Musculoskeletal disorders have been a main concern
in milkers for many years. To improve posture, a formula
was developed in a previous study to calculate
ergonomically optimal working heights for various milking
parlor types. However, the working height recommendations
based on the formula for the herringbone
30° parlor were broad. To clarify the recommendations
for the optimal working height, we investigated the
effect of working height on upper limb and shoulder
muscle contraction intensities. We evaluated 60 milking
cluster attachment procedures in a herringbone
30° milking parlor in 7 men and 9 women. Specifically,
we examined the effect of working height on muscle
contraction intensity of 4 arm and shoulder muscles bilaterally
(flexor carpi ulnaris, biceps brachii, deltoideus
anterior, and upper trapezius) by using surface electromyography.
The working heights (low, medium, and
high), which reflect the ratio of the subject’s height to
the height of the udder base, were used in the milking
health formula to determine and fit individual depth
of pits. Data were evaluated for each muscle and arm
side in the functions holding and attaching. Statistical
analysis was performed using linear mixed effects
models, where muscle contraction intensity served as
a target variable, whereas working height coefficient,
sex, subject height, and repetition were treated as
fixed effects, and repetition group nested in working
height nested in subject was considered a random effect.
Contraction intensities decreased with decreasing
working height for the deltoideus anterior and upper
trapezius, but not for the flexor carpi ulnaris or the
biceps brachii muscles in both holding and attaching
arm functions. We found that milking at a lower working
height reduced muscle contraction intensities of the
shoulder muscles. Women showed higher contraction
intensities than men, whereas subject height had no
effect. The study demonstrated that a lower working
height decreased muscular load during milking. These
lower working heights should be used within the recommendations
made by the milking health formula for
the herringbone 30°. Working heights could be adjusted
effectively for milkers of varying body height. Future
studies should therefore use the milking health formula
as a tool to objectively compare and improve the accuracy
of the working height coefficients.