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Both the hamstrings and the gastrocnemius are flexors of the knee
joint. Actually, gastrocnemius is the muscle that initiates knee flexion
when the knee is fully extended (because the hamstrings insert onto the
sides of the tibia they have very little leverage in this posture).
Gastrocnemius is also (primarily) an important plantarflexor of the ankle.
What is being described here is the interaction between two
two-joint muscle groups at their common joint (the knee). When the ankle is
kept in full dorsiflexion the gastrocnemius tries to respond by moving its
free end (at the knee). When a hip extension stretch is done on the
hamstrings, those muscles also try to respond on their free end, which is
also the knee. The pull that is felt in the popliteal fossa is exactly what
would be expected from this simultaneous interaction. The greater strength,
and leverage, of the quadriceps femoris muscles is the only thing that keeps
the knee from collapsing into flexion during this exercise.
That said, there are a few rare individuals that do have a muscle
that extends from ischium to calcaneous. When this condition occurs in
humans the muscle is very gracile and not really capable of providing any
independent movement (it is usually seen as a small bundle of muscle fibers
that link the hamstrings with gastrocnemius). So, while this muscular
condition would contribute to the tightness felt in the popliteal fossa, the
musculoskeletal relationships described above would still provide the
better, and more common, explanation.
Thomas M. Greiner, Ph.D.
Associate Professor of Anatomy
Department of Basic Sciences
New York Chiropractic College
Seneca Falls, NY 13148-0800 USA
Office Phone: (315) 568-3183
Gait Lab Phone: (315) 568-3150
Fax: (315) 568-3017
Email: tgreiner@nycc.edu
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joint. Actually, gastrocnemius is the muscle that initiates knee flexion
when the knee is fully extended (because the hamstrings insert onto the
sides of the tibia they have very little leverage in this posture).
Gastrocnemius is also (primarily) an important plantarflexor of the ankle.
What is being described here is the interaction between two
two-joint muscle groups at their common joint (the knee). When the ankle is
kept in full dorsiflexion the gastrocnemius tries to respond by moving its
free end (at the knee). When a hip extension stretch is done on the
hamstrings, those muscles also try to respond on their free end, which is
also the knee. The pull that is felt in the popliteal fossa is exactly what
would be expected from this simultaneous interaction. The greater strength,
and leverage, of the quadriceps femoris muscles is the only thing that keeps
the knee from collapsing into flexion during this exercise.
That said, there are a few rare individuals that do have a muscle
that extends from ischium to calcaneous. When this condition occurs in
humans the muscle is very gracile and not really capable of providing any
independent movement (it is usually seen as a small bundle of muscle fibers
that link the hamstrings with gastrocnemius). So, while this muscular
condition would contribute to the tightness felt in the popliteal fossa, the
musculoskeletal relationships described above would still provide the
better, and more common, explanation.
Thomas M. Greiner, Ph.D.
Associate Professor of Anatomy
Department of Basic Sciences
New York Chiropractic College
Seneca Falls, NY 13148-0800 USA
Office Phone: (315) 568-3183
Gait Lab Phone: (315) 568-3150
Fax: (315) 568-3017
Email: tgreiner@nycc.edu
-----------------------------------------------------------------
To unsubscribe send SIGNOFF BIOMCH-L to LISTSERV@nic.surfnet.nl
For information and archives: http://isb.ri.ccf.org/biomch-l
Please consider posting your message to the Biomch-L Web-based
Discussion Forum: http://movement-analysis.com/biomch_l
-----------------------------------------------------------------