grip measurements for males and females were:
- mean initial strength of 56.0 kg and 35.7 kg, respectively
- mean final strength of 43.3 kg and 27.3 kg, respectively
- endurance of 22.7% and 23.4%, respectively
The Bottom Line – How This Applies To Ergonomists
This study presents a protocol for obtaining objective hand power grip strength-endurance data which can indicate hand health. Values plotted on the Power Grip Graphic Analyzer easily show practical information relative to maximum initial strength, maximum strength after 60 seconds of repetitive maximum strength contractions, and percent of strength lost.
The described procedure could be useful as part of a pro-active company occupational health employee check-up. Further, the numeric value of hand strength-endurance produced by the Graphic Analyzer which could be useful when comparing the same employee over time and when comparing two employees of the same gender.
Other Key Points
A comparison of study findings to a prior investigation by a different author revealed similar strength endurance values among those with healthy upper extremities. This suggests a validity of the concept that genders lose maximum grip strength at the same rate.
When the statistics of that similar study were used, 46% of injured hands and 29% of uninjured hands had values outside the normal range of the Graphic Analyzer data points. However, overall, the similar study had greater abnormal values for the injured versus the uninjured hands.
A group of 148 (74 males, average age 38.4 years/74females, average age 33.5 years) healthy subjects were recruited from multiple occupations that varied from sedentary to heavy physical workers.
Data was collected by using a Jamar hand dynamometer with force/time values processed through the BTE Technologies functional testing system. Voluntary maximum hand contractions were performed for approximately 1.7 seconds followed by a rest period of approximately 1.7 seconds over a duration of 60 seconds (total of 18 exertions were conducted). The Jamar hand grip position was set at position 3.
The subject’s preferred hand was tested. While standing, the upper extremity was placed in a postural position of mid-axillary line shoulder adduction, elbow flexed to 90 degrees, and forearm midway between pronation/supination.
The mean peak forces generated during contraction cycles 1-3 and 16-12 were identified as initial strength and final strength, respectively. The strength decrement index (SDI) – a measurement of fatigue or loss of strength – was calculated for each volunteer as a percentage. The SDI formula was (initial strength-final strength)/initial strength x 100.
The authors created a “Power Grip Analyzer” – a gender specific graphic chart involving strength scores and endurance scores. Using the chart (details on chart design are supplied within the article and illustrated in Figure 2 of the publication), an individual’s strength/endurance performance was evaluated in two ways:
- Vector Measurement – a subject’s data produced two points on the chart; a line connecting the two points created magnitude and slope which was compared to the gender average
- Strength-Endurance Performance Score – the sum of numeric values associated with initial strength and endurance performance
This study can be acquired at: http://springerlink.com/content/q7j387k9x1628743/?p=8b91313d64004f649a19722cf0fa1dfc&pi=5
Article Title: A Strength-Endurance Index for Power Grip
Publication: Journal of Occupational Rehabilitation, 19:56–63, 2009
Authors: DW Jones, LD Robertson, and SF Figoni
This article originally appeared in The Ergonomics Report™ on 2010-04-27.