Biomechanics responsible for effect of elbow position on biceps tendon reflex

Kaveer Nandigam 
 Jawaharlal Institute of Post Graduate Medical Education and Research, 428, Lister House, Pondicherry, India

Correspondence Address:
Kaveer Nandigam
Jawaharlal Institute of Post Graduate Medical Education and Research, 428, Lister House, Pondicherry
India

Full Text

Sir,

I read with interest the electrophysiological study published by Keles et al[1] quantifying the effect of elbow position on biceps tendon reflex in normal healthy volunteers. The amplitude and reflex onset latency were studied at 3 different elbow positions, at 90º, 120º, and 150º of joint angles. It was interesting to note in this study that the mean amplitude decreased progressively from 90º to 150º. The amplitude of the biceps tendon reflex is proportional to the net forces acting at the elbow in the direction of contraction of the biceps muscle during the biceps tendon reflex, which include the force of contraction (F) of the biceps muscle and the opposing force of the horizontal component of the 'tension' (T) in the forearm due to the weight of the forearm. The force of contraction of the biceps muscle during the reflex is again dependent on various factors such as the strength of tendon tap, influences of reciprocal inhibition etc. As mentioned in this study, if care is exerted to maintain the strength of the tendon taps similar, the amplitude may, hence depend primarily on the opposing force of the forearm tension.

When the elbow is placed at 90º, as seen in [Figure:1], tension 'T' is equal to the component of the weight of the forearm in vertical direction i.e. 'mg' ('mass of the forearm' X 'acceleration due to gravity'). The horizontal component of this force which opposes the force of contraction of muscle is mgCos 90º i.e. zero.

When the elbow is placed at 120º, as seen in [Figure:2], tension 'T' along the direction of the forearm will be mg/cos 30º. The horizontal component of this force opposing 'F' will be equal to (mg/cos 30º) X cos 60º or 0.57 mg or approximately half the weight of the forearm.

When the elbow is placed at 150º, as seen in [Figure:3], tension 'T' along the direction of the forearm will be mg/cos 60º. The horizontal component of this force opposing 'F' will be equal to (mg/Cos 60º) X Cos 30º or 1.73 mg. Therefore at 150º, the opposing force to muscle contraction was almost equal to twice the weight of the forearm, which led to a statistically significant decrease in the amplitude at 150º as reported by Keles et al.[1]

Evidently the opposing force to the muscle contraction due to the horizontal component of the tension in the forearm increased with the increase in the elbow angle and thence led to the decrease in the amplitude of the muscle reflex with increasing angle. Clinically, most commonly appreciated character of a DTR is its amplitude and is thus accordingly noted as exaggerated, normal or diminished reflex. Hence to obtain the maximum amplitude in order to make an appropriate clinical judgment about the biceps tendon reflex, it is best to tap the biceps tendon with the elbow placed at an angle of 90º.

References